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Borrelia burgdorferi (Bb), the causative agent of Lyme disease, must adapt to vastly different environments as the bacterium cycles between the tick vector and a vertebrate host. During a bloodmeal, Bb migrates from the tick midgut to the salivary glands and changes its gene expression, priming Bb for vertebrate infection. These tick-specific transmission events are dependent on key transcriptional regulators; however, the full range of transcriptional changes that occur over several days inside of the tick are technically challenging to capture. We developed an experimental approach to enrich Bb cells from Ixodes ticks during a transmitting bloodmeal to longitudinally define their global transcriptomic landscape. We identified 192 genes that substantially change expression over the course of the tick bloodmeal, most of which were located on plasmids of the Bb genome. The majority of upregulated genes encode proteins found at the cell envelope or proteins of unknown function, including 45 outer surface lipoproteins embedded in the unusual protein-rich coat of Bb. As these proteins may facilitate Bb interactions with host tissues or immune systems, we also utilized mass spectrometry to identify candidate tick proteins that physically associate with Bb. The ex vivo Bb transcriptomes and candidate tick interacting proteins presented here provide an important roadmap for investigating key determinants of Bb priming and transmission during the tick stage of its unique transmission cycle.

Gram-negative bacteria can antagonize neighboring microbes using a type VI secretion system (T6SS) to deliver toxins that target different essential cellular features. Despite the conserved nature of these targets, T6SS potency can vary across recipient species. To understand the molecular basis of intrinsic T6SS susceptibility, we screened for essentialEscherichia coligenes that affect its survival when antagonized by a cell wall-degrading T6SS toxin fromPseudomonas aeruginosa, Tae1. We revealed genes associated with both the cell wall and a separate layer of the cell envelope, surface lipopolysaccharide, that modulate Tae1 toxicityin vivo. Disruption of lipopolysaccharide synthesis providedEscherichia coli (Eco)with novel resistance to Tae1, despite significant cell wall degradation. These data suggest that Tae1 toxicity is determined not only by direct substrate damage, but also by indirect cell envelope homeostasis activities. We also found that Tae1-resistantEcoexhibited reduced cell wall synthesis and overall slowed growth, suggesting that reactive cell envelope maintenance pathways could promote, not prevent, self-lysis. Together, our study highlights the consequences of co-regulating essential pathways on recipient fitness during interbacterial competition, and how antibacterial toxins leverage cellular vulnerabilities that are both direct and indirect to their specific targetsin vivo.

Borrelia burgdorferi(Bb), the causative agent of Lyme disease, adapts to vastly different environments as it cycles between tick vector and vertebrate host. During a tick bloodmeal,Bbalters its gene expression to prepare for vertebrate infection; however, the full range of transcriptional changes that occur over several days inside of the tick are technically challenging to capture. We developed an experimental approach to enrichBbcells to longitudinally define their global transcriptomic landscape inside nymphalIxodes scapularisticks during a transmitting bloodmeal. We identified 192Bbgenes that substantially change expression over the course of the bloodmeal from one to four days after host attachment. The majority of upregulated genes encode proteins found at the cell envelope or proteins of unknown function, including 45 outer surface lipoproteins embedded in the unusual protein-rich coat ofBb. As these proteins may facilitateBbinteractions with the host, we utilized mass spectrometry to identify candidate tick proteins that physically associate withBb. TheBbenrichment methodology along with theex vivo Bbtranscriptomes and candidate tick interacting proteins presented here provide a resource to facilitate investigations into key determinants ofBbpriming and transmission during the tick stage of its unique transmission cycle.

Ticks are increasingly important vectors of human and agricultural diseases. While many studies have focused on tick-borne bacteria, far less is known about tick-associated viruses and their roles in public health or tick physiology. To address this, we investigated patterns of bacterial and viral communities across two field populations of western black-legged ticks (Ixodes pacificus). Through metatranscriptomic analysis of 100 individual ticks, we quantified taxon prevalence, abundance, and co-occurrence with other members of the tick microbiome. Our analysis revealed 11 novel RNA viruses from Rhabdoviridae, Chuviridae, Picornaviridae, Phenuiviridae, Reoviridae, Solemovidiae, Narnaviridae, and 2 highly divergent RNA viruses lacking sequence similarity to known viral families. The majority of these viruses were also detectable in lab-raised ticks at all developmental life stages, localize to tick salivary glands, and show evidence of circulation in mice fed on by ticks. These data suggest that viruses are stable, heritable, and transmissible members of the tick microbiota. We also unexpectedly identified numerous virus-like transcripts that are associated with tick genomic DNA, most of which are distinct from known endogenous viral element-mediated immunity pathways in invertebrates. Together, our work reveals that in addition to potentially serving as vectors for potential viral pathogens, I. pacificus ticks may also have symbiotic partnerships with their own vertically-transmitted viruses or with ancient viruses through evolutionarily acquired virus-like transcripts. Our findings highlight how pervasive and intimate tick–virus interactions are, with major implications for both the fundamental physiology and vector biology of I. pacificus ticks.

In mammals, cyclic dinucleotides (CDNs) bind and activate STING to initiate an antiviral type I interferon response. CDNs and STING originated in bacteria and are present in most animals. By contrast, interferons are believed to have emerged in vertebrates; thus, the function of CDN signaling in invertebrates is unclear. Here, we use a CDN, 2'3' cyclic guanosine monophosphate-adenosine monophosphate (2'3'-cGAMP), to activate immune responses in a model cnidarian invertebrate, the starlet sea anemone Nematostella vectensis Using RNA sequencing, we found that 2'3'-cGAMP induces robust transcription of both antiviral and antibacterial genes in N. vectensis Many of the antiviral genes induced by 2'3'-cGAMP are homologs of vertebrate interferon-stimulated genes, implying that the interferon response predates the evolution of interferons. Knockdown experiments identified a role for NF-κB in specifically inducing antibacterial genes downstream of 2'3'-cGAMP. Some of these putative antibacterial genes were also found to be induced during Pseudomonas aeruginosa infection. We characterized the protein product of one of the putative antibacterial genes, the N. vectensis homolog of Dae4, and found that it has conserved antibacterial activity. This work suggests that a broad antibacterial and antiviral transcriptional response is an evolutionarily ancestral output of 2'3'-cGAMP signaling in animals.

Significance Cyclic dinucleotides are signaling molecules that originated in bacteria and were subsequently acquired and co-opted by animals for immune signaling. The major cyclic dinucleotide signaling pathway in mammals results in the production of antiviral molecules called interferons. Invertebrates such as sea anemones lack interferons, and thus it was unclear whether cyclic dinucleotide signaling would play a role in immunity in these animals. Here, we report that in the anemone Nematostella vectensis, cyclic dinucleotides activate both antiviral and antibacterial immune responses and do so through a conserved pathway. These results provide insights into the evolutionary origins of innate immunity and suggest a broader ancestral role for cyclic dinucleotide signaling that evolved toward more specialized antiviral functions in mammals., In mammals, cyclic dinucleotides (CDNs) bind and activate STING to initiate an antiviral type I interferon response. CDNs and STING originated in bacteria and are present in most animals. By contrast, interferons are believed to have emerged in vertebrates; thus, the function of CDN signaling in invertebrates is unclear. Here, we use a CDN, 2′3′ cyclic guanosine monophosphate-adenosine monophosphate (2′3′-cGAMP), to activate immune responses in a model cnidarian invertebrate, the starlet sea anemone Nematostella vectensis. Using RNA sequencing, we found that 2′3′-cGAMP induces robust transcription of both antiviral and antibacterial genes in N. vectensis. Many of the antiviral genes induced by 2′3′-cGAMP are homologs of vertebrate interferon-stimulated genes, implying that the interferon response predates the evolution of interferons. Knockdown experiments identified a role for NF-κB in specifically inducing antibacterial genes downstream of 2′3′-cGAMP. Some of these putative antibacterial genes were also found to be induced during Pseudomonas aeruginosa infection. We characterized the protein product of one of the putative antibacterial genes, the N. vectensis homolog of Dae4, and found that it has conserved antibacterial activity. This work suggests that a broad antibacterial and antiviral transcriptional response is an evolutionarily ancestral output of 2′3′-cGAMP signaling in animals.

In mammals, cyclic dinucleotides (CDNs) bind and activate STING to initiate an anti-viral type I interferon response. CDNs and STING originated in bacteria and are present in most animals. By contrast, interferons are believed to have emerged in vertebrates; thus, the function of CDN signaling in invertebrates is unclear. Here, we use a CDN, 2’3’-cGAMP, to activate immune responses in a model cnidarian invertebrate, the starlet sea anemone Nematostella vectensis. Using RNA-Seq, we found that 2’3’-cGAMP induces robust transcription of both anti-viral and anti-bacterial genes in N. vectensis. Many of the anti-viral genes induced by 2’3’-cGAMP are homologs of vertebrate interferon stimulated genes, implying that the interferon response predates the evolution of interferons. Knockdown experiments identified a role for NF-κB in specifically inducing anti-bacterial genes downstream of 2’3’-cGAMP. Some of these putative anti-bacterial genes were also found to be induced during Pseudomonas aeruginosa infection. We characterized the protein product of one of the putative anti-bacterial genes, the N. vectensis homolog of Dae4, and found that it has conserved anti-bacterial activity. This work suggests that a broad anti-bacterial and anti-viral transcriptional response is an evolutionarily ancestral output of 2’3’-cGAMP signaling in animals.Significance statementCyclic dinucleotides are signaling molecules that originated in bacteria and were subsequently acquired and co-opted by animals for immune signaling. The major cyclic dinucleotide signaling pathway in mammals results in the production of anti-viral molecules called interferons. Invertebrates such as sea anemones lack interferons, and thus it was unclear whether cyclic dinucleotide signaling would play a role in immunity in these animals. Here we report that in the anemone Nematostella vectensis, cyclic dinucleotides activate both anti-viral and anti-bacterial immune responses, and do so through a conserved pathway. These results provide insights into the evolutionary origins of innate immunity, and suggest a broader ancestral role for cyclic dinucleotide signaling that evolved toward more specialized anti-viral functions in mammals.

Ticks transmit a diverse array of microbes to vertebrate hosts, including human pathogens, which has led to a human-centric focus in this vector system. Far less is known about pathogens of ticks themselves. Here, we discover that a toxin in blacklegged ticks (Ixodes scapularis) horizontally acquired from bacteria-called domesticated amidase effector 2 (dae2)-has evolved to kill mammalian skin microbes with remarkable efficiency. Secreted into the saliva and gut of ticks, Dae2 limits skin-associated staphylococci in ticks while feeding. In contrast, Dae2 has no intrinsic ability to kill Borrelia burgdorferi, the tick-borne Lyme disease bacterial pathogen. These findings suggest ticks resist their own pathogens while tolerating symbionts. Thus, just as tick symbionts can be pathogenic to humans, mammalian commensals can be harmful to ticks. Our study underscores how virulence is context-dependent and bolsters the idea that "pathogen" is a status and not an identity.

SummaryHard ticks are blood-feeding arthropods that carry and transmit microbes to their vertebrate hosts1. Tick-borne disease cases have been on the rise over the last several decades, drawing much-needed attention to the molecular interplay between transmitted pathogens and their human hosts. However, far less is known about how ticks control their own microbes, which is critical for understanding how zoonotic transmission cycles persist. We previously found that ticks horizontally acquired an antimicrobial toxin gene from bacteria known as domesticated amidase effector 2 (dae2)2. Here we show that this effector from the tick vector Ixodes scapularis (Dae2Is) has structurally and biochemically diverged from ancestral bacterial representatives, expanding its antibacterial targeting range to include host skin microbes. Disruption of dae2Is increases the burden of skin-associated staphylococci within I. scapularis and adversely affects tick fitness, suggesting resistance of host microbes may be important for the parasitic blood-feeding lifestyle. In contrast, Dae2Is has no intrinsic ability to kill Borrelia burgdorferi, the tick-borne bacterium of Lyme disease. Our observations suggest that ticks have evolved to tolerate their own symbionts while resisting host skin commensals, which we discover are natural opportunistic pathogens of ticks. This work moves our understanding of vector biology beyond a human-centric view: just as tick commensals are pathogenic to humans, so too do our commensals pose a threat to ticks. These observations illuminate how a complex and mirrored set of interkingdom interactions between blood-feeding vectors, their hosts, and associated microbes can ultimately lead to disease.

Hard ticks of the order Ixodidae serve as vectors for numerous human pathogens, including the causative agent of Lyme DiseaseBorrelia burgdorferi. Tick-associated microbes can influence pathogen colonization, offering the potential to inhibit disease transmission through engineering of the tick microbiota. Here, we investigate whetherB. burgdorferiencounters abundant bacteria within the midgut of wild adultIxodes scapularis, its primary vector. Through the use of controlled sequencing methods and confocal microscopy, we find that the majority of field-collected adultI. scapularisharbor limited internal microbial communities that are dominated by endosymbionts. A minority ofI. scapularisticks harbor abundant midgut bacteria and lackB. burgdorferi. We find that the lack of a stable resident midgut microbiota is not restricted toI. scapularissince extension of our studies toI. pacificus, Amblyomma maculatum, andDermacentorspp showed similar patterns. Finally, bioinformatic examination of theB. burgdorferigenome revealed the absence of genes encoding known interbacterial interaction pathways, a feature unique to theBorreliagenus within the phylumSpirochaetes. Our results suggest that reduced selective pressure from limited microbial populations within ticks may have facilitated the evolutionary loss of genes encoding interbacterial competition pathways fromBorrelia.

Globally, nearly 2 billion people are infected with the intracellular protozoan Toxoplasma gondii1. This persistent infection can cause severe disease in immunocompromised people and is epidemiologically linked to major mental illnesses2 and cognitive impairment3. There are currently no options for curing this infection. The lack of effective therapeutics is due partly to a poor understanding of the essential pathways that maintain long-term infection. Although it is known that Toxoplasma replicates slowly within intracellular cysts demarcated with a cyst wall, precisely how it sustains itself and remodels organelles in this niche is unknown. Here, we identify a key role for proteolysis within the parasite lysosomal organelle (the vacuolar compartment or VAC) in turnover of autophagosomes and persistence during neural infection. We found that disrupting a VAC-localized cysteine protease compromised VAC digestive function and markedly reduced chronic infection. Death of parasites lacking the VAC protease was preceded by accumulation of undigested autophagosomes in the parasite cytoplasm. These findings suggest an unanticipated function for parasite lysosomal degradation in chronic infection, and identify an intrinsic role for autophagy in the T. gondii parasite and its close relatives. This work also identifies a key element of Toxoplasma persistence and suggests that VAC proteolysis is a prospective target for pharmacological development.

Volume 5, no. 2, doi:[10.1128/mBio.01017-14][1], 2014. In [Fig. 3B][2], the wrong lanes were inadvertently included in the upper left panel, which resulted in misidentification of the samples. [Figure 3][2] should appear as shown below. ![Figure][3] [1]: /lookup/doi/10.1128/mBio.01017-14

The Lyme disease spirochete Borrelia burgdorferi senses and responds to environmental cues as it transits between the tick vector and vertebrate host. Failure to properly adapt can block transmission of the spirochete and persistence in either vector or host. We previously identified BBD18, a novel plasmid-encoded protein of B. burgdorferi, as a putative repressor of the host-essential factor OspC. In this study, we investigate the in vivo role of BBD18 as a regulatory protein, using an experimental mouse-tick model system that closely resembles the natural infectious cycle of B. burgdorferi. We show that spirochetes that have been engineered to constitutively produce BBD18 can colonize and persist in ticks but do not infect mice when introduced by either tick bite or needle inoculation. Conversely, spirochetes lacking BBD18 can persistently infect mice but are not acquired by feeding ticks. Through site-directed mutagenesis, we have demonstrated that abrogation of spirochete infection in mice by overexpression of BBD18 occurs only with bbd18 alleles that can suppress OspC synthesis. Finally, we demonstrate that BBD18-mediated regulation does not utilize a previously described ospC operator sequence required by B. burgdorferi for persistence in immunocompetent mice. These data lead us to conclude that BBD18 does not represent the putative repressor utilized by B. burgdorferi for the specific downregulation of OspC in the mammalian host. Rather, we suggest that BBD18 exhibits features more consistent with those of a global regulatory protein whose critical role occurs during spirochete acquisition by feeding ticks., IMPORTANCE Lyme disease, caused by Borrelia burgdorferi, is the most common arthropod-borne disease in North America. B. burgdorferi is transmitted to humans and other vertebrate hosts by ticks as they take a blood meal. Transmission between vectors and hosts requires the bacterium to sense changes in the environment and adapt. However, the mechanisms involved in this process are not well understood. By determining how B. burgdorferi cycles between two very different environments, we can potentially establish novel ways to interfere with transmission and limit infection of this vector-borne pathogen. We are studying a regulatory protein called BBD18 that we recently described. We found that too much BBD18 interferes with the spirochete’s ability to establish infection in mice, whereas too little BBD18 appears to prevent colonization in ticks. Our study provides new insight into key elements of the infectious cycle of the Lyme disease spirochete.

Borrelia burgdorferi, the agent of Lyme disease, is a vector-borne pathogen that transits between Ixodes ticks and vertebrate hosts. During the natural infectious cycle, spirochetes must globally adjust their transcriptome to survive in these dissimilar environments. One way B. burgdorferi accomplishes this is through the use of alternative sigma factors to direct transcription of specific genes. RpoS, one of only three sigma factors in B. burgdorferi, controls expression of genes required during tick-transmission and infection of the mammalian host. How spirochetes switch between different sigma factors during the infectious cycle has remained elusive. Here we establish a role for a novel protein, BBD18, in the regulation of the virulence-associated sigma factor RpoS. Constitutive expression of BBD18 repressed transcription of RpoS-dependent genes to levels equivalent to those observed in an rpoS mutant. Consistent with the global loss of RpoS-dependent transcripts, we were unable to detect RpoS protein. However, constitutive expression of BBD18 did not diminish the amount of rpoS transcript, indicating post-transcriptional regulation of RpoS by BBD18. Interestingly, BBD18-mediated repression of RpoS is independent of both the rpoS promoter and the 5' untranslated region, suggesting a mechanism of protein destabilization rather than translational control. We propose that BBD18 is a novel regulator of RpoS and its activity likely represents a first step in the transition from an RpoS-ON to an RpoS-OFF state, when spirochetes transition from the host to the tick vector.

Persistent infection of a mammalian host by Borrelia burgdorferi , the spirochete that causes Lyme disease, requires specific downregulation of an immunogenic outer surface protein, OspC. Although OspC is an essential virulence factor needed by the spirochete to establish infection in the mammal, it represents a potent target for the host acquired immune response, and constitutive expression of OspC results in spirochete clearance. In this study, we demonstrate that a factor encoded on a linear plasmid of B. burgdorferi , lp17, can negatively regulate ospC transcription from the endogenous gene on the circular plasmid cp26 and from an ospC promoter- lacZ fusion on a shuttle vector. Furthermore, we have identified bbd18 as the gene on lp17 that is responsible for this effect. These data identify a novel component of ospC regulation and provide the basis for determining the molecular mechanisms of ospC repression in vivo .