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Bacterial pathogens in theRalstonia solanacearumspecies complex (RSSC) infect the water-transporting xylem vessels of plants, causing bacterial wilt disease. Strains in RSSC phylotypes I and III can reduce nitrate to dinitrogen via complete denitrification. The four-step denitrification pathway enables bacteria to use inorganic nitrogen species as terminal electron acceptors, supporting their growth in oxygen-limited environments like biofilms or plant xylem. Reduction of nitrate, nitrite, and nitric oxide all contribute to virulence of a model phylotype I strain. However, little is known about the physiological role of the last denitrification step, the reduction of nitrous oxide to dinitrogen by NosZ. We found that phylotypes I and III need NosZ for full virulence. However, strains in phylotypes II and IV are highly virulent despite lacking NosZ. The ability to respire by reducing nitrate to nitrous oxide does not greatly enhance growth of phylotype II and IV strains. These partial denitrifying strains reach high cell densities during plant infection and cause typical wilt disease. However, unlike phylotype I and III strains, partial denitrifiers cannot grow well under anaerobic conditions or form thick biofilms in culture or in tomato xylem vessels. Furthermore, aerotaxis assays show that strains from different phylotypes have different oxygen and nitrate preferences. Together, these results indicate that the RSSC contains two subgroups that occupy the same habitat but have evolved divergent energy metabolism strategies to exploit distinct metabolic niches in the xylem.IMPORTANCEPlant pathogenicRalstonia spp. are a heterogeneous globally distributed group of bacteria that colonize plant xylem vessels.Ralstoniacells multiply rapidly in plants and obstruct water transport, causing fatal wilting and serious economic losses of many key food security crops. Virulence of these pathogens depends on their ability to grow to high cell densities in the low-oxygen xylem environment. Plant pathogenicRalstoniacan use denitrifying respiration to generate ATP. The last denitrification step, nitrous oxide reduction by NosZ, contributes to energy production and virulence for only one of the three phytopathogenicRalstoniaspecies. These complete denitrifiers form thicker biofilms in culture and in tomato xylem, suggesting they are better adapted to hypoxic niches. Strains with partial denitrification physiology form less biofilm and are more often planktonic. They are nonetheless highly virulent. Thus, these closely related bacteria have adapted their core metabolic functions to exploit distinct micro-niches in the same habitat.

Helicobacter pylori encodes a large number of Restriction-Modification (R-M) systems despite its small genome.R-M systems have been described as “primitive immune systems” in bacteria, but the role of methylation in bacterial gene regulation and other processes is increasingly accepted. Every H.pylori strain harbours a unique set of R-M systems resulting in a highly diverse methylome. We identified a highly conserved GCGC-specific m5C MTase (JHP1050) that was predicted to be active in all of 459 H.pylori genome sequences analyzed. Transcriptome analysis of two H.pylori strains and their respective MTase mutants showed that inactivation of the MTase led to changes in the expression of 225 genes in strain J99, and 29 genes in strain BCM-300.10 genes were differentially expressed in both mutated strains. Combining bioinformatic analysis and site-directed mutagenesis, we demonstrated that motifs overlapping the promoter influence the expression of genes directly, while methylation of other motifs might cause secondary effects.Thus, m5C methylation modifies the transcription of multiple genes, affecting important phenotypic traits that include adherence to host cells, natural competence for DNA uptake, bacterial cell shape, and susceptibility to copper.

Helicobacter pylori is a Gram-negative, spiral shaped bacterium that selectively and chronically infects the gastric mucosa of humans. The clinical course of this infection can range from lifelong asymptomatic infection to severe disease, including peptic ulcers or gastric cancer. The high mutation rate and natural competence typical of this species are responsible for massive inter-strain genetic variation exceeding that observed in all other bacterial human pathogens. The adaptive value of such a plastic genome is thought to derive from a rapid exploration of the fitness landscape resulting in fast adaptation to the changing conditions of the gastric environment. Nevertheless, diversity is also lost through recurrent bottlenecks and H. pylori’s lifestyle is thus a perpetual race to maintain an appropriate pool of standing genetic variation able to withstand selection events. Another aspect of H. pylori’s diversity is a large and variable repertoire of restriction-modification systems. While not yet completely understood, methylome evolution could generate enough transcriptomic variation to provide another intricate layer of adaptive potential. This review provides an up to date synopsis of this rapidly emerging area of H. pylori research that has been enabled by the ever-increasing throughput of Omics technologies and a multitude of other technological advances.

Exceptional genetic diversity and variability are hallmarks of Helicobacter pylori, but the biological role of this plasticity remains incompletely understood. Here, we had the rare opportunity to investigate the molecular evolution during the first weeks of H. pylori infection by comparing the genomes and epigenomes of H. pylori strain BCS 100 used to challenge human volunteers in a vaccine trial with those of bacteria reisolated from the volunteers 10 weeks after the challenge. The data provide molecular insights into the process of establishment of this highly versatile pathogen in 10 different human individual hosts, showing, for example, selection for changes in host-interaction molecules as well as changes in epigenetic methylation patterns. The data provide important clues to the early adaptation of H. pylori to new host niches after transmission, which we believe is vital to understand its success as a chronic pathogen and develop more efficient treatments and vaccines., Multiple studies have demonstrated rapid bacterial genome evolution during chronic infection with Helicobacter pylori. In contrast, little was known about genetic changes during the first stages of infection, when selective pressure is likely to be highest. Using single-molecule, real-time (SMRT) and Illumina sequencing technologies, we analyzed genome and methylome evolution during the first 10 weeks of infection by comparing the cag pathogenicity island (cagPAI)-negative H. pylori challenge strain BCS 100 with pairs of H. pylori reisolates from gastric antrum and corpus biopsy specimens of 10 human volunteers who had been infected with this strain as part of a vaccine trial. Most genetic changes detected in the reisolates affected genes with a surface-related role or a predicted function in peptide uptake. Apart from phenotypic changes of the bacterial envelope, a duplication of the catalase gene was observed in one reisolate, which resulted in higher catalase activity and improved survival under oxidative stress conditions. The methylomes also varied in some of the reisolates, mostly by activity switching of phase-variable methyltransferase (MTase) genes. The observed in vivo mutation spectrum was remarkable for a very high proportion of nonsynonymous mutations. Although the data showed substantial within-strain genome diversity in the challenge strain, most antrum and corpus reisolates from the same volunteers were highly similar to each other, indicating that the challenge infection represents a major selective bottleneck shaping the transmitted population. Our findings suggest rapid in vivo selection of H. pylori during early-phase infection providing adaptation to different individuals by common mechanisms of genetic and epigenetic alterations.

The human pathogen Helicobacter pylori displays extensive genetic diversity. While H. pylori is known to evolve during infection, population dynamics inside the gastric environment have not been extensively investigated. Here we obtained gastric biopsies from multiple stomach regions of 16 H. pylori-infected adults, and analyze the genomes of 10 H. pylori isolates from each biopsy. Phylogenetic analyses suggest location-specific evolution and bacterial migration between gastric regions. Migration is significantly more frequent between the corpus and the fundus than with the antrum, suggesting that physiological differences between antral and oxyntic mucosa contribute to spatial partitioning of H. pylori populations. Associations between H. pylori gene polymorphisms and stomach niches suggest that chemotaxis, regulatory functions and outer membrane proteins contribute to specific adaptation to the antral and oxyntic mucosa. Moreover, we show that antibiotics can induce severe population bottlenecks and likely play a role in shaping the population structure of H. pylori., Helicobacter pylori, a bacterial pathogen that infects human stomachs, has high genetic diversity across hosts. Here, Ailloud et al. reveal genetic structuring of H. pylori populations among different stomach regions of individual hosts and find signals of genetic associations with stomach region.

A single multiplex PCR assay targeting seven virulence factors and the wzi gene specific for the K1 and K2 capsular serotypes of Klebsiella pneumoniae was developed and tested on 65 clinical isolates, which included 45 isolates responsible for community-acquired severe human infections. The assay is useful for the surveillance of emerging highly virulent strains.

Plants use the signaling molecule salicylic acid (SA) to trigger defenses against diverse pathogens, including the bacterial wilt pathogen Ralstonia solanacearum. SA can also inhibit microbial growth. Most sequenced strains of the heterogeneous R. solanacearum species complex can degrade SA via gentisic acid to pyruvate and fumarate. R. solanacearum strain GMI1000 expresses this SA degradation pathway during tomato pathogenesis. Transcriptional analysis revealed that subinhibitory SA levels induced expression of the SA degradation pathway, toxin efflux pumps, and some general stress responses. Interestingly, SA treatment repressed expression of virulence factors, including the type III secretion system, suggesting that this pathogen may suppress virulence functions when stressed. A GMI1000 mutant lacking SA degradation activity was much more susceptible to SA toxicity but retained the wild-type colonization ability and virulence on tomato. This may be because SA is less important than gentisic acid in tomato defense signaling. However, another host, tobacco, responds strongly to SA. To test the hypothesis that SA degradation contributes to virulence on tobacco, we measured the effect of adding this pathway to the tobacco-pathogenic R. solanacearum strain K60, which lacks SA degradation genes. Ectopic addition of the GMI1000 SA degradation locus, including adjacent genes encoding two porins and a LysR-type transcriptional regulator, significantly increased the virulence of strain K60 on tobacco. Together, these results suggest that R. solanacearum degrades plant SA to protect itself from inhibitory levels of this compound and also to enhance its virulence on plant hosts like tobacco that use SA as a defense signal molecule., IMPORTANCE Plant pathogens such as the bacterial wilt agent Ralstonia solanacearum threaten food and economic security by causing significant losses for small- and large-scale growers of tomato, tobacco, banana, potato, and ornamentals. Like most plants, these crop hosts use salicylic acid (SA) both indirectly as a signal to activate defenses and directly as an antimicrobial chemical. We found that SA inhibits growth of R. solanacearum and induces a general stress response that includes repression of multiple bacterial wilt virulence factors. The ability to degrade SA reduces the pathogen’s sensitivity to SA toxicity and increases its virulence on tobacco.

Background The increased availability of genome sequences has advanced the development of genomic distance methods to describe bacterial diversity. Results of these fast-evolving methods are highly correlated with those of the historically standard DNA-DNA hybridization technique. However, these genomic-based methods can be done more rapidly and less expensively and are less prone to technical and human error. They are thus a technically accessible replacement for species delineation. Here, we use several genomic comparison methods, supported by our own proteomic analyses and metabolic characterization as well as previously published DNA-DNA hybridization analyses, to differentiate members of the Ralstonia solanacearum species complex into three species. This pathogen group consists of diverse and widespread strains that cause bacterial wilt disease on many different plants. Results We used three different methods to compare the complete genomes of 29 strains from the R. solanacearum species complex. In parallel we profiled the proteomes of 73 strains using Matrix-Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry (MALDI-TOF-MS). Proteomic profiles together with genomic sequence comparisons consistently and comprehensively described the diversity of the R. solanacearum species complex. In addition, genome-driven functional phenotypic assays excitingly supported an old hypothesis (Hayward et al. (J Appl Bacteriol 69:269–80, 1990)), that closely related members of the R. solanacearum could be identified through a simple assay of anaerobic nitrate metabolism. This assay allowed us to clearly and easily differentiate phylotype II and IV strains from phylotype I and III strains. Further, genomic dissection of the pathway distinguished between proposed subspecies within the current phylotype IV. The assay revealed large scale differences in energy production within the R. solanacearum species complex, indicating coarse evolutionary distance and further supporting a repartitioning of this group into separate species. Conclusions Together, the results of these studies support the proposed division of the R. solanacearum species complex into three species, consistent with recent literature, and demonstrate the utility of proteomic and genomic approaches to delineate bacterial species. Electronic supplementary material The online version of this article (doi:10.1186/s12864-016-2413-z) contains supplementary material, which is available to authorized users.

Background.Ralstonia solanacearumis an economically important plant pathogen with an unusually large host range. The Moko (banana) and NPB (not pathogenic to banana) strain groups are closely related but are adapted to distinct hosts. Previous comparative genomics studies uncovered very few differences that could account for the host range difference between these pathotypes. To better understand the basis of this host specificity, we used RNAseq to profile the transcriptomes of anR. solanacearumMoko strain and an NPB strain underin vitroandin plantaconditions.Results.RNAs were sequenced from bacteria grown in rich and minimal media, and from bacteria extracted from mid-stage infected tomato, banana and melon plants. We computed differential expression between each pair of conditions to identify constitutive and host-specific gene expression differences between Moko and NPB. We found that type III secreted effectors were globally up-regulated upon plant cell contact in the NPB strain compared with the Moko strain. Genes encoding siderophore biosynthesis and nitrogen assimilation genes were highly up-regulated in the NPB strain during melon pathogenesis, while denitrification genes were up-regulated in the Moko strain during banana pathogenesis. The relatively lower expression of oxidases and the denitrification pathway during banana pathogenesis suggests thatR. solanacearumexperiences higher oxygen levels in banana pseudostems than in tomato or melon xylem.Conclusions.This study provides the first report of differential gene expression associated with host range variation. Despite minimal genomic divergence, the pathogenesis of Moko and NPB strains is characterized by striking differences in expression of virulence- and metabolism-related genes.

Background Ralstonia solanacearum is a vascular soil-borne plant pathogen with an unusually broad host range. This economically destructive and globally distributed bacterium has thousands of distinct lineages within a heterogeneous and taxonomically disputed species complex. Some lineages include highly host-adapted strains (ecotypes), such as the banana Moko disease-causing strains, the cold-tolerant potato brown rot strains (also known as R3bv2) and the recently emerged Not Pathogenic to Banana (NPB) strains. Results These distinct ecotypes offer a robust model to study host adaptation and the emergence of ecotypes because the polyphyletic Moko strains include lineages that are phylogenetically close to the monophyletic brown rot and NPB strains. Draft genomes of eight new strains belonging to these three model ecotypes were produced to complement the eleven publicly available R. solanacearum genomes. Using a suite of bioinformatics methods, we searched for genetic and evolutionary features that distinguish ecotypes and propose specific hypotheses concerning mechanisms of host adaptation in the R. solanacearum species complex. Genome-wide, few differences were identified, but gene loss events, non-synonymous polymorphisms, and horizontal gene transfer were identified among type III effectors and were associated with host range differences. Conclusions This extensive comparative genomics analysis uncovered relatively few divergent features among closely related strains with contrasting biological characteristics; however, several virulence factors were associated with the emergence of Moko, NPB and brown rot and could explain host adaptation. Electronic supplementary material The online version of this article (doi:10.1186/s12864-015-1474-8) contains supplementary material, which is available to authorized users.

International audience; Banana wilt outbreaks that are attributable to Moko disease-causing strains of the pathogen Ralstonia solanacearum (Rs) remain a social and economic burden for both multinational corporations and subsistence farmers. All known Moko strains belong to the phylotype II lineage, which has been previously recognized for its broad genetic basis. Moko strains are paraphyletic and are distributed among seven related but distinct phylogenetic clusters (sequevars) that are potentially major threats to Musaceae, Solanaceae, and ornamental crops in many countries. Although clustered within the Moko IIB-4 sequevar, strains of the epidemiologically variant IIB-4NPB do not cause wilt on Cavendish or plantain bananas; instead, they establish a latent infection in the vascular tissues of plantains and demonstrate an expanded host range and high aggressiveness toward Solanaceae and Cucurbitaceae. Although most molecular diagnostic methods focus on strains that wilt Solanaceae (particularly potato), no relevant protocol has been described that universally detects strains of the Musaceae-infecting Rs phylotype II. Thus, a duplex PCR assay targeting Moko and IIB-4NPB variant strains was developed, and its performance was assessed using an extensive collection of 111 strains representing the known diversity of Rs Moko-related strains and IIB-4NPB variant strains along with certain related strains and families. The proposed diagnostic protocol demonstrated both high accuracy (inclusivity and exclusivity) and high repeatability, detected targets on either pure culture or spiked plant extracts. Although they did not belong to the Moko clusters described at the time of the study, recently discovered banana-infecting strains from Brazil were also detected. According to our comprehensive evaluation, this duplex PCR assay appears suitable for both research and diagnostic laboratories and provides reliable detection of phylotype II Rs strains that infect Musaceae.

We created a Web-accessible genome database to enable rapid extraction of genotype, virulence, and resistance information from sequences., Multidrug-resistant and highly virulent Klebsiella pneumoniae isolates are emerging, but the clonal groups (CGs) corresponding to these high-risk strains have remained imprecisely defined. We aimed to identify K. pneumoniae CGs on the basis of genome-wide sequence variation and to provide a simple bioinformatics tool to extract virulence and resistance gene data from genomic data. We sequenced 48 K. pneumoniae isolates, mostly of serotypes K1 and K2, and compared the genomes with 119 publicly available genomes. A total of 694 highly conserved genes were included in a core-genome multilocus sequence typing scheme, and cluster analysis of the data enabled precise definition of globally distributed hypervirulent and multidrug-resistant CGs. In addition, we created a freely accessible database, BIGSdb-Kp, to enable rapid extraction of medically and epidemiologically relevant information from genomic sequences of K. pneumoniae. Although drug-resistant and virulent K. pneumoniae populations were largely nonoverlapping, isolates with combined virulence and resistance features were detected.

Hypervirulent Klebsiella pneumoniae isolates of capsular serotype K2 (hvKP-K2) that cause community-acquired invasive infections represent several unrelated clones, which all belong to phylogenetic group KpI. These clones can be recognized using multilocus sequence typing and genomic analyses, but no rapid method currently exists to differentiate them. In this work, a multiplex PCR assay was developed to identify three hvKP-K2 groups: (i) sequence type (ST)86; (ii) ST380 and ST679 (i.e. clonal group 380); and (iii) ST65 and ST375. A specific genetic marker, Kp50233, allowing K. pneumoniae sensu stricto (corresponding to phylogroup KpI) to be distinguished from closely related species, was included in the assay. This PCR assay will be useful in better defining the epidemiology and clinical features of emerging virulent K. pneumoniae clones.

Ralstonia solanacearum requires a Type III (T3) secretion system for bacterial wilt pathogenesis, but the biological functions of individual effectors remain unknown. During tomato wilt, R. solanacearum expresses popS, which encodes an AvrE-family T3 effector. popS homologs were present in all 17 sequenced R. solanacearum strains, and the phylogeny of popS parallels that of the R. solanacearum species complex, suggesting that PopS is an ancient effector needed for association with plants. We determined that popS is required for full virulence on multiple Solanum crop hosts (susceptible potato and susceptible and quantitatively resistant tomato), but not for virulence on a related epidemiologically relevant weed, S. dulcamara. The popS mutant was also significantly delayed in tomato stem colonization following direct inoculation through cut petioles. AvrE-type effectors in other plant pathogenic bacteria suppress plant defenses triggered by the plant signaling molecule salicylic acid (SA). The popS mutant induced higher expression of SAresponsive tomato PR genes than its wild-type parent. Further, pretreating plant roots with SA exacerbated the popS virulence defect. Finally, PopS was dispensable for bacterial colonization of SA-deficient NahG transgenic tomato plants. These results indicate that this conserved T3 effector suppresses SAmediated defenses in tomato roots and stems, which are the natural infection courts of this soilborne vascular pathogen. (texte integral)