Gene fitness landscapes of Vibrio cholerae at important stages of its life cycle

The activity of the quorum sensing regulator HapR is modulated by the bacterial extracellular vesicle (BEV)-associated protein ObfA of Vibrio cholerae

Vibrio cholerae, a facultative human pathogen and causative agent of the severe diarrheal disease cholera, transits between the human intestinal tract and aquatic reservoirs. Like other bacterial species, V. cholerae continuously releases bacterial extracellular vesicles (BEVs) from its surface, which have been recently characterised for their role during in vivo colonisation. However, between epidemic outbreaks, V. cholerae persists in the biofilm mode for extended periods in aquatic reservoirs, which enhances environmental fitness and host transition. In this study, we investigated the effect of V. cholerae BEVs on biofilm formation, a critical feature for ex vivo survival. In contrast to BEVs from planktonic cultures, our results show that physiological concentrations of BEVs from dynamic biofilm cultures facilitate V. cholerae biofilm formation, which could be linked to a proteinaceous factor. Comparative proteomic analyses of planktonic- and biofilm-derived BEVs identified a previously uncharacterised outer membrane protein as an abundant component of dynamic biofilm-derived BEVs, which was found to be responsible for the BEV-dependent enhancement of biofilm production. Consequently, this protein was named outer membrane-associated biofilm facilitating protein A (ObfA). Comprehensive molecular studies unravelled ObfA as a negative modulator of HapR activity. HapR is a key transcriptional regulator of the V. cholerae quorum sensing (QS) cascade acting as a potent repressor of biofilm formation and virulence. Consistently, obfA mutants not only exhibited reduced biofilm production but also reduced colonisation fitness. Surprisingly, our results demonstrate that ObfA does not affect HapR through the canonical QS system but via the Csr-cascade altering the expression of the small regulatory RNAs CsrC and CsrD. In summary, this study elucidates a novel intraspecies BEV-based communication in V. cholerae that influences biofilm formation and colonisation fitness via a new regulatory pathway involving HapR, Csr-cascade and the BEV-associated protein ObfA.

Nucleoid-associated proteins shape the global protein occupancy and transcriptional landscape of a clinical isolate of Vibrio cholerae

Vibrio cholerae, the causative agent of the diarrheal disease cholera, poses an ongoing health threat due to its wide repertoire of horizontally acquired elements (HAEs) and virulence factors. New clinical isolates of the bacterium with improved fitness abilities, often associated with HAEs, frequently emerge. The appropriate control and expression of such genetic elements is critical for the bacteria to thrive in the different environmental niches they occupy. H-NS, the histone-like nucleoid structuring protein, is the best-studied xenogeneic silencer of HAEs in gamma-proteobacteria. Although H-NS and other highly abundant nucleoid-associated proteins (NAPs) have been shown to play important roles in regulating HAEs and virulence in model bacteria, we still lack a comprehensive understanding of how different NAPs modulate transcription in V. cholerae. By obtaining genome-wide measurements of protein occupancy and active transcription in a clinical isolate of V. cholerae, harboring recently discovered HAEs encoding for phage defense systems, we show that a lack of H-NS causes a robust increase in the expression of genes found in many HAEs. We further found that TsrA, a protein with partial homology to H-NS, regulates virulence genes primarily through modulation of H-NS activity. We also identified few sites that are affected by TsrA independently of H-NS, suggesting TsrA may act with diverse regulatory mechanisms. Our results demonstrate how the combinatorial activity of NAPs is employed by a clinical isolate of an important pathogen to regulate recently discovered HAEs.

IMPORTANCE

New strains of the bacterial pathogen Vibrio cholerae, bearing novel horizontally acquired elements (HAEs), frequently emerge. HAEs provide beneficial traits to the bacterium, such as antibiotic resistance and defense against invading bacteriophages. Xenogeneic silencers are proteins that help bacteria harness new HAEs and silence those HAEs until they are needed. H-NS is the best-studied xenogeneic silencer; it is one of the nucleoid-associated proteins (NAPs) in gamma-proteobacteria and is responsible for the proper regulation of HAEs within the bacterial transcriptional network. We studied the effects of H-NS and other NAPs on the HAEs of a clinical isolate of V. cholerae. Importantly, we found that H-NS partners with a small and poorly characterized protein, TsrA, to help domesticate new HAEs involved in bacterial survival and in causing disease. A proper understanding of the regulatory state in emerging isolates of V. cholerae will provide improved therapies against new isolates of the pathogen.

Mobile-CRISPRi as a powerful tool for modulating Vibrio gene expression

CRISPRi (Clustered Regularly Interspaced Palindromic Repeats interference) is a gene knockdown method that uses a deactivated Cas9 protein (dCas9) that binds a specific gene target locus dictated by an encoded guide RNA (sgRNA) to block transcription. Mobile-CRISPRi is a suite of modular vectors that enable CRISPRi knockdowns in diverse bacteria by integrating IPTG-inducible dcas9 and sgRNA genes into the genome using Tn7 transposition. Here, we show that the Mobile-CRISPRi system functions robustly and specifically in multiple Vibrio species: Vibrio cholerae, Vibrio fischeri, Vibrio vulnificus, Vibrio parahaemolyticus, and Vibrio campbellii. We demonstrate efficacy by targeting both essential and non-essential genes that function to produce defined, measurable phenotypes: bioluminescence, quorum sensing, cell division, and growth arrest. We anticipate that Mobile-CRISPRi will be used in Vibrio species to systematically probe gene function and essentiality in various behaviors and native environments.IMPORTANCEThe genetic manipulation of bacterial genomes is an invaluable tool in experimental microbiology. The development of CRISPRi (Clustered Regularly Interspaced Palindromic Repeats interference) tools has revolutionized genetics in many organisms, including bacteria. Here, we optimized the use of Mobile-CRISPRi in five Vibrio species, each of which has significant impacts on marine environments and organisms that include squid, shrimp, shellfish, finfish, corals, and multiple of which pose direct threats to human health. The Mobile-CRISPRi technology is easily adaptable, moveable from strain to strain, and enables researchers to selectively turn off gene expression. Our experiments demonstrate Mobile-CRISPRi is effective and robust at repressing gene expression of both essential and non-essential genes in Vibrio species.

Nucleoid-associated proteins shape the global protein occupancy and transcriptional landscape of a clinical isolate of Vibrio cholerae

Vibrio cholerae, the causative agent of the diarrheal disease cholera, poses an ongoing health threat due to its wide repertoire of horizontally acquired elements (HAEs) and virulence factors. New clinical isolates of the bacterium with improved fitness abilities, often associated with HAEs, frequently emerge. The appropriate control and expression of such genetic elements is critical for the bacteria to thrive in the different environmental niches it occupies. H-NS, the histone-like nucleoid structuring protein, is the best studied xenogeneic silencer of HAEs in gamma-proteobacteria. Although H-NS and other highly abundant nucleoid-associated proteins (NAPs) have been shown to play important roles in regulating HAEs and virulence in model bacteria, we still lack a comprehensive understanding of how different NAPs modulate transcription in V. cholerae. By obtaining genome-wide measurements of protein occupancy and active transcription in a clinical isolate of V. cholerae, harboring recently discovered HAEs encoding for phage defense systems, we show that a lack of H-NS causes a robust increase in the expression of genes found in many HAEs. We further found that TsrA, a protein with partial homology to H-NS, regulates virulence genes primarily through modulation of H-NS activity. We also identified a few sites that are affected by TsrA independently of H-NS, suggesting TsrA may act with diverse regulatory mechanisms. Our results demonstrate how the combinatorial activity of NAPs is employed by a clinical isolate of an important pathogen to regulate recently discovered HAEs.

Importance

New strains of the bacterial pathogen Vibrio cholerae, bearing novel horizontally acquired elements (HAEs), frequently emerge. HAEs provide beneficial traits to the bacterium, such as antibiotic resistance and defense against invading bacteriophages. Xenogeneic silencers are proteins that help bacteria harness new HAEs and silence those HAEs until they are needed. H-NS is the best-studied xenogeneic silencer; it is one of the nucleoid-associated proteins (NAPs) in gamma-proteobacteria and is responsible for the proper regulation of HAEs within the bacterial transcriptional network. We studied the effects of H-NS and other NAPs on the HAEs of a clinical isolate of V. cholerae. Importantly, we found that H-NS partners with a small and poorly characterized protein, TsrA, to help domesticate new HAEs involved in bacterial survival and in causing disease. Proper understanding of the regulatory state in emerging isolates of V. cholerae will provide improved therapies against new isolates of the pathogen.

Mobile-CRISPRi as a powerful tool for modulating Vibrio gene expression

CRISPRi (Clustered Regularly Interspaced Palindromic Repeats interference) is a gene knockdown method that uses a deactivated Cas9 protein (dCas9) that binds a specific gene target locus dictated by an encoded guide RNA (sgRNA) to block transcription. Mobile-CRISPRi is a suite of modular vectors that enable CRISPRi knockdowns in diverse bacteria by integrating IPTG-inducible dcas9 and sgRNA genes into the genome using Tn 7 transposition. Here, we show that the Mobile-CRISPRi system functions robustly and specifically in multiple Vibrio species: Vibrio cholerae, Vibrio fischeri, Vibrio vulnificus, Vibrio parahaemolyticus , and Vibrio campbellii . We demonstrate efficacy by targeting both essential and non-essential genes that function to produce defined, measurable phenotypes: bioluminescence, quorum sensing, cell division, and growth arrest. We anticipate that Mobile-CRISPRi will be used in Vibrio species to systematically probe gene function and essentiality in various behaviors and native environments.

Adaptation to bile and anaerobicity limits Vibrio cholerae phage adsorption

IMPORTANCE

Vibrio cholerae is the bacterial pathogen responsible for cholera, a diarrheal disease that impacts people in areas without access to potable water. In regions that lack such infrastructure, cholera represents a large proportion of disease outbreaks. Bacteriophages (phages, viruses that infect bacteria) have recently been examined as potential therapeutic and prophylactic agents to treat and prevent bacterial disease outbreaks like cholera due to their specificity and stability. This work examines the interaction between V. cholerae and vibriophages in consideration for a cholera prophylaxis regimen (M. Yen, L. S. Cairns, and A. Camilli, Nat Commun 8:14187, 2017, https://doi.org/10.1038/ncomms14187) in the context of stimuli found in the intestinal environment. We discover that common signals in the intestinal environment induce cell surface modifications in V. cholerae that also restrict some phages from binding and initiating infection. These findings could impact considerations for the design of phage-based treatments, as phage infection appears to be limited by bacterial adaptations to the intestinal environment.

Recent insights into the world of dual-function bacterial sRNAs

Dual-function sRNAs refer to a small subgroup of small regulatory RNAs that merges base-pairing properties of antisense RNAs with peptide-encoding properties of mRNA. Both functions can be part of either same or in another metabolic pathway. Here, we want to update the knowledge of to the already known dual-function sRNAs and review the six new sRNAs found since 2017 regarding their structure, functional mechanisms, evolutionary conservation, and role in the regulation of distinct biological/physiological processes. The increasing identification of dual-function sRNAs through bioinformatics approaches, RNomics and RNA-sequencing and the associated increase in regulatory understanding will likely continue to increase at the same rate in the future. This may improve our understanding of the physiology, virulence and resistance of bacteria, as well as enable their use in technical applications. This article is categorized under: Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs.

SARS-CoV-2 Orphan Gene ORF10 Contributes to More Severe COVID-19 Disease

The orphan gene of SARS-CoV-2, ORF10, is the least studied gene in the virus responsible for the COVID-19 pandemic. Recent experimentation indicated ORF10 expression moderates innate immunity in vitro. However, whether ORF10 affects COVID-19 in humans remained unknown. We determine that the ORF10 sequence is identical to the Wuhan-Hu-1 ancestral haplotype in 95% of genomes across five variants of concern (VOC). Four ORF10 variants are associated with less virulent clinical outcomes in the human host: three of these affect ORF10 protein structure, one affects ORF10 RNA structural dynamics. RNA-Seq data from 2070 samples from diverse human cells and tissues reveals ORF10 accumulation is conditionally discordant from that of other SARS-CoV-2 transcripts. Expression of ORF10 in A549 and HEK293 cells perturbs immune-related gene expression networks, alters expression of the majority of mitochondrially-encoded genes of oxidative respiration, and leads to large shifts in levels of 14 newly-identified transcripts. We conclude ORF10 contributes to more severe COVID-19 clinical outcomes in the human host.

KSHV 3.0: A State-of-the-Art Annotation of the Kaposi's Sarcoma-Associated Herpesvirus Transcriptome Using Cross-Platform Sequencing

Kaposi's sarcoma-associated herpesvirus (KSHV) is a large, oncogenic DNA virus belonging to the gammaherpesvirus subfamily. KSHV has been extensively studied with various high-throughput RNA-sequencing approaches to map the transcription start and end sites, the splice junctions, and the translation initiation sites. Despite these efforts, the comprehensive annotation of the viral transcriptome remains incomplete. In the present study, we generated a long-read sequencing dataset of the lytic and latent KSHV transcriptome using native RNA and direct cDNA sequencing methods. This was supplemented with CAGE sequencing based on a short-read platform. We also utilized datasets from previous publications for our analysis. As a result of this combined approach, we have identified a number of novel viral transcripts and RNA isoforms and have either corroborated or improved the annotation of previously identified viral RNA molecules, thereby notably enhancing our comprehension of the transcriptomic architecture of the KSHV genome. We also evaluated the coding capability of transcripts previously thought to be non-coding, by integrating our data on the viral transcripts with translatomic information from other publications.

D-amino acids signal a stress-dependent run-away response in Vibrio cholerae

To explore favourable niches while avoiding threats, many bacteria use a chemotaxis navigation system. Despite decades of studies on chemotaxis, most signals and sensory proteins are still unknown. Many bacterial species release D-amino acids to the environment; however, their function remains largely unrecognized. Here we reveal that D-arginine and D-lysine are chemotactic repellent signals for the cholera pathogen Vibrio cholerae. These D-amino acids are sensed by a single chemoreceptor MCPDRK co-transcribed with the racemase enzyme that synthesizes them under the control of the stress-response sigma factor RpoS. Structural characterization of this chemoreceptor bound to either D-arginine or D-lysine allowed us to pinpoint the residues defining its specificity. Interestingly, the specificity for these D-amino acids appears to be restricted to those MCPDRK orthologues transcriptionally linked to the racemase. Our results suggest that D-amino acids can shape the biodiversity and structure of complex microbial communities under adverse conditions.

Precisely Controlling Csr sRNA Levels by MshH Enhances Vibrio cholerae Colonization in Adult Mice

Vibrio cholerae is the causative agent of cholera. Effective intestinal colonization is a key step for V. cholerae pathogenicity and transmission. In this study, we found that deleting mshH, a homolog of the Escherichia coli CsrD protein, caused a V. cholerae colonization defect in the intestine of adult mice. By analyzing the RNA levels of CsrB, CsrC, and CsrD, we found that deleting mshH increased the levels of CsrB and CsrD but decreased the level of CsrC. However, deleting CsrB and -D not only recovered the mshH deletion mutant colonization defect but also recovered CsrC to wild-type levels. These results indicated that controlling the RNA levels of CsrB, -C, and -D is crucial for V. cholerae colonization of adult mice. We further demonstrated that the RNA levels of CsrB and CsrD were mainly controlled by MshH-dependent degradation, yet the level of CsrC was mainly determined by the CsrA-dependent stabilization. Our data show that V. cholerae differentially controls CsrB, -C, and -D abundance through the MshH-CsrB/C/D-CsrA regulatory pathway to finely regulate the activity of CsrA targets such as ToxR, so as to better survive in adult mouse intestine. IMPORTANCE The ability of V. cholerae to colonize the intestine is a key factor for its fitness and transmissibility between hosts. Here, we investigated the mechanism of V. cholerae colonization of adult mammal intestine and found that precisely controlling the CsrB, -C, and -D contents by MshH and CsrA plays an essential role for V. cholerae colonization in the adult mouse intestine. These data expand our knowledge on the mechanism of V. cholerae controlling the RNA level of CsrB, -C, and -D and highlight the importance that the different strategies used by V. cholerae to regulate the RNA level of CsrB, -C, and -D confer the bacterium with a survival advantage.

Phylogenomic analysis of the Porphyromonas gingivalis - Porphyromonas gulae duo: approaches to the origin of periodontitis

Porphyromonas gingivalis is an oral human pathogen associated with the onset and progression of periodontitis, a chronic immune-inflammatory disease characterized by the destruction of the teeth-supporting tissue. P. gingivalis belongs to the genus Porphyromonas, which is characterized by being composed of Gram-negative, asaccharolytic, non-spore-forming, non-motile, obligatory anaerobic species, inhabiting niches such as the oral cavity, urogenital tract, gastrointestinal tract and infected wound from different mammals including humans. Among the Porphyromonas genus, P. gingivalis stands out for its specificity in colonizing the human oral cavity and its keystone pathogen role in periodontitis pathogenesis. To understand the evolutionary process behind P. gingivalis in the context of the Pophyoromonas genus, in this study, we performed a comparative genomics study with publicly available Porphyromonas genomes, focused on four main objectives: (A) to confirm the phylogenetic position of P. gingivalis in the Porphyromonas genus by phylogenomic analysis; (B) the definition and comparison of the pangenomes of P. gingivalis and its relative P. gulae; and (C) the evaluation of the gene family gain/loss events during the divergence of P. gingivalis and P. gulae; (D) the evaluation of the evolutionary pressure (represented by the calculation of Tajima-D values and dN/dS ratios) comparing gene families of P. gingivalis and P. gulae. Our analysis found 84 high-quality assemblies representing P. gingivalis and 14 P. gulae strains (from a total of 233 Porphyromonas genomes). Phylogenomic analysis confirmed that P. gingivalis and P. gulae are highly related lineages, close to P. loveana. Both organisms harbored open pangenomes, with a strong core-to-accessory ratio for housekeeping genes and a negative ratio for unknown function genes. Our analyses also characterized the gene set differentiating P. gulae from P. gingivalis, mainly associated with unknown functions. Relevant virulence factors, such as the FimA, Mfa1, and the hemagglutinins, are conserved in P. gulae, P. gingivalis, and P. loveana, suggesting that the origin of those factors occurred previous to the P. gulae - P. gingivalis divergence. These results suggest an unexpected evolutionary relationship between the P. gulae - P. gingivalis duo and P. loveana, showing more clues about the origin of the role of those organisms in periodontitis.

Advances in cholera research: from molecular biology to public health initiatives

The aquatic bacterium Vibrio cholerae is the etiological agent of the diarrheal disease cholera, which has plagued the world for centuries. This pathogen has been the subject of studies in a vast array of fields, from molecular biology to animal models for virulence activity to epidemiological disease transmission modeling. V. cholerae genetics and the activity of virulence genes determine the pathogenic potential of different strains, as well as provide a model for genomic evolution in the natural environment. While animal models for V. cholerae infection have been used for decades, recent advances in this area provide a well-rounded picture of nearly all aspects of V. cholerae interaction with both mammalian and non-mammalian hosts, encompassing colonization dynamics, pathogenesis, immunological responses, and transmission to naïve populations. Microbiome studies have become increasingly common as access and affordability of sequencing has improved, and these studies have revealed key factors in V. cholerae communication and competition with members of the gut microbiota. Despite a wealth of knowledge surrounding V. cholerae, the pathogen remains endemic in numerous countries and causes sporadic outbreaks elsewhere. Public health initiatives aim to prevent cholera outbreaks and provide prompt, effective relief in cases where prevention is not feasible. In this review, we describe recent advancements in cholera research in these areas to provide a more complete illustration of V. cholerae evolution as a microbe and significant global health threat, as well as how researchers are working to improve understanding and minimize impact of this pathogen on vulnerable populations.

Archetypal analysis of COVID-19 in Montana, USA, March 13, 2020 to April 26, 2022

Given the potential consequences of infectious diseases, it is important to understand how broad scale incidence variability influences the probability of localized outbreaks. Often, these infectious disease data can involve complex spatial patterns intermixed with temporal trends. Archetypal Analysis is a method to mine complex spatiotemporal epidemiological data, and can be used to discover the dynamics of spatial patterns. The application of Archetypal Analysis to epistemological data is relatively new, and here we present one of the first applications using COVID-19 data from March 13, 2020 to April 26, 2022, in the counties of Montana, USA. We present three views of the data set with Archetypal Analysis. First, we evaluate the entire 56 county data set. Second, we compute mutual information of the 56 counties' time series to remove counties whose dynamics are mainly independent from most of the other counties. We choose the top 17 counties ranked in terms of increasing total mutual information. Finally, to compare how population size might influence results, we conducted an analysis with 10 of the largest counties. Using the Archetypal Analysis results, we analyze the disease outbreaks across Montana, comparing and contrasting the three different cases and showing how certain counties can be found in distinct sets of archetypes. Using the reconstruction time series, we show how each outbreak had a unique trajectory across the state in terms of the archetypes.

The Role of Nutrients and Nutritional Signals in the Pathogenesis of Vibrio cholerae

Vibrio cholerae, the agent of cholera, is a natural inhabitant of aquatic environments. Over the past decades, the importance of specific nutrients and micronutrients in the environmental survival, host colonization, and pathogenesis of this species has become increasingly clear. For instance, V. cholerae has evolved ingenious mechanisms that allow the bacterium to colonize and establish a niche in the intestine of human hosts, where it competes with commensals (gut microbiota) and other pathogenic bacteria for available nutrients. Here, we discuss the carbon and energy sources utilized by V. cholerae and what is known about the role of nutrition in V. cholerae colonization. We examine how nutritional signals affect virulence gene regulation and how interactions with intestinal commensal species can affect intestinal colonization.

Cholera Dynamics and the Emergence of Pandemic Vibrio cholerae

Cholera is a severe diarrheal disease caused by the aquatic bacterium Vibrio cholerae. Interestingly, to date, only one major clade has emerged to cause pandemic disease in humans: the clade that encompasses the strains from the O1 and O139 serogroups. In this chapter, we provide a comprehensive perspective on the virulence factors and mobile genetic elements (MGEs) associated with the emergence of pandemic V. cholerae strains and highlight novel findings such as specific genomic background or interactions between MGEs that explain their confined distribution. Finally, we discuss pandemic cholera dynamics contextualizing them within the evolution of the bacterium.

Vibrio parahaemolyticus and Vibrio vulnificus in vitro colonization on plastics influenced by temperature and strain variability

Marine bacteria often exist in biofilms as communities attached to surfaces, like plastic. Growing concerns exist regarding marine plastics acting as potential vectors of pathogenic Vibrio, especially in a changing climate. It has been generalized that Vibrio vulnificus and Vibrio parahaemolyticus often attach to plastic surfaces. Different strains of these Vibrios exist having different growth and biofilm-forming properties. This study evaluated how temperature and strain variability affect V. parahaemolyticus and V. vulnificus biofilm formation and characteristics on glass (GL), low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS). All strains of both species attached to GL and all plastics at 25, 30, and 35°C. As a species, V. vulnificus produced more biofilm on PS (p ≤ 0.05) compared to GL, and biofilm biomass was enhanced at 25°C compared to 30° (p ≤ 0.01) and 35°C (p ≤ 0.01). However, all individual strains' biofilm biomass and cell densities varied greatly at all temperatures tested. Comparisons of biofilm-forming strains for each species revealed a positive correlation (r = 0.58) between their dry biomass weight and OD₅₇₀ values from crystal violet staining, and total dry biofilm biomass for both species was greater (p ≤ 0.01) on plastics compared to GL. It was also found that extracellular polymeric substance (EPS) chemical characteristics were similar on all plastics of both species, with extracellular proteins mainly contributing to the composition of EPS. All strains were hydrophobic at 25, 30, and 35°C, further illustrating both species' affinity for potential attachment to plastics. Taken together, this study suggests that different strains of V. parahaemolyticus and V. vulnificus can rapidly form biofilms with high cell densities on different plastic types in vitro. However, the biofilm process is highly variable and is species-, strain-specific, and dependent on plastic type, especially under different temperatures.

Undecaprenyl phosphate translocases confer conditional microbial fitness

The microbial cell wall is essential for maintenance of cell shape and resistance to external stressors¹. The primary structural component of the cell wall is peptidoglycan, a glycopolymer with peptide crosslinks located outside of the cell membrane¹. Peptidoglycan biosynthesis and structure are responsive to shifting environmental conditions such as pH and salinity^2-6, but the mechanisms underlying such adaptations are incompletely understood. Precursors of peptidoglycan and other cell surface glycopolymers are synthesized in the cytoplasm and then delivered across the cell membrane bound to the recyclable lipid carrier undecaprenyl phosphate⁷ (C55-P, also known as UndP). Here we identify the DUF368-containing and DedA transmembrane protein families as candidate C55-P translocases, filling a critical gap in knowledge of the proteins required for the biogenesis of microbial cell surface polymers. Gram-negative and Gram-positive bacteria lacking their cognate DUF368-containing protein exhibited alkaline-dependent cell wall and viability defects, along with increased cell surface C55-P levels. pH-dependent synthetic genetic interactions between DUF368-containing proteins and DedA family members suggest that C55-P transporter usage is dynamic and modulated by environmental inputs. C55-P transporter activity was required by the cholera pathogen for growth and cell shape maintenance in the intestine. We propose that conditional transporter reliance provides resilience in lipid carrier recycling, bolstering microbial fitness both inside and outside the host.

Structural investigation and gene deletion studies of mycobacterial oligoribonuclease reveal modulation of c-di-GMP-mediated phenotypes

Cyclic-di-GMP (c-di-GMP) is a ubiquitous bacterial second messenger required for normal physiology as well as survival under hypoxic and reductive stress conditions of mycobacterial cells. Complete degradation of c-di-GMP is necessary for signal termination and maintaining its homeostasis inside the cells. Homeostasis of c-di-GMP in mycobacteria is brought about by the bifunctional diguanylate cyclase (DGC) that synthesizes c-di-GMP from two molecules of GTP and also catalyses the asymmetric cleavage of c-di-GMP to linear pGpG through its phosphodiesterase activity. However, the mycobacterial enzyme for the last step of degradation from pGpG to GMP has not been characterized thus far. Here, we present the identification of oligoribonuclease (Orn) as the most likely phosphodiesterase to degrade pGpG to GMP through AlphaFold-empowered structural homology that exhibited in vitro phosphodiesterase activity on pGpG substrates. In order to understand the physiological role of Orn in mycobacteria, we created a deletion mutant of orn in M. smegmatis and analysed the phenotypes that are associated with c-di-GMP signaling. We find that orn plays important roles in vivo and is required not only for proper growth of M. smegmatis in normal and stress conditions but also for biofilm formation.

Analysis of multipartite bacterial genomes using alignment free and alignment-based pipelines

Since the discovery of second chromosome in Rhodobacter sphaeroides 2.4.1 in 1989, multipartite genomes have been reported in over three hundred bacterial species under nine different phyla. This has shattered the unipartite (single chromosome) genome dogma in bacteria. Since then, many questions on various aspects of multipartite genomes in bacteria have been addressed. However, our understanding of how multipartite genomes emerge and evolve is still lacking. Importantly, the knowledge of genetic factors underlying the differences in multipartite and single-chromosome genomes is lacking. In this work, we have performed comparative evolutionary and functional genomics analyses to identify molecular factors that discriminate multipartite from unipartite bacteria, with the goal to decipher taxon-specific factors, and those that are prevalent across the taxa, underlying these traits. We assessed the roles of evolutionary mechanisms, specifically gene gain, in driving the divergence of bacteria with single and multiple chromosomes. In addition, we performed functional genomic analysis to garner support for our findings from comparative evolutionary analysis. We found genes such as those encoding conserved hypothetical proteins in Deinococcus radiodurans R1, and putative phage phi-C31 gp36 major capsid like and hypothetical proteins in Rhodobacter sphaeroides 2.4.1, which are located on accessory chromosomes in these bacteria but were not found in the inferred ancestral sequences, and on the primary chromosomes, as well as were not found in their closest relatives with single chromosome within the same clade. Our study shines a new light on the potential roles of the secondary chromosomes in helping bacteria with multipartite genomes to adapt to specialized environments or growth conditions.

Emergence and Evolutionary Response of Vibrio cholerae to Novel Bacteriophage, Democratic Republic of the Congo1

Cholera causes substantial illness and death in Africa. We analyzed 24 toxigenic Vibrio cholerae O1 strains isolated in 2015-2017 from patients in the Great Lakes region of the Democratic Republic of the Congo. Strains originating in southern Asia appeared to be part of the T10 introduction event in eastern Africa. We identified 2 main strain lineages, most recently a lineage corresponding to sequence type 515, a V. cholerae cluster previously reported in the Lake Kivu region. In 41% of fecal samples from cholera patients, we also identified a novel ICP1 (Bangladesh cholera phage 1) bacteriophage, genetically distinct from ICP1 isolates previously detected in Asia. Bacteriophage resistance occurred in distinct clades along both internal and external branches of the cholera phylogeny. This bacteriophage appears to have served as a major driver for cholera evolution and spread, and its appearance highlights the complex evolutionary dynamic that occurs between predatory phage and bacterial host.

PAS Domain-Containing Chemoreceptors Influence the Signal Sensing and Intestinal Colonization of Vibrio cholerae

Bacterial chemotaxis is the phenomenon in which bacteria migrate toward a more favorable niche in response to chemical cues in the environment. The methyl-accepting chemotaxis proteins (MCPs) are the principal sensory receptors of the bacterial chemotaxis system. Aerotaxis is a special form of chemotaxis in which oxygen serves as the signaling molecule; the process is dependent on the aerotaxis receptors (Aer) containing the Per-Arnt-Sim (PAS) domain. Over 40 MCPs are annotated on the genome of Vibrio cholerae; however, little is known about their functions. We investigated six MCPs containing the PAS domain in V. cholerae El Tor C6706, namely aer2, aer3, aer4, aer5, aer6, and aer7. Deletion analyses of each aer homolog gene indicated that these Aer receptors are involved in aerotaxis, chemotaxis, biofilm formation, and intestinal colonization. Swarming motility assay indicated that the aer2 gene was responsible for sensing the oxygen gradient independent of the other five homologs. When bile salts and mucin were used as chemoattractants, each Aer receptor influenced the chemotaxis differently. Biofilm formation was enhanced by overexpression of the aer6 and aer7 genes. Moreover, deletion of the aer2 gene resulted in better bacterial colonization of the mutant in adult mice; however, virulence gene expression was unaffected. These data suggest distinct roles for different Aer homologs in V. cholerae physiology.

Nano-RNases: oligo- or dinucleases?

Diribonucleotides arise from two sources: turnover of RNA transcripts (rRNA, tRNA, mRNA, and others) and linearization of cyclic-di-nucleotide signaling molecules. In both cases, there appears to be a requirement for a dedicated set of enzymes that will cleave these diribonucleotides into mononucleotides. The first enzyme discovered to mediate this activity is oligoribonuclease (Orn) from Escherichia coli. In addition to being the enzyme that cleaves dinucleotides and potentially other short oligoribonucleotides, Orn is also the only known exoribonuclease enzyme that is essential for E. coli, suggesting that removal of the shortest RNAs is an essential cellular function. Organisms naturally lacking the orn gene encode other nanoRNases (nrn) that can complement the conditional E. coli orn mutant. This review covers the history and recent advances in our understanding of these enzymes and their substrates. In particular, we focus on (i) the sources of diribonucleotides; (ii) the discovery of exoribonucleases; (iii) the structural features of Orn, NrnA/NrnB, and NrnC; (iv) the enzymatic activity of these enzymes against diribonucleotides versus other substrates; (v) the known physiological consequences of accumulation of linear dinucleotides; and (vi) outstanding biological questions for diribonucleotides and diribonucleases.

Emerging Concepts in Cholera Vaccine Design

Cholera is a severe diarrheal disease caused by the bacterium Vibrio cholerae and constitutes a significant public health threat in many areas of the world. V. cholerae infection elicits potent and long-lasting immunity, and efforts to develop cholera vaccines have been ongoing for more than a century. Currently available inactivated two-dose oral cholera vaccines are increasingly deployed to both prevent and actively curb cholera outbreaks, and they are key components of the global effort to eradicate cholera. However, these killed whole-cell vaccines have several limitations, and a variety of new oral and nonoral cholera vaccine platforms have recently been developed. Here, we review emerging concepts in cholera vaccine design and implementation that have been driven by insights from human and animal studies. As a prototypical vaccine-preventable disease, cholera continues to be an excellent target for the development and application of cutting-edge technologies and platforms that may transform vaccinology. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The VarA-CsrA regulatory pathway influences cell shape in Vibrio cholerae

Despite extensive studies on the curve-shaped bacterium Vibrio cholerae, the causative agent of the diarrheal disease cholera, its virulence-associated regulatory two-component signal transduction system VarS/VarA is not well understood. This pathway, which mainly signals through the downstream protein CsrA, is highly conserved among gamma-proteobacteria, indicating there is likely a broader function of this system beyond virulence regulation. In this study, we investigated the VarA-CsrA signaling pathway and discovered a previously unrecognized link to the shape of the bacterium. We observed that varA-deficient V. cholerae cells showed an abnormal spherical morphology during late-stage growth. Through peptidoglycan (PG) composition analyses, we discovered that these mutant bacteria contained an increased content of disaccharide dipeptides and reduced peptide crosslinks, consistent with the atypical cellular shape. The spherical shape correlated with the CsrA-dependent overproduction of aspartate ammonia lyase (AspA) in varA mutant cells, which likely depleted the cellular aspartate pool; therefore, the synthesis of the PG precursor amino acid meso-diaminopimelic acid was impaired. Importantly, this phenotype, and the overall cell rounding, could be prevented by means of cell wall recycling. Collectively, our data provide new insights into how V. cholerae use the VarA-CsrA signaling system to adjust its morphology upon unidentified external cues in its environment.

Animal models for dissecting Vibrio cholerae intestinal pathogenesis and immunity

The human diarrheal disease cholera is caused by the bacterium Vibrio cholerae. Efforts to develop animal models that closely mimic cholera to study the pathogenesis of this disease began >125 years ago. Here, we review currently used non-surgical, oral inoculation-based animal models for investigation of V. cholerae intestinal colonization and disease and highlight recent discoveries that have illuminated mechanisms of cholera pathogenesis and immunity, particularly in the area of how V. cholerae interacts with the gut microbiome to influence infection. The emergence of high-throughput tools for studies of pathogen-host interactions, along with continued advances in host genetic engineering and manipulation in animal models of V. cholerae will deepen understanding of cholera pathogenesis, uncovering knowledge important for control of this globally important bacterial pathogen.

Thiol-based functional mimicry of phosphorylation of the two-component system response regulator ArcA promotes pathogenesis in enteric pathogens

Pathogenic bacteria can rapidly respond to stresses such as reactive oxygen species (ROS) using reversible redox-sensitive oxidation of cysteine thiol (-SH) groups in regulators. Here, we use proteomics to profile reversible ROS-induced thiol oxidation in Vibrio cholerae, the etiologic agent of cholera, and identify two modified cysteines in ArcA, a regulator of global carbon oxidation that is phosphorylated and activated under low oxygen. ROS abolishes ArcA phosphorylation but induces the formation of an intramolecular disulfide bond that promotes ArcA-ArcA interactions and sustains activity. ArcA cysteines are oxidized in cholera patient stools, and ArcA thiol oxidation drives in vitro ROS resistance, colonization of ROS-rich guts, and environmental survival. In other pathogens, such as Salmonella enterica, oxidation of conserved cysteines of ArcA orthologs also promotes ROS resistance, suggesting a common role for ROS-induced ArcA thiol oxidation in modulating ArcA activity, allowing for a balance of expression of stress- and pathogenesis-related genetic programs.

Proteomic analysis of the host-pathogen interface in experimental cholera

The microbial cell surface is a site of critical microbe-host interactions that often control infection outcomes. Defining the set of host proteins present at this interface has been challenging. Here we used a surface-biotinylation approach coupled to quantitative mass spectrometry to identify and quantify both bacterial and host proteins present on the surface of diarrheal fluid-derived Vibrio cholerae in an infant rabbit model of cholera. The V. cholerae surface was coated with numerous host proteins, whose abundance were driven by the presence of cholera toxin, including the C-type lectin SP-D. Mice lacking SP-D had enhanced V. cholerae intestinal colonization, and SP-D production shaped both host and pathogen transcriptomes. Additional host proteins (AnxA1, LPO and ZAG) that bound V. cholerae were also found to recognize distinct taxa of the murine intestinal microbiota, suggesting that these host factors may play roles in intestinal homeostasis in addition to host defense.

Essential Genes of Vibrio anguillarum and Other Vibrio spp. Guide the Development of New Drugs and Vaccines

Essential genes in bacterial pathogens are potential drug targets and vaccine candidates because disrupting their function is lethal. The development of new antibiotics, in addition to effective prevention measures such as vaccination, contributes to addressing the global problem of bacterial antibiotic resistance. The aim of this present study was to determine the essential genes of Vibrio anguillarum, a bacterial pathogen of aquatic animals, as a means to identify putative targets for novel drugs and to assist the prioritisation of potential vaccine candidates. Essential genes were characterised by a Tn-seq approach using the TnSC189 mariner transposon to construct a library of 52,662 insertion mutants. In total, 329 essential genes were identified, with 34.7% found within the core genome of this species; each of these genes represents a strong potential drug target. Seven essential gene products were predicted to reside in the cell membrane or be released extracellularly, thus serving as putative vaccine candidates. Comparison to essential gene data from five other studies of Vibrio species revealed 13 proteins to be conserved across the studies, while 25 genes were specific to V. anguillarum and not found to be essential in the other Vibrio spp. This study provides new information on the essential genes of Vibrio species and the methodology may be applied to other pathogens to guide the development of new drugs and vaccines, which will assist efforts to counter antibiotic resistance.

A dual-function RNA balances carbon uptake and central metabolism in Vibrio cholerae

Bacterial small RNAs (sRNAs) are well known to modulate gene expression by base pairing with trans‐encoded transcripts and are typically non‐coding. However, several sRNAs have been reported to also contain an open reading frame and thus are considered dual‐function RNAs. In this study, we discovered a dual‐function RNA from Vibrio cholerae, called VcdRP, harboring a 29 amino acid small protein (VcdP), as well as a base‐pairing sequence. Using a forward genetic screen, we identified VcdRP as a repressor of cholera toxin production and link this phenotype to the inhibition of carbon transport by the base‐pairing segment of the regulator. By contrast, we demonstrate that the VcdP small protein acts downstream of carbon transport by binding to citrate synthase (GltA), the first enzyme of the citric acid cycle. Interaction of VcdP with GltA results in increased enzyme activity and together VcdR and VcdP reroute carbon metabolism. We further show that transcription of vcdRP is repressed by CRP allowing us to provide a model in which VcdRP employs two different molecular mechanisms to synchronize central metabolism in V. cholerae.

Structural characterization of NrnC identifies unifying features of dinucleotidases

RNA degradation is fundamental for cellular homeostasis. The process is carried out by various classes of endolytic and exolytic enzymes that together degrade an RNA polymer to mono-ribonucleotides. Within the exoribonucleases, nano-RNases play a unique role as they act on the smallest breakdown products and hence catalyze the final steps in the process. We recently showed that oligoribonuclease (Orn) acts as a dedicated diribonuclease, defining the ultimate step in RNA degradation that is crucial for cellular fitness (Kim et al., 2019). Whether such a specific activity exists in organisms that lack Orn-type exoribonucleases remained unclear. Through quantitative structure-function analyses, we show here that NrnC-type RNases share this narrow substrate length preference with Orn. Although NrnC and Orn employ similar structural features that distinguish these two classes of dinucleases from other exonucleases, the key determinants for dinuclease activity are realized through distinct structural scaffolds. The structures, together with comparative genomic analyses of the phylogeny of DEDD-type exoribonucleases, indicate convergent evolution as the mechanism of how dinuclease activity emerged repeatedly in various organisms. The evolutionary pressure to maintain dinuclease activity further underlines the important role these analogous proteins play for cell growth.

Bacteriophage ICP1: A Persistent Predator of Vibrio cholerae

Bacteriophages or phages—viruses of bacteria—are abundant and considered to be highly diverse. Interestingly, a particular group of lytic Vibrio cholerae–specific phages (vibriophages) of the International Centre for Diarrheal Disease Research, Bangladesh cholera phage 1 (ICP1) lineage show high levels of genome conservation over large spans of time and geography, despite a constant coevolutionary arms race with their host. From a collection of 67 sequenced ICP1 isolates, mostly from clinical samples, we find these phages have mosaic genomes consisting of large, conserved modules disrupted by variable sequences that likely evolve mostly through mobile endonuclease-mediated recombination during coinfection. Several variable regions have been associated with adaptations against antiphage elements in V. cholerae; notably, this includes ICP1’s CRISPR-Cas system. The ongoing association of ICP1 and V. cholerae in cholera-endemic regions makes this system a rich source for discovery of novel defense and counterdefense strategies in bacteria-phage conflicts in nature.

Vibrio cholerae's mysterious Seventh Pandemic island (VSP-II) encodes novel Zur-regulated zinc starvation genes involved in chemotaxis and cell congregation

Vibrio cholerae is the causative agent of cholera, a notorious diarrheal disease that is typically transmitted via contaminated drinking water. The current pandemic agent, the El Tor biotype, has undergone several genetic changes that include horizontal acquisition of two genomic islands (VSP-I and VSP-II). VSP presence strongly correlates with pandemicity; however, the contribution of these islands to V. cholerae's life cycle, particularly the 26-kb VSP-II, remains poorly understood. VSP-II-encoded genes are not expressed under standard laboratory conditions, suggesting that their induction requires an unknown signal from the host or environment. One signal that bacteria encounter under both host and environmental conditions is metal limitation. While studying V. cholerae's zinc-starvation response in vitro, we noticed that a mutant constitutively expressing zinc starvation genes (Δzur) congregates at the bottom of a culture tube when grown in a nutrient-poor medium. Using transposon mutagenesis, we found that flagellar motility, chemotaxis, and VSP-II encoded genes were required for congregation. The VSP-II genes encode an AraC-like transcriptional activator (VerA) and a methyl-accepting chemotaxis protein (AerB). Using RNA-seq and lacZ transcriptional reporters, we show that VerA is a novel Zur target and an activator of the nearby AerB chemoreceptor. AerB interfaces with the chemotaxis system to drive oxygen-dependent congregation and energy taxis. Importantly, this work suggests a functional link between VSP-II, zinc-starved environments, and energy taxis, yielding insights into the role of VSP-II in a metal-limited host or aquatic reservoir.

A Tail Fiber Protein and a Receptor-Binding Protein Mediate ICP2 Bacteriophage Interactions with Vibrio cholerae OmpU

ICP2 is a virulent bacteriophage (phage) that preys on Vibrio cholerae. ICP2 was first isolated from cholera patient stool samples. Some of these stools also contained ICP2-resistant isogenic V. cholerae strains harboring missense mutations in the trimeric outer membrane porin protein OmpU, identifying it as the ICP2 receptor. In this study, we identify the ICP2 proteins that mediate interactions with OmpU by selecting for ICP2 host range mutants within infant rabbits infected with a mixture of wild-type and OmpU mutant strains. ICP2 host range mutants that can now infect OmpU mutant strains have missense mutations in the putative tail fiber gene gp25 and the putative adhesin gene gp23. Using site-specific mutagenesis, we show that single or double mutations in gp25 are sufficient to generate the host range mutant phenotype. However, at least one additional mutation in gp23 is required for robust plaque formation on specific OmpU mutants. Mutations in gp23 alone were insufficient to produce a host range mutant phenotype. All ICP2 host range mutants retained the ability to form plaques on wild-type V. cholerae cells. The strength of binding of host range mutants to V. cholerae correlated with plaque morphology, indicating that the selected mutations in gp25 and gp23 restore molecular interactions with the receptor. We propose that ICP2 host range mutants evolve by a two-step process. First, gp25 mutations are selected for their broad host range, albeit accompanied by low-level phage adsorption. Subsequent selection occurs for gp23 mutations that further increase productive binding to specific OmpU alleles, allowing for near-wild-type efficiencies of adsorption and subsequent phage multiplication. IMPORTANCE Concern over multidrug-resistant bacterial pathogens, including Vibrio cholerae, has led to renewed interest in phage biology and the potential for phage therapy. ICP2 is a genetically unique virulent phage isolated from cholera patient stool samples. It is also one of three phages in a prophylactic cocktail that have been shown to be effective in animal models of infection and the only one of the three that requires a protein receptor (OmpU). This study identifies an ICP2 tail fiber and a receptor binding protein and examines how ICP2 responds to the selective pressures of phage-resistant OmpU mutants. We found that this particular coevolutionary arms race presents fitness costs to both ICP2 and V. cholerae.

Transcriptional regulation of the mannitol phosphotransferase system operon by the ferric uptake regulator (Fur) in Vibrio cholerae El Tor serogroup O1

The phosphoenolpyruvate (PEP): carbohydrate phosphotransferase system (PTS) allows bacteria to use various carbohydrates as energy resources including mannitol. The mannitol-specific PTS transporter in Vibrio cholerae is encoded by the mtlADR operon. Expression of the mtl operon has been shown to be strictly regulated by CRP, MtlS, and MtlR. In the present study, we investigated the regulation of mtlADR by the ferric uptake regulator (Fur). The results showed that Fur binds to the promoter-proximal DNA region of mtlADR to repress its transcription independent of iron, in mannitol-containing growth medium. The capacity for mannitol fermentation was significantly increased in Δfur relative to that of WT for normal and iron-replete growth media. The level of organic acids produced by Δfur was significantly enhanced relative to that produced by the WT strain in the normal and iron-replete media but not in an iron-starved medium. The results provided for a deeper understanding of the regulation of mtlADR in V. cholerae.

Next-generation microbiology: from comparative genomics to gene function

Microbiology is at a turning point in its 120-year history. Widespread next-generation sequencing has revealed genetic complexity among bacteria that could hardly have been imagined by pioneers such as Pasteur, Escherich and Koch. This data cascade brings enormous potential to improve our understanding of individual bacterial cells and the genetic basis of phenotype variation. However, this revolution in data science cannot replace established microbiology practices, presenting the challenge of how to integrate these new techniques. Contrasting comparative and functional genomic approaches, we evoke molecular microbiology theory and established practice to present a conceptual framework and practical roadmap for next-generation microbiology.

Suppressor Mutations in Type II Secretion Mutants of Vibrio cholerae: Inactivation of the VesC Protease

Genome-wide transposon mutagenesis has identified the genes encoding the T2SS in Vibrio cholerae as essential for viability, but the reason for this is unclear. Mutants with deletions or insertions in these genes can be isolated, suggesting that they have acquired secondary mutations that suppress their growth defect.

The type II secretion system (T2SS) is a conserved transport pathway responsible for the secretion of a range of virulence factors by many pathogens, including Vibrio cholerae. Disruption of the T2SS genes in V. cholerae results in loss of secretion, changes in cell envelope function, and growth defects. While T2SS mutants are viable, high-throughput genomic analyses have listed these genes among essential genes. To investigate whether secondary mutations arise as a consequence of T2SS inactivation, we sequenced the genomes of six V. cholerae T2SS mutants with deletions or insertions in either the epsG, epsL, or epsM genes and identified secondary mutations in all mutants. Two of the six T2SS mutants contain distinct mutations in the gene encoding the T2SS-secreted protease VesC. Other mutations were found in genes coding for V. cholerae cell envelope proteins. Subsequent sequence analysis of the vesC gene in 92 additional T2SS mutant isolates identified another 19 unique mutations including insertions or deletions, sequence duplications, and single-nucleotide changes resulting in amino acid substitutions in the VesC protein. Analysis of VesC variants and the X-ray crystallographic structure of wild-type VesC suggested that all mutations lead to loss of VesC production and/or function. One possible mechanism by which V. cholerae T2SS mutagenesis can be tolerated is through selection of vesC-inactivating mutations, which may, in part, suppress cell envelope damage, establishing permissive conditions for the disruption of the T2SS. Other mutations may have been acquired in genes encoding essential cell envelope proteins to prevent proteolysis by VesC.

IMPORTANCE Genome-wide transposon mutagenesis has identified the genes encoding the T2SS in Vibrio cholerae as essential for viability, but the reason for this is unclear. Mutants with deletions or insertions in these genes can be isolated, suggesting that they have acquired secondary mutations that suppress their growth defect. Through whole-genome sequencing and phenotypic analysis of T2SS mutants, we show that one means by which the growth defect can be suppressed is through mutations in the gene encoding the T2SS substrate VesC. VesC homologues are present in other Vibrio species and close relatives, and this may be why inactivation of the T2SS in species such as Vibrio vulnificus, Vibrio sp. strain 60, and Aeromonas hydrophila also results in a pleiotropic effect on their outer membrane assembly and integrity.

Interactions of replication initiator RctB with single- and double-stranded DNA in origin opening of Vibrio cholerae chromosome 2

Studies of bacterial chromosomes and plasmids indicate that their replication initiator proteins bind to origins of replication at many double-stranded sites and also at AT-rich regions where single-stranded DNA is exposed during origin opening. Single-strand binding apparently promotes origin opening by stabilizing an open structure, but how the initiator participates in this process and the contributions of the several binding sites remain unclear. Here, we show that the initiator protein of Vibrio cholerae specific to chromosome 2 (Chr2) also has single-strand binding activity in the AT-rich region of its origin. Binding is strand specific, depends on repeats of the sequence 5'ATCA and is greatly stabilized in vitro by specific double-stranded sites of the origin. The stability derives from the formation of ternary complexes of the initiator with the single- and double-stranded sites. An IHF site lies between these two kinds of sites in the Chr2 origin and an IHF-induced looping out of the intervening DNA mediates their interaction. Simultaneous binding to two kinds of sites in the origin appears to be a common mechanism by which bacterial replication initiators stabilize an open origin.

Tracking and predicting the African COVID-19 pandemic

The ongoing coronavirus disease 2019 (COVID-19) pandemic is heterogeneous throughout Africa and threatening millions of lives. Surveillance and short-term modeling forecasts are critical to provide timely information for decisions on control strategies. We use a model that explains the evolution of the COVID-19 pandemic over time in the entire African continent, parameterized by socioeconomic and geoeconomic variations and the lagged effects of social policy and meteorological history. We observed the effect of the human development index, containment policies, testing capacity, specific humidity, temperature and landlocked status of countries on the local within-country and external between-country transmission. One week forecasts of case numbers from the model were driven by the quality of the reported data. Seeking equitable behavioral and social interventions, balanced with coordinated country-specific strategies in infection suppression, should be a continental priority to control the COVID-19 pandemic in Africa.

A modular chromosomally integrated toolkit for ectopic gene expression in Vibrio cholerae

The ability to express genes ectopically in bacteria is essential for diverse academic and industrial applications. Two major considerations when utilizing regulated promoter systems for ectopic gene expression are (1) the ability to titrate gene expression by addition of an exogenous inducer and (2) the leakiness of the promoter element in the absence of the inducer. Here, we describe a modular chromosomally integrated platform for ectopic gene expression in Vibrio cholerae. We compare the broadly used promoter elements P_tac and P_BAD to versions that have an additional theophylline-responsive riboswitch (P_tac-riboswitch and P_BAD-riboswitch). These constructs all exhibited unimodal titratable induction of gene expression, however, max induction varied with P_tac > P_BAD > P_BAD-riboswitch > P_tac-riboswitch. We also developed a sensitive reporter system to quantify promoter leakiness and show that leakiness for P_tac > P_tac-riboswitch > P_BAD; while the newly developed P_BAD-riboswitch exhibited no detectable leakiness. We demonstrate the utility of the tightly inducible P_BAD-riboswitch construct using the dynamic activity of type IV competence pili in V. cholerae as a model system. The modular chromosomally integrated toolkit for ectopic gene expression described here should be valuable for the genetic study of V. cholerae and could be adapted for use in other species.

Integrated Metabolic Modeling, Culturing, and Transcriptomics Explain Enhanced Virulence of Vibrio cholerae during Coinfection with Enterotoxigenic Escherichia coli

Most studies proposing new strategies to manage and treat infections have been largely focused on identifying druggable targets that can inhibit a pathogen's growth when it is the single cause of infection. In vivo, however, infections can be caused by multiple species. This is important to take into account when attempting to develop or use current antibacterials since their efficacy can change significantly between single infections and coinfections. In this study, we used genome-scale metabolic models (GEMs) to interrogate the growth capabilities of Vibrio cholerae in single infections and coinfections with enterotoxigenic E. coli (ETEC), which cooccur in a large fraction of diarrheagenic patients. Coinfection model predictions showed that V. cholerae growth capabilities are enhanced in the presence of ETEC relative to V. cholerae single infection, through cross-fed metabolites made available to V. cholerae by ETEC. In vitro, cocultures of the two enteric pathogens further confirmed model predictions showing an increased growth of V. cholerae in coculture relative to V. cholerae single cultures while ETEC growth was suppressed. Dual RNAseq analysis of the cocultures also confirmed that the transcriptome of V. cholerae was distinct during coinfection compared to single-infection scenarios where processes related to metabolism were significantly perturbed. Further, in silico gene-knockout simulations uncovered discrepancies in gene essentiality for V. cholerae growth between single infections and coinfections. Integrative model-guided analysis thus identified druggable targets that would be critical for V. cholerae growth in both single infections and coinfections; thus, designing inhibitors against those targets would provide a broader spectrum of coverage against cholera infections.

Gene essentiality is altered during polymicrobial infections. Nevertheless, most studies rely on single-species infections to assess pathogen gene essentiality. Here, we use genome-scale metabolic models (GEMs) to explore the effect of coinfection of the diarrheagenic pathogen Vibrio cholerae with another enteric pathogen, enterotoxigenic Escherichia coli (ETEC). Model predictions showed that V. cholerae metabolic capabilities were increased due to ample cross-feeding opportunities enabled by ETEC. This is in line with increased severity of cholera symptoms known to occur in patients with dual infections by the two pathogens. In vitro coculture systems confirmed that V. cholerae growth is enhanced in cocultures relative to single cultures. Further, expression levels of several V. cholerae metabolic genes were significantly perturbed as shown by dual RNA sequencing (RNAseq) analysis of its cocultures with different ETEC strains. A decrease in ETEC growth was also observed, probably mediated by nonmetabolic factors. Single gene essentiality analysis predicted conditionally independent genes that are essential for the pathogen’s growth in both single-infection and coinfection scenarios. Our results reveal growth differences that are of relevance to drug targeting and efficiency in polymicrobial infections.

IMPORTANCE Most studies proposing new strategies to manage and treat infections have been largely focused on identifying druggable targets that can inhibit a pathogen's growth when it is the single cause of infection. In vivo, however, infections can be caused by multiple species. This is important to take into account when attempting to develop or use current antibacterials since their efficacy can change significantly between single infections and coinfections. In this study, we used genome-scale metabolic models (GEMs) to interrogate the growth capabilities of Vibrio cholerae in single infections and coinfections with enterotoxigenic E. coli (ETEC), which cooccur in a large fraction of diarrheagenic patients. Coinfection model predictions showed that V. cholerae growth capabilities are enhanced in the presence of ETEC relative to V. cholerae single infection, through cross-fed metabolites made available to V. cholerae by ETEC. In vitro, cocultures of the two enteric pathogens further confirmed model predictions showing an increased growth of V. cholerae in coculture relative to V. cholerae single cultures while ETEC growth was suppressed. Dual RNAseq analysis of the cocultures also confirmed that the transcriptome of V. cholerae was distinct during coinfection compared to single-infection scenarios where processes related to metabolism were significantly perturbed. Further, in silico gene-knockout simulations uncovered discrepancies in gene essentiality for V. cholerae growth between single infections and coinfections. Integrative model-guided analysis thus identified druggable targets that would be critical for V. cholerae growth in both single infections and coinfections; thus, designing inhibitors against those targets would provide a broader spectrum of coverage against cholera infections.

Transposon Insertion Sequencing, a Global Measure of Gene Function

The goal of genomics and systems biology is to understand how complex systems of factors assemble into pathways and structures that combine to form living organisms. Great advances in understanding biological processes result from determining the function of individual genes, a process that has classically relied on characterizing single mutations. Advances in DNA sequencing has made available the complete set of genetic instructions for an astonishing and growing number of species. To understand the function of this ever-increasing number of genes, a high-throughput method was developed that in a single experiment can measure the function of genes across the genome of an organism. This occurred approximately 10 years ago, when high-throughput DNA sequencing was combined with advances in transposon-mediated mutagenesis in a method termed transposon insertion sequencing (TIS). In the subsequent years, TIS succeeded in addressing fundamental questions regarding the genes of bacteria, many of which have been shown to play central roles in bacterial infections that result in major human diseases. The field of TIS has matured and resulted in studies of hundreds of species that include significant innovations with a number of transposons. Here, we summarize a number of TIS experiments to provide an understanding of the method and explanation of approaches that are instructive when designing a study. Importantly, we emphasize critical aspects of a TIS experiment and highlight the extension and applicability of TIS into nonbacterial species such as yeast.

Aerobic Metabolism in Vibrio cholerae Is Required for Population Expansion during Infection

Vibrio cholerae remains a challenge in the developing world and incidence of the disease it causes, cholera, is anticipated to increase with rising global temperatures and with emergent, highly infectious strains. At present, the underlying metabolic processes that support V. cholerae growth during infection are less well understood than specific virulence traits, such as production of a toxin or pilus. In this study, we determined that oxidative metabolism of host substrates such as mucin contribute significantly to V. cholerae population expansion in vivo. Identifying metabolic pathways critical for growth can provide avenues for controlling V. cholerae infection and the knowledge may be translatable to other pathogens of the gastrointestinal tract.

Vibrio cholerae replicates to high cell density in the human small intestine, leading to the diarrheal disease cholera. During infection, V. cholerae senses and responds to environmental signals that govern cellular responses. Spatial localization of V. cholerae within the intestine affects nutrient availability and metabolic pathways required for replicative success. Metabolic processes used by V. cholerae to reach such high cell densities are not fully known. We sought to better define the metabolic traits that contribute to high levels of V. cholerae during infection. By disrupting the pyruvate dehydrogenase (PDH) complex and pyruvate formate-lyase (PFL), we could differentiate aerobic and anaerobic metabolic pathway involvement in V. cholerae proliferation. We demonstrate that oxidative metabolism is a key contributor to the replicative success of V. choleraein vivo using an infant mouse model in which PDH mutants were attenuated 100-fold relative to the wild type for colonization. Additionally, metabolism of host substrates, including mucin, was determined to support V. cholerae growth in vitro as a sole carbon source, primarily under aerobic growth conditions. Mucin likely contributes to population expansion during human infection as it is a ubiquitous source of carbohydrates. These data highlight oxidative metabolism as important in the intestinal environment and warrant further investigation of how oxygen and other host substrates shape the intestinal landscape that ultimately influences bacterial disease. We conclude from our results that oxidative metabolism of host substrates is a key driver of V. cholerae proliferation during infection, leading to the substantial bacterial burden exhibited in cholera patients.

Landscape of in vivo Fitness-Associated Genes of Enterobacter cloacae Complex

Species of the Enterobacter cloacae complex (ECC) represent an increasing cause of hospital-acquired infections and commonly exhibit multiple antibiotic resistances. In order to identify genes that may play a role in its ability to colonize the host, we used the transposon-sequencing (Tn-seq) approach. To this end, a high-density random transposon insertion library was obtained from E. cloacae subsp. cloacae ATCC 13047, which was used to analyze the fitness of ca. 300,000 mutants in Galleria mellonella colonization model. Following massively parallel sequencing, we identified 624 genes that seemed essential for the optimal growth and/or the fitness within the host. Moreover, 63 genes where mutations resulted in positive selection were found, while 576 genes potentially involved in the in vivo fitness were observed. These findings pointed out the role of some transcriptional regulators, type VI secretion system, and surface-associated proteins in the in vivo fitness of E. cloacae ATCC 13047. We then selected eight genes based on their high positive or negative fold changes (FCs) and tested the corresponding deletion mutants for their virulence and ability to cope with stresses. Thereby, we showed that ECL_02247 (encoding the NAD-dependent epimerase/dehydratase) and ECL_04444 (coding for a surface antigen-like protein) may correspond to new virulence factors, and that the regulator ECL_00056 was involved in in vivo fitness. In addition, bacterial cells lacking the flagellum-specific ATP synthase FliI (ECL_03223) and the hypothetical protein ECL_01421 were affected for mobility and resistance to H₂O₂, respectively. All these results yield valuable information regarding genes important for infection process and stress response of E. cloacae ATCC 13047 and participate to a better understanding of the opportunistic traits in this bacterial pathogen.

A Comprehensive Coexpression Network Analysis in Vibrio cholerae

Cholera is a devastating illness that kills tens of thousands of people annually. Vibrio cholerae, the causative agent of cholera, is an important model organism to investigate both bacterial pathogenesis and the impact of horizontal gene transfer on the emergence and dissemination of new virulent strains. Despite the importance of this pathogen, roughly one-third of V. cholerae genes are functionally unannotated, leaving large gaps in our understanding of this microbe. Through coexpression network analysis of existing RNA sequencing data, this work develops an approach to uncover novel gene-gene relationships and contextualize genes with no known function, which will advance our understanding of V. cholerae virulence and evolution.

Research into the evolution and pathogenesis of Vibrio cholerae has benefited greatly from the generation of high-throughput sequencing data to drive molecular analyses. The steady accumulation of these data sets now provides a unique opportunity for in silico hypothesis generation via coexpression analysis. Here, we leverage all published V. cholerae RNA sequencing data, in combination with select data from other platforms, to generate a gene coexpression network that validates known gene interactions and identifies novel genetic partners across the entire V. cholerae genome. This network provides direct insights into genes influencing pathogenicity, metabolism, and transcriptional regulation, further clarifies results from previous sequencing experiments in V. cholerae (e.g., transposon insertion sequencing [Tn-seq] and chromatin immunoprecipitation sequencing [ChIP-seq]), and expands upon microarray-based findings in related Gram-negative bacteria.

IMPORTANCE Cholera is a devastating illness that kills tens of thousands of people annually. Vibrio cholerae, the causative agent of cholera, is an important model organism to investigate both bacterial pathogenesis and the impact of horizontal gene transfer on the emergence and dissemination of new virulent strains. Despite the importance of this pathogen, roughly one-third of V. cholerae genes are functionally unannotated, leaving large gaps in our understanding of this microbe. Through coexpression network analysis of existing RNA sequencing data, this work develops an approach to uncover novel gene-gene relationships and contextualize genes with no known function, which will advance our understanding of V. cholerae virulence and evolution.

Vibrio cholerae OmpR Contributes to Virulence Repression and Fitness at Alkaline pH

Vibrio cholerae is a Gram-negative human pathogen and the causative agent of the life-threatening disease cholera. V. cholerae is a natural inhabitant of marine environments and enters humans through the consumption of contaminated food or water. The ability to transition between aquatic ecosystems and the human host is paramount to the pathogenic success of V. cholerae The transition between these two disparate environments requires the expression of adaptive responses, and such responses are most often regulated by two-component regulatory systems such as the EnvZ/OmpR system, which responds to osmolarity and acidic pH in many Gram-negative bacteria. Previous work in our laboratory indicated that V. cholerae OmpR functioned as a virulence regulator through repression of the LysR-family transcriptional regulator aphB; however, the role of OmpR in V. cholerae biology outside virulence regulation remained unknown. In this work, we sought to further investigate the function of OmpR in V. cholerae biology by defining the OmpR regulon through RNA sequencing. This led to the discovery that V. cholerae ompR was induced at alkaline pH to repress genes involved in acid tolerance and virulence factor production. In addition, OmpR was required for V. cholerae fitness during growth under alkaline conditions. These findings indicate that V. cholerae OmpR has evolved the ability to respond to novel signals during pathogenesis, which may play a role in the regulation of adaptive responses to aid in the transition between the human gastrointestinal tract and the marine ecosystem.

Exception to the exception rule: synthetic and naturally occurring single chromosome Vibrio cholerae

The genome of Vibrio cholerae, the etiological agent of cholera, is an exception to the single chromosome rule found in the vast majority of bacteria and has its genome partitioned between two unequally sized chromosomes. This unusual two-chromosome arrangement in V. cholerae has sparked considerable research interest since its discovery. It was demonstrated that the two chromosomes could be fused by deliberate genome engineering or forced to fuse spontaneously by blocking the replication of Chr2, the secondary chromosome. Recently, natural isolates of V. cholerae with chromosomal fusion have been found. Here, we summarize the pertinent findings on this exception to the exception rule and discuss the potential utility of single-chromosome V. cholerae to address fundamental questions on chromosome biology in general and DNA replication in particular.

Shedding Light on Specificity: Population Genomic Structure of a Symbiosis Between a Coral Reef Fish and Luminous Bacterium

All organisms depend on symbiotic associations with bacteria for their success, yet how these interspecific interactions influence the population structure, ecology, and evolution of microbial symbionts is not well understood. Additionally, patterns of genetic variation in interacting species can reveal ecological traits that are important to gene flow and co-evolution. In this study, we define patterns of spatial and temporal genetic variation of a coral reef fish, Siphamia tubifer, and its luminous bacterial symbiont, Photobacterium mandapamensis in the Okinawa Islands, Japan. Using restriction site-associated sequencing (RAD-Seq) methods, we show that populations of the facultative light organ symbiont of S. tubifer exhibit genetic structure at fine spatial scales of tens of kilometers despite the absence of physical barriers to dispersal and in contrast to populations of the host fish. These results suggest that the host’s behavioral ecology and environmental interactions between host and symbiont help to structure symbiont populations in the region, consequently fostering the specificity of the association between host generations. Our approach also revealed several symbiont genes that were divergent between host populations, including hfq and a homolog of varS, both of which play a role in host association in Vibrio cholerae. Overall, this study highlights the important role that a host animal can play in structuring the distribution of its bacterial symbiont, particularly in highly connected marine environments, thereby promoting specificity of the symbiosis between host generations.