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Human untargeted metabolomics studies annotate only ~10% of molecular features. We introduce reference-data-driven analysis to match metabolomics tandem mass spectrometry (MS/MS) data against metadata-annotated source data as a pseudo-MS/MS reference library. Applying this approach to food source data, we show that it increases MS/MS spectral usage 5.1-fold over conventional structural MS/MS library matches and allows empirical assessment of dietary patterns from untargeted data.

Bacterial-fungal interactions (BFIs) can shape the structure of microbial communities, but the small molecules mediating these BFIs are often understudied. We explored various optimization steps for our microbial culture and chemical extraction protocols for bacterial-fungal co-cultures, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) revealed that metabolomic profiles are mainly comprised of fungi derived features, indicating that fungi are the key contributors to small molecule mediated BFIs. LC-inductively coupled plasma MS (LC-ICP-MS) and MS/MS based dereplication using database searching revealed the presence of several known fungal specialized metabolites and structurally related analogues in these extracts, including siderophores such as desferrichrome, desferricoprogen, and palmitoylcoprogen. Among these analogues, a novel putative coprogen analogue possessing a terminal carboxylic acid motif was identified fromScopulariopsisspp. JB370, a common cheese rind fungus, and its structure was elucidated via MS/MS fragmentation. Based on these findings, filamentous fungal species appear to be capable of producing multiple siderophores with potentially different biological roles (i.e. various affinities for different forms of iron). These findings highlight that fungal species are important contributors to microbiomes via their production of abundant specialized metabolites and their role in complex communities should continue to be a priority.

Microbial interactions are key aspects of the biology of microbiomes. Recently, there has been a shift in the field towards studying interactions in more representative contexts, whether using multispecies model microbial communities or by looking at interactions in situ. Across diverse microbial systems, these studies have begun to identify common interaction mechanisms. These mechanisms include interactions related to toxic molecules, nutrient competition and cross-feeding, access to metals, signaling pathways, pH changes, and interactions within biofilms. Leveraging technological innovations, many of these studies have used an interdisciplinary approach combining genetic, metabolomic, imaging, and/or microfluidic techniques to gain insight into mechanisms of microbial interactions and into the impact of these interactions on microbiomes.

Microbial interactions are expected to be major determinants of microbiome structure and function. Although fungi are found in diverse microbiomes, their interactions with bacteria remain largely uncharacterized. In this work, we characterize interactions in 16 different bacterial-fungal pairs, examining the impacts of 8 different fungi isolated from cheese rind microbiomes on 2 bacteria (Escherichia coli and a cheese-isolated Pseudomonas psychrophila). Using random barcode transposon site sequencing (RB-TnSeq) with an analysis pipeline that allows statistical comparisons between different conditions, we observed that fungal partners caused widespread changes in the fitness of bacterial mutants compared to growth alone. We found that all fungal species modulated the availability of iron and biotin to bacterial species, suggesting that these may be conserved drivers of bacterial-fungal interactions. Species-specific interactions were also uncovered, a subset of which suggest fungal antibiotic production. Changes in both conserved and species-specific interactions resulted from deletion of a global regulator of fungal specialized metabolite production. This work highlights the potential for broad impacts of fungi on bacterial species within microbiomes.

Although metals are essential for the molecular machineries of life, systematic methods for discovering metal–small molecule complexes from biological samples are limited. Here, we describe a two-step native electrospray ionization–mass spectrometry method, in which post-column pH adjustment and metal infusion are combined with ion identity molecular networking, a rule-based data analysis workflow. This method enabled the identification of metal-binding compounds in complex samples based on defined mass (m/z) offsets of ion species with the same chromatographic profiles. As this native electrospray metabolomics approach is suited to the use of any liquid chromatography–mass spectrometry system to explore the binding of any metal, this method has the potential to become an essential strategy for elucidating metal-binding molecules in biology. The systemic discovery of metal–small-molecule complexes from biological samples is a difficult challenge. Now, a method based on liquid chromatography and native electrospray ionization mass spectrometry has been developed. The approach uses post-column pH adjustment and metal infusion combined with ion identity molecular networking, and a rule-based informatics workflow, to interrogate small-molecule–metal binding.

Human untargeted metabolomics studies succeed in annotating only ~10% of molecular features. We, therefore, introduce reference data-driven analysis that uses the source data as a pseudo-MS/MS reference library to match against human metabolomics MS/MS data. We demonstrate this approach with food source data, allowing an empirical assessment of dietary patterns from untargeted data but is broadly applicable and provides an additional layer of interpretability to metabolomics data.

The vast majority of bacteria, including human pathogens and microbiome species, lack genetic tools needed to systematically associate genes with phenotypes. This is the major impediment to understanding the fundamental contributions of genes and gene networks to bacterial physiology and human health. Clustered regularly interspaced short palindromic repeats interference (CRISPRi), a versatile method of blocking gene expression using a catalytically inactive Cas9 protein (dCas9) and programmable single guide RNAs, has emerged as a powerful genetic tool to dissect the functions of essential and non-essential genes in species ranging from bacteria to humans1-6. However, the difficulty of establishing effective CRISPRi systems across bacteria is a major barrier to its widespread use to dissect bacterial gene function. Here, we establish 'Mobile-CRISPRi', a suite of CRISPRi systems that combines modularity, stable genomic integration and ease of transfer to diverse bacteria by conjugation. Focusing predominantly on human pathogens associated with antibiotic resistance, we demonstrate the efficacy of Mobile-CRISPRi in gammaproteobacteria and Bacillales Firmicutes at the individual gene scale, by examining drug-gene synergies, and at the library scale, by systematically phenotyping conditionally essential genes involved in amino acid biosynthesis. Mobile-CRISPRi enables genetic dissection of non-model bacteria, facilitating analyses of microbiome function, antibiotic resistances and sensitivities, and comprehensive screens for host-microorganism interactions.

International audience; Genomics and metabolomics are widely used to explore specialized metabolite diversity. The Paired Omics Data Platform is a community initiative to systematically document links between metabolome and (meta)genome data, aiding identification of natural product biosynthetic origins and metabolite structures.

We engineered a machine learning approach, MSHub, to enable auto-deconvolution of gas chromatography–mass spectrometry (GC–MS) data. We then designed workflows to enable the community to store, process, share, annotate, compare and perform molecular networking of GC–MS data within the Global Natural Product Social (GNPS) Molecular Networking analysis platform. MSHub/GNPS performs auto-deconvolution of compound fragmentation patterns via unsupervised non-negative matrix factorization and quantifies the reproducibility of fragmentation patterns across samples., Fil: Aksenov, Alexander A. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Aksenov, Alexander A. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Laponogov, Ivan. South Kensington Campus. Imperial College London. Department of Surgery and Cancer; Reino Unido, Fil: Zhang, Zheng. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Doran, Sophie L. F. South Kensington Campus. Imperial College London. Department of Surgery and Cancer; Reino Unido, Fil: Belluomo, Ilaria. South Kensington Campus. Imperial College London. Department of Surgery and Cancer; Reino Unido, Fil: Veselkov, Dennis. Intelligify Limited. Reino Unido. South Kensington Campus. Imperial College. Department of Computing; Reino Unido, Fil: Bittremieux, Wout. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Bittremieux, Wout. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Bittremieux, Wout. University of Antwerp. Department of Computer Science; Belgica, Fil: Nothias, Louis Felix. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Nothias, Louis Felix. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Nothias-Esposito, Mélissa. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Nothias-Esposito, Mélissa. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Maloney, Katherine N. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Maloney, Katherine N. Point Loma Nazarene University. Department of Chemistry; Estados Unidos, Fil: Misra, Biswapriya B. Wake Forest School of Medicine. Section of Molecular Medicine. Department of Internal Medicine. Center for Precision Medicine; Estados Unidos, Fil: Melnik, Alexey V. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Smirnov, Aleksandr. University of North Carolina at Charlotte. Department of Bioinformatics and Genomics; Estados Unidos, Fil: Du, Xiuxia. University of North Carolina at Charlotte. Department of Bioinformatics and Genomics; Estados Unidos, Fil: Jones, Kenneth L. University of North Carolina at Charlotte. Department of Bioinformatics and Genomics; Estados Unidos, Fil: Dorrestein, Kathleen. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Dorrestein, Kathleen. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Panitchpakdi, Morgan. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Ernst, Madeleine. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Ernst, Madeleine. Statens Serum Institut. Danish Center for Neonatal Screening. Department of Congenital Disorders. Section for Clinical Mass Spectrometry; Dinamarca, Fil: van der Hooft, Justin J. J. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: van der Hooft, Justin J. J. Wageningen University. Bioinformatics Group; Países Bajos, Fil: Gonzalez, Mabel. Universidad de los Andes. Department of Chemistry; Colombia, Fil: Carazzone, Chiara. Universidad de los Andes. Department of Chemistry; Colombia, Fil: Amézquita, Adolfo. Universidad de los Andes. Department of Biological Sciences; Colombia, Fil: Callewaert, Chris. Center for Microbial Ecology and Technology; Belgica, Fil: Callewaert, Chris. University of California. Department of Pediatrics; Estados Unidos, Fil: Morton, James T. University of California. Department of Pediatrics; Estados Unidos, Fil: Morton, James T. Simons Foundation. Flatiron Institute. Center for Computational Biology; Estados Unidos, Fil: Quinn, Robert A. Michigan State University. Department of Biochemistry and Molecular Biology; Estados Unidos, Fil: Bouslimani, Amina. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Bouslimani, Amina. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Orio, Andrea Albarracín. Universidad Católica de Córdoba. IRNASUS. CONICET. Facultad de Ciencias Agropecuarias; Argentina, Fil: Petras, Daniel. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Petras, Daniel. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Smania, Andrea M. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Departamento de Química Biológica Ranwel Caputto; Argentina, Fil: Smania, Andrea M. Centro de Investigaciones en Química Biológica de Córdoba. Universidad Nacional de Córdoba. CONICET; Argentina, Fil: Couvillion, Sneha P. Pacific Northwest National Laboratory. Biological Sciences Division; Estados Unidos, Fil: Burnet, Meagan C. Pacific Northwest National Laboratory. Biological Sciences Division; Estados Unidos, Fil: Nicora, Carrie D. Pacific Northwest National Laboratory. Biological Sciences Division; Estados Unidos, Fil: Zink, Erika. Pacific Northwest National Laboratory. Biological Sciences Division; Estados Unidos, Fil: Metz, Thomas O. Pacific Northwest National Laboratory. Biological Sciences Division; Estados Unidos, Fil: Artaev, Viatcheslav. LECO Corporation; Estados Unidos, Fil: Humston-Fulmer, Elizabeth. LECO Corporation; Estados Unidos, Fil: Gregor, Rachel. Ben-Gurion University of the Negev. Department of Chemistry and the National Institute for Biotechnology in the Negev; Israel, Fil: Meijler, Michael M. Ben-Gurion University of the Negev. Department of Chemistry and the National Institute for Biotechnology in the Negev; Israel, Fil: Mizrahi, Itzhak. Ben-Gurion University of the Negev. Department of Life Sciences and the National Institute for Biotechnology in the Negev; Israel, Fil: Eyal, Stav. Ben-Gurion University of the Negev. Department of Life Sciences and the National Institute for Biotechnology in the Negev; Israel, Fil: Anderson, Brooke. University of California. Division of Biological Sciences; Estados Unidos, Fil: Dutton, Rachel. University of California. Division of Biological Sciences; Estados Unidos, Fil: Lugan, Raphaël. Université d’Avignon et des Pays du Vaucluse, Agrosciences. UMR Qualisud; Francia, Fil: Boulch, Pauline Le. Université d’Avignon et des Pays du Vaucluse, Agrosciences. UMR Qualisud; Francia, Fil: Guitton, Yann. INRAe. Oniris. Laboratoire d’Etude des Résidus et Contaminants dans les Aliments (LABERCA); Francia, Fil: Prevost, Stephanie. INRAe. Oniris. Laboratoire d’Etude des Résidus et Contaminants dans les Aliments (LABERCA); Francia, Fil: Poirier, Audrey. INRAe. Oniris. Laboratoire d’Etude des Résidus et Contaminants dans les Aliments (LABERCA); Francia, Fil: Dervilly, Gaud. INRAe. Oniris. Laboratoire d’Etude des Résidus et Contaminants dans les Aliments (LABERCA); Francia, Fil: Le Bizec, Bruno. Oniris. Laboratoire d’Etude des Résidus et Contaminants dans les Aliments (LABERCA); Francia, Fil: Fait, Aaron. Ben Gurion University of the Negev. Sede Boqer Campus. The Jacob Blaustein Institutes for Desert Research. The French Associates Institute for Agriculture and Biotechnology of Dryland; Israel, Fil: Persi, Noga Sikron. Ben Gurion University of the Negev. Sede Boqer Campus. The Jacob Blaustein Institutes for Desert Research. The French Associates Institute for Agriculture and Biotechnology of Dryland; Israel, Fil: Song, Chao. Ben Gurion University of the Negev. Sede Boqer Campus. The Jacob Blaustein Institutes for Desert Research. The French Associates Institute for Agriculture and Biotechnology of Dryland; Israel, Fil: Gashu, Kelem. Ben Gurion University of the Negev. Sede Boqer Campus. The Jacob Blaustein Institutes for Desert Research. The French Associates Institute for Agriculture and Biotechnology of Dryland; Israel, Fil: Coras, Roxana. University of California. Department of Medicine. Division of Rheumatology; Estados Unidos, Fil: Guma, Monica. University of California. Department of Medicine. Division of Rheumatology; Estados Unidos, Fil: Manasson, Julia. New York University School of Medicine. Department of Medicine. Division of Rheumatology; Estados Unidos, Fil: Scher, Jose U. New York University School of Medicine. Department of Medicine. Division of Rheumatology; Estados Unidos, Fil: Barupal, Dinesh Kumar. Icahn School of Medicine at Mount Sinai. Department of Environmental Medicine and Public Health; Estados Unidos, Fil: Alseekh, Saleh. Max Planck Institute for Molecular Plant Physiology; Alemania, Fil: Alseekh, Saleh. Center of Plant Systems Biology and Biotechnology (CPSBB); Bulgaria, Fil: Fernie, Alisdair R. Max Planck Institute for Molecular Plant Physiology; Alemania, Fil: Fernie, Alisdair R. Center of Plant Systems Biology and Biotechnology (CPSBB); Bulgaria, Fil: Mirnezami, Reza. Royal Free Hospital NHS Foundation Trust. Department of Colorectal Surgery; Reino Unido, Fil: Vasiliou, Vasilis. Yale University. Yale School of Public Health. Department of Environmental Health Sciences; Estados Unidos, Fil: Schmid, Robin. University of Münster. Institute of Inorganic and Analytical Chemistry; Alemania, Fil: Borisov, Roman S. A.V. Topchiev Institute of Petrochemical Synthesis RAS; Rusia, Fil: Kulikova, Larisa N. Рeoples’ Friendship University of Russia (RUDN University); Rusia, Fil: Knight, Rob. University of California. Department of Pediatrics; Estados Unidos, Fil: Knight, Rob. University of California. UCSD Center for Microbiome Innovation; Estados Unidos, Fil: Knight, Rob. University of California. Department of Bioengineering; Estados Unidos, Fil: Wang, Mingxun. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Wang, Mingxun. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Hanna, George B. South Kensington Campus. Imperial College London. Department of Surgery and Cancer; Reino Unido, Fil: Dorrestein, Pieter C. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences; Estados Unidos, Fil: Dorrestein, Pieter C. University of California. Skaggs School of Pharmacy and Pharmaceutical Sciences. Collaborative Mass Spectrometry Innovation Center; Estados Unidos, Fil: Dorrestein, Pieter C. University of California. Department of Pediatrics; Estados Unidos, Fil: Veselkov, Kirill. South Kensington Campus. Imperial College London. Department of Surgery and Cancer; Reino Unido

SummaryThe human metabolome has remained largely unknown, with most studies annotating ∼10% of features. In nucleic acid sequencing, annotating transcripts by source has proven essential for understanding gene function. Here we generalize this concept to stool, plasma, urine and other human metabolomes, discovering that food-based annotations increase the interpreted fraction of molecular features 7-fold, providing a general framework for expanding the interpretability of human metabolomic “dark matter.”

The human metabolome has remained largely unknown, with most studies annotating ~10% of features. In nucleic acid sequencing, annotating transcripts by source has proven essential for understanding gene function. Here we generalize this concept to stool, plasma, urine and other human metabolomes, discovering that food-based annotations increase the interpreted fraction of molecular features 7-fold, providing a general framework for expanding the interpretability of human metabolomic “dark matter.”

Multispecies microbiome systems are known to be closely linked to human, animal, and plant life processes. The growing field of metabolomics presents the opportunity to detect changes in overall metabolomic profiles of microbial species interactions. These metabolomic changes provide insight into function of metabolites as they correlate to different species presence and the observed phenotypic changes, but detection of subtle changes is often difficult in samples with complex backgrounds. Natural environments such as soil and food contain many molecules that convolute mass spectrometry-based analyses, and identification of microbial metabolites amongst environmental metabolites is an informatics problem we begin to address here. Our microbes are grown on solid or liquid cheese curd media. This medium, which is necessary for microbial growth, contains high amounts of salts, lipids, and casein breakdown products which make statistical analyses using LC-MS/MS data difficult due to the high background from the media. We have developed a simple algorithm to carry out background subtraction from microbes grown on solid or liquid cheese curd media to aid in our ability to conduct statistical analyses so that we may prioritize metabolites for further structure elucidation. Graphical Abstract .

International audience; The potential of the diverse chemistries present in natural products (NP) for biotechnology and medicine remains untapped because NP databases are not searchable with raw data and the NP community has no way to share data other than in published papers. Although mass spectrometry (MS) techniques are well-suited to high-throughput characterization of NP, there is a pressing need for an infrastructure to enable sharing and curation of data. We present Global Natural Products Social Molecular Networking (GNPS; http://gnps.ucsd.edu), an open-access knowledge base for community-wide organization and sharing of raw, processed or identified tandem mass (MS/MS) spectrometry data. In GNPS, crowdsourced curation of freely available community-wide reference MS libraries will underpin improved annotations. Data-driven social-networking should facilitate identification of spectra and foster collaborations. We also introduce the concept of 'living data' through continuous reanalysis of deposited data.

Introductory paragraphThe vast majority of bacteria, including human pathogens and microbiome species, lack genetic tools needed to systematically associate genes with phenotypes. This is the major impediment to understanding the fundamental contributions of genes and gene networks to bacterial physiology and human health. CRISPRi, a versatile method of blocking gene expression using a catalytically inactive Cas9 protein (dCas9) and programmable single guide RNAs (sgRNAs), has emerged as a powerful genetic tool to dissect the functions of essential and non-essential genes in species ranging from bacteria to human. However, the difficulty of establishing effective CRISPRi systems in non-model bacteria is a major barrier to its widespread use to dissect bacterial gene function. Here, we establish “Mobile-CRISPRi”, a suite of CRISPRi systems that combine modularity, stable genomic integration and ease of transfer to diverse bacteria by conjugation. Focusing predominantly on human pathogens associated with antibiotic resistance, we demonstrate the efficacy of Mobile-CRISPRi in Proteobacteria and Firmicutes at the individual gene scale by examining drug-gene synergies and at the library scale by systematically phenotyping conditionally essential genes involved in amino acid biosynthesis. Mobile-CRISPRi enables genetic dissection of non-model bacteria, facilitating analyses of microbiome function, antibiotic resistances and sensitivities, and comprehensive screens for host-microbe interactions.

Microbial communities of fermented foods have provided humans with tools for preservation and flavor development for thousands of years. These simple, reproducible, accessible, culturable, and easy-to-manipulate systems also provide opportunities for dissecting the mechanisms of microbial community formation. Fermented foods can be valuable models for processes in less tractable microbiota.

In bacteria, disulfide bonds confer stability on many proteins exported to the cell envelope or beyond. These proteins include numerous bacterial virulence factors, and thus bacterial enzymes that promote disulfide bond formation represent targets for compounds inhibiting bacterial virulence. Here, we describe a new target- and cell-based screening methodology for identifying compounds that inhibit the disulfide bond-forming enzymes Escherichia coli DsbB (EcDsbB) or Mycobacterium tuberculosis VKOR (MtbVKOR), which can replace EcDsbB, although the two are not homologs. Initial screening of 51,487 compounds yielded six specifically inhibiting EcDsbB. These compounds share a structural motif and do not inhibit MtbVKOR. A medicinal chemistry approach led us to select related compounds, some of which are much more effective DsbB inhibitors than those found in the screen. These compounds inhibit purified DsbB and prevent anaerobic growth of E. coli. Furthermore, these compounds inhibit all but one of the DsbBs of nine other Gram-negative pathogenic bacteria tested.

Many metagenomic sequencing studies have observed the presence of closely related bacterial species or genotypes in the same microbiome. Previous attempts to explain these patterns of microdiversity have focused on the abiotic environment, but few have considered how biotic interactions could drive patterns of microbiome diversity. We dissected the patterns, processes, and mechanisms shaping the ecological distributions of three closely related Staphylococcus species in cheese rind biofilms. Paradoxically, the most abundant species ( S. equorum ) is the slowest colonizer and weakest competitor based on growth and competition assays in the laboratory. Through in vitro community reconstructions, we determined that biotic interactions with neighboring fungi help resolve this paradox. Species-specific stimulation of the poor competitor by fungi of the genus Scopulariopsis allows S. equorum to dominate communities in vitro as it does in situ . Results of comparative genomic and transcriptomic experiments indicate that iron utilization pathways, including a homolog of the S. aureus staphyloferrin B siderophore operon pathway, are potential molecular mechanisms underlying Staphylococcus - Scopulariopsis interactions. Our integrated approach demonstrates that fungi can structure the ecological distributions of closely related bacterial species, and the data highlight the importance of bacterium-fungus interactions in attempts to design and manipulate microbiomes. IMPORTANCE Decades of culture-based studies and more recent metagenomic studies have demonstrated that bacterial species in agriculture, medicine, industry, and nature are unevenly distributed across time and space. The ecological processes and molecular mechanisms that shape these distributions are not well understood because it is challenging to connect in situ patterns of diversity with mechanistic in vitro studies in the laboratory. Using tractable cheese rind biofilms and a focus on coagulase-negative staphylococcus (CNS) species, we demonstrate that fungi can mediate the ecological distributions of closely related bacterial species. One of the Staphylococcus species studied, S. saprophyticus , is a common cause of urinary tract infections. By identifying processes that control the abundance of undesirable CNS species, cheese producers will have more precise control on the safety and quality of their products. More generally, Staphylococcus species frequently co-occur with fungi in mammalian microbiomes, and similar bacterium-fungus interactions may structure bacterial diversity in these systems.

Polymicrobial infections are difficult to diagnose due to the challenge in comprehensively cultivating the microbes present. Omics methods, such as 16S rRNA sequencing, metagenomics, and metabolomics, can provide a more complete picture of a microbial community and its metabolite production, without the biases and selectivity of microbial culture. However, these advanced methods have not been applied to clinical or industrial microbiology or other areas where complex microbial dysbioses require immediate intervention. The reason for this is the length of time required to generate and analyze omics data. Here, we describe the development and application of a pipeline for multi-omics data analysis in time frames matching those of the culture-based approaches often used for these applications. This study applied multi-omics methods effectively in clinically relevant time frames and sets a precedent toward their implementation in clinical medicine and industrial microbiology., Multi-omics methods have greatly advanced our understanding of the biological organism and its microbial associates. However, they are not routinely used in clinical or industrial applications, due to the length of time required to generate and analyze omics data. Here, we applied a novel integrated omics pipeline for the analysis of human and environmental samples in under 48 h. Human subjects that ferment their own foods provided swab samples from skin, feces, oral cavity, fermented foods, and household surfaces to assess the impact of home food fermentation on their microbial and chemical ecology. These samples were analyzed with 16S rRNA gene sequencing, inferred gene function profiles, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) metabolomics through the Qiita, PICRUSt, and GNPS pipelines, respectively. The human sample microbiomes clustered with the corresponding sample types in the American Gut Project (http://www.americangut.org), and the fermented food samples produced a separate cluster. The microbial communities of the household surfaces were primarily sourced from the fermented foods, and their consumption was associated with increased gut microbial diversity. Untargeted metabolomics revealed that human skin and fermented food samples had separate chemical ecologies and that stool was more similar to fermented foods than to other sample types. Metabolites from the fermented foods, including plant products such as procyanidin and pheophytin, were present in the skin and stool samples of the individuals consuming the foods. Some food metabolites were modified during digestion, and others were detected in stool intact. This study represents a first-of-its-kind analysis of multi-omics data that achieved time intervals matching those of classic microbiological culturing. IMPORTANCE Polymicrobial infections are difficult to diagnose due to the challenge in comprehensively cultivating the microbes present. Omics methods, such as 16S rRNA sequencing, metagenomics, and metabolomics, can provide a more complete picture of a microbial community and its metabolite production, without the biases and selectivity of microbial culture. However, these advanced methods have not been applied to clinical or industrial microbiology or other areas where complex microbial dysbioses require immediate intervention. The reason for this is the length of time required to generate and analyze omics data. Here, we describe the development and application of a pipeline for multi-omics data analysis in time frames matching those of the culture-based approaches often used for these applications. This study applied multi-omics methods effectively in clinically relevant time frames and sets a precedent toward their implementation in clinical medicine and industrial microbiology.

Humans harbor microbial communities throughout the gastrointestinal tract that both respond to and modify orally ingested macronutrients, bioactive compounds, and xenobiotics; for example, the metabolism of polyphenols, heterocyclic amines, and phosphatidylcholine. However, the composition and physiological impact of our diet is also linked to the methods of food production, preparation, and consumption, which are altered by environmental and food-borne microbial communities. Metagenomic analyses spanning these various steps in human nutrition will be critical for a more comprehensive view.Studies in humans and animal models have highlighted the key role that diet plays in shaping gut microbial ecology, and how the trillions of microbes in the gut (microbiota) enable the digestion of substrates inaccessible to our own human enzymes. These transformations have been implicated in a variety of diseases and disorders, ranging from obesity, inflammatory bowel disease, heart disease, to cancer.In order to move towards personalized nutrition and medicine, it is important to take into account both our host and microbial genomes. The resulting metagenomic view of human nutrition, ranging from the initial biotransformations of food to digestion and the end result on human physiology, could have wide-ranging implications for food science, human evolutionary biology, and microbial ecology.

Escherichia coli uses the DsbA/DsbB system for introducing disulphide bonds into proteins in the cell envelope. Deleting either dsbA or dsbB or both reduces disulphide bond formation but does not entirely eliminate it. Whether such background disulphide bond forming activity is enzyme-catalysed is not known. To identify possible cellular factors that might contribute to the background activity, we studied the effects of overexpressing endogenous proteins on disulphide bond formation in the periplasm. We find that overexpressing PspE, a periplasmic rhodanese, partially restores substantial disulphide bond formation to a dsbA strain. This activity depends on DsbC, the bacterial disulphide bond isomerase, but not on DsbB. We show that overexpressed PspE is oxidized to the sulphenic acid form and reacts with substrate proteins to form mixed disulphide adducts. DsbC either prevents the formation of these mixed disulphides or resolves these adducts subsequently. In the process, DsbC itself gets oxidized and proceeds to catalyse disulphide bond formation. Although this PspE/DsbC system is not responsible for the background disulphide bond forming activity, we suggest that it might be utilized in other organisms lacking the DsbA/DsbB system.

We have presented evidence that a homologue of vertebrate membrane protein vitamin K epoxide reductase (VKOR) is an important component of the protein disulfide bond-forming pathway in many bacteria. Bacterial VKOR appears to take the place of the nonhomologous DsbB found in Escherichia coli. We also determined the structure of a VKOR from a Cyanobacterium and showed that two or four conserved cysteines are required, according to different reductants for activity in an in vitro assay. Here we present evidence for the topologic arrangement in the cytoplasmic membrane of the VKOR from Mycobacterium tuberculosis (Mtb). The results show that Mtb VKOR is a membrane protein that spans the membrane 5 times with its N-terminus in the cytoplasm, C-terminus in the periplasm, and all four cysteines facing the periplasm. The essentiality of the four conserved cysteine residues has also been demonstrated in promoting disulfide bond formation in vivo and a mixed disulfide between a cysteine of DsbA of E. coli, and one of the cysteines (Cys57) of the VKOR homologue has been identified to be a likely intermediate in the disulfide bond-forming pathway. These studies may inform future resolution of issues surrounding the functioning of human VKOR. Antioxid. Redox Signal. 14, 1413–1420.

Protein disulfide bond formation contributes to the folding and activity of many exported proteins in bacteria. However, information about disulfide bond formation is limited to only a few bacterial species. We used a multifaceted bioinformatic approach to assess the capacity for disulfide bond formation across this biologically diverse group of organisms. We combined data from a cysteine counting method, in which a significant bias for even numbers of cysteine in proteins is taken as an indicator of disulfide bond formation, with data on the presence of homologs of known disulfide bond formation enzymes. These combined data enabled us to make predictions about disulfide bond formation in the cell envelope across bacterial species. Our bioinformatic and experimental results suggest that many bacteria may not generally oxidatively fold proteins, and implicate the bacterial homolog of the enzyme vitamin K epoxide reductase, a protein required for blood clotting in humans, as part of a disulfide bond formation pathway present in several major bacterial phyla.

Cheese is an ideal environment to serve as a model for the behavior of microbes in complex communities and at the same time allow detailed genetic analysis. Linking organisms, and their genes, to their role in the environment becomes possible in the case of cheese since cheese microbial communities have been “in culture” for thousands of years, with the knowledge of how to grow these organisms passed down by generations of cheesemakers. Recent reviews have described several emerging approaches to link molecular systems biology to ecosystem-scale processes, known as ecosystems biology. These approaches integrate massive datasets now available through high-throughput sequencing technologies with measurements of ecosystem properties. High-throughput datasets uncover the “parts list” (e.g., the species and all the genes within each species) of an ecosystem as well as the molecular basis of interactions within this parts list. Novel computational frameworks make it possible to link species and their interactions to ecosystem properties. Applying these approaches across multiple temporal and spatial scales makes it possible to understand how changes in the parts lists over space and time lead to changes in ecosystems processes. By manipulating the species present within model systems, we can test hypotheses related to the role of microbes in ecosystem function. Due to the tractability of cheese microbial communities, we have the opportunity to use an ecosystems biology approach from the scale of individual microbial cells within a cheese to replicated cheese microbial communities across continents. Using cheese as a model microbial ecosystem can provide a way to answer important questions concerning the form, function, and evolution of microbial communities.

Background: Taxonomic classification of marker-gene (i.e., amplicon) sequences represents an important step for molecular identification of microorganisms. Results: We present three advances in our ability to assign and interpret taxonomic classifications of short marker gene sequences: two new methods for taxonomy assignment, which reduce runtime up to two-fold and achieve high-precision genus-level assignments; an evaluation of classification methods that highlights differences in performance with different marker genes and at different levels of taxonomic resolution; and an extensible framework for evaluating and optimizing new classification methods, which we hope will serve as a model for standardized and reproducible bioinformatics methods evaluations. Conclusions: Our new methods are accessible in QIIME 1.9.0, and our evaluation framework will support ongoing optimization of classification methods to complement rapidly evolving short-amplicon sequencing and bioinformatics technologies. Static versions of all of the analysis notebooks generated with this framework, which contain all code and analysis results, can be viewed at http://bit.ly/srta-012 .

During Bacillus subtilis sporulation, SpoIIIE is required for translocation of the trapped forespore chromosome across the sporulation septum, for compartmentalization of cell-specific gene expression, and for membrane fusion after engulfment. We isolated mutations within the SpoIIIE membrane domain that block localization and function. One mutant protein initially localizes normally and completes DNA translocation, but shows reduced membrane fusion after engulfment. Fluorescence recovery after photobleaching experiments demonstrate that in this mutant the sporulation septum remains open, allowing cytoplasmic contents to diffuse between daughter cells, suggesting that it blocks membrane fusion after cytokinesis as well as after engulfment. We propose that SpoIIIE catalyses these topologically opposite fusion events by assembling or disassembling a proteinaceous fusion pore. Mutants defective in SpoIIIE assembly also demonstrate that the ability of SpoIIIE to provide a diffusion barrier is directly proportional to its ability to assemble a focus at the septal midpoint during DNA translocation. Thus, SpoIIIE mediates compartmentalization by two distinct mechanisms: the SpoIIIE focus first provides a temporary diffusion barrier during DNA translocation, and then mediates the completion of membrane fusion after division to provide a permanent diffusion barrier. SpoIIIE-like proteins might therefore serve to couple the final step in cytokinesis, septal membrane fusion, to the completion of chromosome segregation.

Summary In Caulobacter crescentus , the temporal and spatial expression of late flagellar genes is regulated by the σ 54 transcriptional activator, FlbD. Genetic experi- ments have indicated that the trans -acting factor FliX regulates FlbD in response to the progression of flagellar assembly, repressing FlbD activity until an early flagellar basal body structure is assembled. Fol- lowing assembly of this structure, FliX is thought to function as an activator of FlbD. Here we have inves- tigated the mechanism of FliX-mediated regulation of FlbD activity. In vitro transcription experiments showed that purified FliX could function as a repres- sor of FlbD-activated transcription. Transcription acti- vated by a gain-of-function mutant of FlbD (FlbD-1204) that is active in vivo in the absence of an early flagel- lar structure, was resistant to the repressive effects of FliX. DNA binding studies showed that FliX inhib- ited the interaction of wild-type FlbD with enhancer DNA but did not effect FlbD-catalysed ATPase activity. DNA binding activity of FlbD-1204 was relatively unaf- fected by FliX indicating that this mutant protein bypasses the transcriptional requirement for early flagellar assembly by escaping FliX-mediated nega- tive regulation. Gel filtration and co-immunoprecipita- tion experiments indicated that FliX formed a stable complex with FlbD. These experiments demonstrate that regulation of FlbD activity is unusual among the well-studied σ 54

The expression of the flagellin proteins in Caulobacter crescentus is regulated by the progression of flagellar assembly both at the transcriptional and post-transcriptional levels. An early basal body structure is required for the transcription of flagellin genes, whereas the ensuing assembly of a hook structure is required for flagellin protein synthesis. Previous experiments have shown that the negative regulatory protein, FlbT, operates this second post-transcriptional checkpoint by associating with the 5' untranslated region (UTR) of the fljK flagellin transcript, inhibiting translation and destabilizing the mRNA. In this paper we examine the role of flaF in flagellar biogenesis. The flaF gene, which is conserved in several speices of flagellated alpha-proteobacteria, is required for motility and flagellin protein synthesis. A deletion of flbT in a DeltaflaF strain restored flagellin protein expression, but not motility, indicating that FlaF functions in filament assembly. Mutant strains with a deletion in flaF had no detectable fljK mRNA, the levels of which were restored by an additional mutation in flbT. Assay of fljK gene expression using transcription and translation reporter fusions indicated that FlaF was essential for the translation of fljK mRNA. FlaF protein levels were under cell cycle control, peaking during the period of flagellin expression and filament assembly, whereas FlbT was present throughout the cell cycle. These results suggest that FlbT and FlaF activities oppose one another in the regulation of flagellin expression in response to both the progression of flagellar assembly and the cell cycle.

Meat scavenged by early Homo could have contributed importantly to a higher–quality diet. However, it has been suggested that because carrion would normally have been contaminated by bacteria it would have been dangerous and therefore eaten rarely prior to the advent of cooking. In this study, we quantified bacterial loads on two tissues apparently eaten by hominins, meat and bone marrow. We tested the following three hypotheses: (1) the bacterial loads on exposed surfaces of raw meat increase within 24 h to potentially dangerous levels, (2) simple roasting of meat on hot coals kills most bacteria, and (3) fewer bacteria grow on marrow than on meat, making marrow a relatively safe food. Our results supported all three hypotheses. Our experimental data imply that early hominins would have found it difficult to scavenge safely without focusing on marrow, employing strategies of carrion selection to minimize pathogen load, or cooking.

Long-term dietary intake influences the structure and activity of the trillions of microorganisms residing in the human gut, but it remains unclear how rapidly and reproducibly the human gut microbiome responds to short-term macronutrient change. Here we show that the short-term consumption of diets composed entirely of animal or plant products alters microbial community structure and overwhelms inter-individual differences in microbial gene expression. The animal-based diet increased the abundance of bile-tolerant microorganisms (Alistipes, Bilophila and Bacteroides) and decreased the levels of Firmicutes that metabolize dietary plant polysaccharides (Roseburia, Eubacterium rectale and Ruminococcus bromii). Microbial activity mirrored differences between herbivorous and carnivorous mammals, reflecting trade-offs between carbohydrate and protein fermentation. Foodborne microbes from both diets transiently colonized the gut, including bacteria, fungi and even viruses. Finally, increases in the abundance and activity of Bilophila wadsworthia on the animal-based diet support a link between dietary fat, bile acids and the outgrowth of microorganisms capable of triggering inflammatory bowel disease. In concert, these results demonstrate that the gut microbiome can rapidly respond to altered diet, potentially facilitating the diversity of human dietary lifestyles.

SummaryTractable microbial communities are needed to bridge the gap between observations of patterns of microbial diversity and mechanisms that can explain these patterns. We developed cheese rinds as model microbial communities by characterizing in situ patterns of diversity and by developing an in vitro system for community reconstruction. Sequencing of 137 different rind communities across 10 countries revealed 24 widely distributed and culturable genera of bacteria and fungi as dominant community members. Reproducible community types formed independent of geographic location of production. Intensive temporal sampling demonstrated that assembly of these communities is highly reproducible. Patterns of community composition and succession observed in situ can be recapitulated in a simple in vitro system. Widespread positive and negative interactions were identified between bacterial and fungal community members. Cheese rind microbial communities represent an experimentally tractable system for defining mechanisms that influence microbial community assembly and function.PaperFlick

Microorganisms such as bacteria and fungi produce a variety of specialized metabolites that are invaluable for agriculture, biological research, and drug discovery. However, the screening of microbial metabolic output is usually a time-intensive task. Here, we utilize a liquid microjunction surface sampling probe for electrospray ionization-mass spectrometry to extract and ionize metabolite mixtures directly from living microbial colonies grown on soft nutrient agar in Petri-dishes without any sample pretreatment. To demonstrate the robustness of the method, this technique was applied to observe the metabolic output of more than 30 microorganisms, including yeast, filamentous fungi, pathogens, and marine-derived bacteria, that were collected worldwide. Diverse natural products produced from different microbes, including Streptomyces coelicolor , Bacillus subtilis , and Pseudomonas aeruginosa are further characterized.

The Escherichia coli membrane protein DsbD functions as an electron hub that dispatches electrons received from the cytoplasmic thioredoxin system to periplasmic oxidoreductases involved in protein disulfide isomerization, cytochrome c biogenesis, and sulfenic acid reduction. Here, we describe a new class of DsbD proteins, named ScsB, whose members are found in proteobacteria and Chlamydia. ScsB has a domain organization similar to that of DsbD, but its amino-terminal domain differs significantly. In DsbD, this domain directly interacts with substrates to reduce them, which suggests that ScsB acts on a different array of substrates. Using Caulobacter crescentus as a model organism, we searched for the substrates of ScsB. We discovered that ScsB provides electrons to the first peroxide reduction pathway identified in the bacterial cell envelope. The reduction pathway comprises a thioredoxin-like protein, TlpA, and a peroxiredoxin, PprX. We show that PprX is a thiol-dependent peroxidase that efficiently reduces both hydrogen peroxide and organic peroxides. Moreover, we identified two additional proteins that depend on ScsB for reduction, a peroxiredoxin-like protein, PrxL, and a novel protein disulfide isomerase, ScsC. Altogether, our results reveal that the array of proteins involved in reductive pathways in the oxidative cell envelope is significantly broader than was previously thought. Moreover, the identification of a new periplasmic peroxiredoxin indicates that in some bacteria, it is important to directly scavenge peroxides in the cell envelope even before they reach the cytoplasm., IMPORTANCE Peroxides are reactive oxygen species (ROS) that damage cellular components such as lipids, proteins, and nucleic acids. The presence of protection mechanisms against ROS is essential for cell survival. Bacteria express cytoplasmic catalases and thiol-dependent peroxidases to directly scavenge harmful peroxides. We report the identification of a peroxide reduction pathway active in the periplasm of Caulobacter crescentus, which reveals that, in some bacteria, it is important to directly scavenge peroxides in the cell envelope even before they reach the cytoplasm. The electrons required for peroxide reduction are delivered to this pathway by ScsB, a new type of membrane electron transporter. We also identified two additional likely ScsB substrates, including a novel protein disulfide isomerase. Our results reveal that the array of proteins involved in reductive pathways in the oxidative environment of the cell envelope is significantly broader than was previously thought.

Blood coagulation in humans requires the activity of vitamin K epoxide reductase (VKOR), the target of the anticoagulant warfarin (Coumadin). Bacterial homologs of VKOR were recently found to participate in a pathway leading to disulfide bond formation in secreted proteins of many bacteria. Here we show that the VKOR homolog from the bacteriumMycobacterium tuberculosis, the causative agent of human tuberculosis, is inhibited by warfarin and that warfarin-resistant mutations of mycobacterial VKOR appear in similar locations to mutations found in human patients who require higher doses of warfarin. Deletion of VKOR results in a severe growth defect in mycobacteria, and the growth ofM. tuberculosisis inhibited by warfarin. The bacterial VKOR homolog may represent a target for antibiotics and a model for genetic studies of human VKOR. We present a simple assay inEscherichia coli, based on a disulfide-sensitive β-galactosidase, which can be used to screen for stronger inhibitors of theM. tuberculosisVKOR homolog.

Vitamin K epoxide reductase (VKOR) generates vitamin K hydroquinone to sustain γ-carboxylation of many blood coagulation factors. Here, we report the 3.6 A crystal structure of a bacterial homologue of VKOR from Synechococcus sp. The structure shows VKOR in complex with its naturally fused redox partner, a thioredoxin-like domain, and corresponds to an arrested state of electron transfer. The catalytic core of VKOR is a four transmembrane helix bundle that surrounds a quinone, connected through an additional transmembrane segment with the periplasmic thioredoxin-like domain. We propose a pathway for how VKOR uses electrons from cysteines of newly synthesized proteins to reduce a quinone, a mechanism confirmed by in vitro reconstitution of vitamin K-dependent disulphide bridge formation. Our results have implications for the mechanism of the mammalian VKOR and explain how mutations can cause resistance to the VKOR inhibitor warfarin, the most commonly used oral anticoagulant. Mammalian vitamin K epoxide reductase (VKOR) catalyses the generation of vitamin K hydroquinone, a decisive step in the vitamin K cycle that is required to sustain blood coagulation. The X-ray crystal structure of a bacterial homologue of VKOR has now been determined. It shows VKOR in a complex with its redox partner, a thioredoxin-like domain, and corresponds to an arrested state of electron transfer. This points to a possible mechanism by which VKOR uses electrons from newly synthesized proteins to reduce the quinone. This work may help explain how mutations to VKOR cause resistance to warfarin, the ubiquitous anticoagulant that acts by inhibiting VKOR. The γ-carboxylation of many blood coagulation factors relies on the generation of vitamin K hydroquinone by the enzyme vitamin K epoxide reductase (VKOR), of which the anticoagulant warfarin is an inhibitor. Here, the X-ray crystal structure of a bacterial homologue of VKOR is presented; the results have implications for the mechanism of action of mammalian VKOR and explain how mutations can cause warfarin resistance.

The Chlamydia family of human pathogens uses outer envelope proteins that are highly cross-linked by disulfide bonds but nevertheless keeps an unusually high number of unpaired cysteines in its secreted proteins. To gain insight into chlamydial disulfide bond catalysis, the structure, function, and substrate interaction of a novel periplasmic oxidoreductase, termed DsbH, were determined. The structure of DsbH, its redox potential of -269 mV, and its functional properties are similar to thioredoxin and the C-terminal domain of DsbD, i.e. characteristic of a disulfide reductase. As compared with these proteins, the two central residues of the DsbH catalytic motif (CMWC) shield the catalytic disulfide bond and are selectively perturbed by a peptide ligand. This shows that these oxidoreductase family characteristic residues are not only important in determining the redox potential of the catalytic disulfide bond but also in influencing substrate interactions. For DsbH, three functional roles are conceivable; that is, reducing intermolecular disulfides between proteins and small molecules, keeping a specific subset of exported proteins reduced, or maintaining the periplasm of Chlamydia in a generally reducing state.

Chemoautotrophic endosymbionts are the metabolic cornerstone of hydrothermal vent communities, providing invertebrate hosts with nearly all of their nutrition. The Calyptogena magnifica (Bivalvia: Vesicomyidae) symbiont, Candidatus Ruthia magnifica , is the first intracellular sulfur-oxidizing endosymbiont to have its genome sequenced, revealing a suite of metabolic capabilities. The genome encodes major chemoautotrophic pathways as well as pathways for biosynthesis of vitamins, cofactors, and all 20 amino acids required by the clam.

In Caulobacter crescentus, the temporal and spatial expression of late flagellar genes is regulated by the sigma54 transcriptional activator, FlbD. Genetic experiments have indicated that the trans-acting factor FliX regulates FlbD in response to the progression of flagellar assembly, repressing FlbD activity until an early flagellar basal body structure is assembled. Following assembly of this structure, FliX is thought to function as an activator of FlbD. Here we have investigated the mechanism of FliX-mediated regulation of FlbD activity. In vitro transcription experiments showed that purified FliX could function as a repressor of FlbD-activated transcription. Transcription activated by a gain-of-function mutant of FlbD (FlbD-1204) that is active in vivo in the absence of an early flagellar structure, was resistant to the repressive effects of FliX. DNA binding studies showed that FliX inhibited the interaction of wild-type FlbD with enhancer DNA but did not effect FlbD-catalysed ATPase activity. DNA binding activity of FlbD-1204 was relatively unaffected by FliX indicating that this mutant protein bypasses the transcriptional requirement for early flagellar assembly by escaping FliX-mediated negative regulation. Gel filtration and co-immunoprecipitation experiments indicated that FliX formed a stable complex with FlbD. These experiments demonstrate that regulation of FlbD activity is unusual among the well-studied sigma54 transcriptional activators, apparently combining a two-component receiver domain with additional control imposed via interaction with a partner protein, FliX.

Background The temporal and spatial expression of late flagellar genes in Caulobacter crescentus is activated by the transcription factor FlbD and its partner trans-acting factor FliX. The physical interaction of these two proteins represents an alternative mechanism for regulating the activity of σ54 transcription factors. This study is to characterize the interaction of the two proteins and the consequences of the interaction on their regulatory activity. Results FliX and FlbD form stable complexes, which can stand the interference of 2.65 M NaCl. The stability of FliX and FlbD was affected by the co-existence of each other. Five FliX mutants (R71A, L85K, Δ117-118, T130L, and L136K) were created by site-directed mutagenesis in conserved regions of the protein. All mutants were successfully expressed in both wild-type and ΔfliX Caulobacter strains. All but FliXL85K could rescue the motility and cell division defects of a ΔfliX mutant strain. The ability of FliX to regulate the transcription of class II and class III/IV flagellar promoters was fully diminished due to the L85K mutation. Co-immunoprecipitation experiment revealed that FliXL85K was unable to physically interact with FlbD. Conclusions FliX interacts with FlbD and thereby directly regulates the activity of FlbD in response to flagellar assembly. Mutations in highly conserved regions of FliX could severely affect the recognition between FliX and FlbD and hence interrupt the normal progression of flagellar synthesis and other developmental events in Caulobacter.