Nucleoside uptake in Vibrio cholerae and its role in the transition fitness from host to environment

Regulatory mechanisms of two-component systems in Vibrio cholerae: Enhancing pathogenicity and environmental adaptation

Cholera, which is caused by the bacterium Vibrio cholerae, is a highly dangerous disease characterized by severe symptoms such as watery diarrhea, dehydration, and even death. V. cholerae can both colonize the host intestine and survive in environmental reservoirs. Two-component systems (TCSs) are essential regulatory mechanisms that allow bacteria to adapt to changing environments. This review focuses on the regulatory mechanisms of TCS-mediated gene expression in V. cholerae. We first summarize the composition and classification of TCSs in V. cholerae N16961. We then discuss the roles of TCSs in facilitating adaptation to diverse environmental stimuli and increasing pathogenicity. Furthermore, we analyze the distribution of TCSs in pandemic and nonpandemic-V. cholerae strains, demonstrating their indispensable role in promoting virulence and facilitating the widespread dissemination of pandemic strains. Elucidation of these mechanisms is crucial for devising new strategies to combat cholera and prevent future outbreaks, ultimately contributing to improved public health outcomes.

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.

Multitasking functions of bacterial extracellular DNA in biofilms

Bacterial biofilms are intricate ecosystems of microbial communities that adhere to various surfaces and are enveloped by an extracellular matrix composed of polymeric substances. Within the context of bacterial biofilms, extracellular DNA (eDNA) originates from cell lysis or is actively secreted, where it exerts a significant influence on the formation, stability, and resistance of biofilms to environmental stressors. The exploration of eDNA within bacterial biofilms holds paramount importance in research, with far-reaching implications for both human health and the environment. An enhanced understanding of the functions of eDNA in biofilm formation and antibiotic resistance could inspire the development of strategies to combat biofilm-related infections and improve the management of antibiotic resistance. This comprehensive review encapsulates the latest discoveries concerning eDNA, encompassing its origins, functions within bacterial biofilms, and significance in bacterial pathogenesis.

An atypical GdpP enzyme linking cyclic nucleotide metabolism to osmotic tolerance and gene regulation in Mycoplasma bovis

Nucleotide second messengers play an important role in bacterial adaptation to environmental changes. Recent evidence suggests that some of these regulatory molecular pathways were conserved upon the degenerative evolution of the wall-less mycoplasmas. We have recently reported the occurrence of a phosphodiesterase (PDE) in the ruminant pathogen Mycoplasma bovis, which was involved in c-di-AMP metabolism. In the present study, we demonstrate that the genome of this mycoplasma species encodes a PDE of the GdpP family with atypical DHH domains. Characterization of M. bovis GdpP (MbovGdpP) revealed a multifunctional PDE with unusual nanoRNase and single-stranded DNase activities. The alarmone ppGpp was found unable to inhibit c-di-NMP degradation by MbovGdpP but efficiently blocked its nanoRNase activity. Remarkably, MbovGdpP was found critical for the osmotic tolerance of M. bovis under K+ and Na+ conditions. Transcriptomic analyses further revealed the biological importance of MbovGdpP in tRNA biosynthesis, pyruvate metabolism, and several steps in genetic information processing. This study is an important step in understanding the role of PDE and nucleotide second messengers in the biology of a minimal bacterial pathogen.

VxrB Influences Antagonism within Biofilms by Controlling Competition through Extracellular Matrix Production and Type 6 Secretion

The human pathogen Vibrio cholerae grows as biofilms, communities of cells encased in an extracellular matrix. When growing in biofilms, cells compete for resources and space. One common competitive mechanism among Gram-negative bacteria is the type six secretion system (T6SS), which can deliver toxic effector proteins into a diverse group of target cells, including other bacteria, phagocytic amoebas, and human macrophages. The response regulator VxrB positively regulates both biofilm matrix and T6SS gene expression. Here, we directly observe T6SS activity within biofilms, which results in improved competition with strains lacking the T6SS. VxrB significantly contributes to both attack and defense via T6SS, while also influencing competition via regulation of biofilm matrix production. We further determined that both Vibrio polysaccharide (VPS) and the biofilm matrix protein RbmA can protect cells from T6SS attack within mature biofilms. By varying the spatial mixing of predator and prey cells in biofilms, we show that a high degree of mixing favors T6SS predator strains and that the presence of extracellular DNA in V. cholerae biofilms is a signature of T6SS killing. VxrB therefore regulates both T6SS attack and matrix-based T6SS defense, to control antagonistic interactions and competition outcomes during mixed-strain biofilm formation.

Mechanisms Underlying Vibrio cholerae Biofilm Formation and Dispersion

Biofilms are a widely observed growth mode in which microbial communities are spatially structured and embedded in a polymeric extracellular matrix. Here, we focus on the model bacterium Vibrio cholerae and summarize the current understanding of biofilm formation, including initial attachment, matrix components, community dynamics, social interactions, molecular regulation, and dispersal. The regulatory network that orchestrates the decision to form and disperse from biofilms coordinates various environmental inputs. These cues are integrated by several transcription factors, regulatory RNAs, and second-messenger molecules, including bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP). Through complex mechanisms, V. cholerae weighs the energetic cost of forming biofilms against the benefits of protection and social interaction that biofilms provide. 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.

Physicochemical and Biological Insights Into the Molecular Interactions Between Extracellular DNA and Exopolysaccharides in Myxococcus xanthus Biofilms

Extracellular DNA (eDNA) is a critical component in the extracellular matrix (ECM) of bacterial biofilms, while little is known about the mechanisms underlying how eDNA integrates into the ECM through potential macromolecular interactions. Myxococcus xanthus biofilm was employed as a suitable model for the investigation due to the co-distribution of eDNA and exopolysaccharides (EPS) owing to their direct interactions in the ECM. DNA is able to combine with M. xanthus EPS to form a macromolecular conjugate, which is dominated by the electrostatic forces participating in the polymer-polymer interactions. Without intercalation binding, DNA-EPS interactions exhibit a certain degree of reversibility. Acting as a strong extracellular framework during biofilm formation process, the eDNA-EPS complex not only facilitates the initial cell adhesion and subsequent establishment of ECM architecture, but also renders cells within biofilms stress resistances that are relevant to the survival of M. xanthus in some hostile environments. Furthermore, the EPS protects the conjugated DNA from the degradation by nucleic acid hydrolases, which leads to the continuous and stable existence of eDNA in the native ECM of M. xanthus biofilms. These results will shed light on developing prevention and treatment strategies against biofilm-related risks.

Nucleotide biosynthesis: the base of bacterial pathogenesis

Most free-living organisms require the synthesis and/or acquisition of purines and pyrimidines, which form the basis of nucleotides, to survive. In most bacteria, the nucleotides are synthesized de novo and the products are used in many cell functions, including DNA replication, energy storage, and as signaling molecules. Due to their central role in the metabolism of bacteria, both nucleotide biosynthesis pathways have strong links with the virulence of opportunistic and bona fide bacterial pathogens. Recent findings have established a new, shared link in the control of nucleotide biosynthesis and the production of virulence factors. Furthermore, targeting of these pathways forms the basis of interspecies competition and can provide an open source for new antimicrobial compounds. Here, we highlight the contribution of nucleotide biosynthesis to bacterial pathogenesis in a plethora of different diseases and speculate on how they can be targeted by intervention strategies.

A fructose/H+ symporter controlled by a LacI-type regulator promotes survival of pandemic Vibrio cholerae in seawater

The bacterium Vibrio cholerae can colonize the human intestine and cause cholera, but spends much of its life cycle in seawater. The pathogen must adapt to substantial environmental changes when moving between seawater and the human intestine, including different availability of carbon sources such as fructose. Here, we use in vitro experiments as well as mouse intestinal colonization assays to study the mechanisms used by pandemic V. cholerae to adapt to these environmental changes. We show that a LacI-type regulator (FruI) and a fructose/H+ symporter (FruT) are important for fructose uptake at low fructose concentrations, as those found in seawater. FruT is downregulated by FruI, which is upregulated when O2 concentrations are low (as in the intestine) by ArcAB, a two-component system known to respond to changes in oxygen levels. As a result, the bacteria predominantly use FruT for fructose uptake under seawater conditions (low fructose, high O2), and use a known fructose phosphotransferase system (PTS, Fpr) for fructose uptake under conditions found in the intestine. PTS activity leads to reduced levels of intracellular cAMP, which in turn upregulate virulence genes. Our results indicate that the FruT/FruI system may be important for survival of pandemic V. cholerae in seawater.

Evolution of Bacterial Cross-Resistance to Lytic Phages and Albicidin Antibiotic

Due to concerns over the global increase of antibiotic-resistant bacteria, alternative antibacterial strategies, such as phage therapy, are increasingly being considered. However, evolution of bacterial resistance to new therapeutics is almost a certainty; indeed, it is possible that resistance to alternative treatments might result in an evolved trade-up such as enhanced antibiotic resistance. Here, we hypothesize that selection for Escherichia coli bacteria to resist phage T6, phage U115, or albicidin, a DNA gyrase inhibitor, should often result in a pleiotropic trade-up in the form of cross-resistance, because all three antibacterial agents interact with the Tsx porin. Selection imposed by any one of the antibacterials resulted in cross-resistance to all three of them, in each of the 29 spontaneous bacterial mutants examined in this study. Furthermore, cross-resistance did not cause measurable fitness (growth) deficiencies for any of the bacterial mutants, when competed against wild-type E. coli in both low-resource and high-resource environments. A combination of whole-genome and targeted sequencing confirmed that mutants differed from wild-type E. coli via change(s) in the tsx gene. Our results indicate that evolution of cross-resistance occurs frequently in E. coli subjected to independent selection by phage T6, phage U115 or albicidin. This study cautions that deployment of new antibacterial therapies such as phage therapy, should be preceded by a thorough investigation of evolutionary consequences of the treatment, to avoid the potential for evolved trade-ups.

Biofilms by bacterial human pathogens: Clinical relevance - development, composition and regulation - therapeutical strategies

Notably, bacterial biofilm formation is increasingly recognized as a passive virulence factor facilitating many infectious disease processes. In this review we will focus on bacterial biofilms formed by human pathogens and highlight their relevance for diverse diseases. Along biofilm composition and regulation emphasis is laid on the intensively studied biofilms of Vibrio cholerae, Pseudomonas aeruginosa and Staphylococcus spp., which are commonly used as biofilm model organisms and therefore contribute to our general understanding of bacterial biofilm (patho-)physiology. Finally, therapeutical intervention strategies targeting biofilms will be discussed.

RNA-mediated control of cell shape modulates antibiotic resistance in Vibrio cholerae

Vibrio cholerae, the cause of cholera disease, exhibits a characteristic curved rod morphology, which promotes infectivity and motility in dense hydrogels. Periplasmic protein CrvA determines cell curvature in V. cholerae, yet the regulatory factors controlling CrvA are unknown. Here, we discover the VadR small RNA (sRNA) as a post-transcriptional inhibitor of the crvA mRNA. Mutation of vadR increases cell curvature, whereas overexpression has the inverse effect. We show that vadR transcription is activated by the VxrAB two-component system and triggered by cell-wall-targeting antibiotics. V. cholerae cells failing to repress crvA by VadR display decreased survival upon challenge with penicillin G indicating that cell shape maintenance by the sRNA is critical for antibiotic resistance. VadR also blocks the expression of various key biofilm genes and thereby inhibits biofilm formation in V. cholerae. Thus, VadR is an important regulator for synchronizing peptidoglycan integrity, cell shape, and biofilm formation in V. cholerae.

Staphylococcus aureus Preferentially Liberates Inorganic Phosphate from Organophosphates in Environments where This Nutrient Is Limiting

Phosphate is an essential nutrient that Staphylococcus aureus and other pathogens must acquire from the host during infection. While inorganic monophosphate (P_i) is the preferred source of this nutrient, bacteria can also obtain it from phosphate-containing organic molecules. The P_i-responsive regulator PhoPR is necessary for S. aureus to cause infection, suggesting that P_i is not freely available during infection and that this nutrient must be obtained from other sources. However, the organophosphates from which S. aureus can obtain phosphate are unknown. We evaluated the ability of 58 phosphorus-containing molecules to serve as phosphate sources for S. aureus Forty-six of these compounds, including phosphorylated amino acids, sugars, and nucleotides, supported growth. Among the organophosphate sources was glycerol-3-phosphate (G3P), which is commonly found in the mammalian host. Differing from the model organism Escherichia coli, S. aureus does not import G3P intact to obtain P_i Instead, S. aureus relies on the phosphatase PhoB to release P_i from G3P, which is subsequently imported by P_i transporters. To determine if this strategy is used by S. aureus to extract phosphate from other phosphate sources, we assessed the ability of PhoB- and P_i transporter-deficient strains to grow on the same library of phosphorus-containing molecules. Sixty percent of the substrates (28/46) relied on the PhoB/P_i transporter pathway, and an additional 10/46 (22%) were PhoB independent but still required P_i transport through the P_i transporters. Cumulatively, these results suggest that in P_i-limited environments, S. aureus preferentially generates P_i from organophosphates and then relies on P_i transporters to import this nutrient.IMPORTANCE For bacteria, the preferred form of the essential nutrient phosphate is inorganic monophosphate (P_i), but phosphate can also be extracted from a variety of phosphocompounds. Pathogens, including Staphylococcus aureus, experience P_i limitation within the host, suggesting that the use of alternative phosphate sources is important during infection. However, the alternative phosphate sources that can be used by S. aureus and others remain largely unexplored. We screened a library of phosphorus-containing compounds for the ability to support growth as a phosphate source. S. aureus could use a variety of phosphocompounds, including nucleotides, phosphosugars, and phosphoamino acids. Subsequent genetic analysis determined that a majority of these alternative phosphate sources are first processed extracellularly to liberate P_i, which is then imported through P_i transporters.

The Effect of the Environmental Temperature on the Adaptation to Host in the Zoonotic Pathogen Vibrio vulnificus

Vibrio vulnificus is a zoonotic pathogen that lives in temperate, tropical and subtropical aquatic ecosystems whose geographical distribution is expanding due to global warming. The species is genetically variable and only the strains that belong to the zoonotic clonal-complex can cause vibriosis in both humans and fish (being its main host the eel). Interestingly, the severity of the vibriosis in the eel and the human depends largely on the water temperature (highly virulent at 28°C, avirulent at 20°C or below) and on the iron content in the blood, respectively. The objective of this work was to unravel the role of temperature in the adaptation to the host through a transcriptomic and phenotypic approach. To this end, we obtained the transcriptome of a zoonotic strain grown in a minimum medium (CM9) at 20, 25, 28, and 37°C, and confirmed the transcriptomic results by RT-qPCR and phenotypic tests. In addition, we compared the temperature stimulon with those previously obtained for iron and serum (from eel and human, respectively). Our results suggest that warm temperatures activate adaptive traits that would prepare the bacteria for host colonization (metabolism, motility, chemotaxis, and the protease activity) and fish septicemia (iron-uptake from transferrin and production of O-antigen of high molecular weight) in a generalized manner, while environmental iron controls the expression of a host-adapted virulent phenotype (toxins and the production of a protective envelope). Finally, our results confirm that beyond the effect of temperature on the V. vulnificus distribution in the environment, it also has an effect on the infectious capability of this pathogen that must be taken into account to predict the real risk of V. vulnificus infection caused by global warming.

The Future of Origin of Life Research: Bridging Decades-Old Divisions

Research on the origin of life is highly heterogeneous. After a peculiar historical development, it still includes strongly opposed views which potentially hinder progress. In the 1st Interdisciplinary Origin of Life Meeting, early-career researchers gathered to explore the commonalities between theories and approaches, critical divergence points, and expectations for the future. We find that even though classical approaches and theories-e.g. bottom-up and top-down, RNA world vs. metabolism-first-have been prevalent in origin of life research, they are ceasing to be mutually exclusive and they can and should feed integrating approaches. Here we focus on pressing questions and recent developments that bridge the classical disciplines and approaches, and highlight expectations for future endeavours in origin of life research.

Outer Membrane Vesiculation Facilitates Surface Exchange and In Vivo Adaptation of Vibrio cholerae

Gram-negative bacteria release outer membrane vesicles into the external milieu to deliver effector molecules that alter the host and facilitate virulence. Vesicle formation is driven by phospholipid accumulation in the outer membrane and regulated by the phospholipid transporter VacJ/Yrb. We use the facultative human pathogen Vibrio cholerae to show that VacJ/Yrb is silenced early during mammalian infection, which stimulates vesiculation that expedites bacterial surface exchange and adaptation to the host environment. Hypervesiculating strains rapidly alter their bacterial membrane composition and exhibit enhanced intestinal colonization fitness. This adaptation is exemplified by faster accumulation of glycine-modified lipopolysaccharide (LPS) and depletion of outer membrane porin OmpT, which confers resistance to host-derived antimicrobial peptides and bile, respectively. The competitive advantage of hypervesiculation is lost upon pre-adaptation to bile and antimicrobial peptides, indicating the importance of these adaptive processes. Thus, bacteria use outer membrane vesiculation to exchange cell surface components, thereby increasing survival during mammalian infection.

Characterization of Vibrio cholerae's Extracellular Nuclease Xds

The Gram-negative bacterium Vibrio cholerae encodes two nucleases, Dns and Xds, which play a major role during the human pathogen's lifecycle. Dns and Xds control three-dimensional biofilm formation and bacterial detachment from biofilms via degradation of extracellular DNA and thus contribute to the environmental, inter-epidemic persistence of the pathogen. During intestinal colonization the enzymes help evade the innate immune response, and therefore promote survival by mediating escape from neutrophil extracellular traps. Xds has the additional function of degrading extracellular DNA down to nucleotides, which are an important nutrient source for V. cholerae. Thus, Xds is a key enzyme for survival fitness during distinct stages of the V. cholerae lifecycle and could be a potential therapeutic target. This study provides detailed information about the enzymatic properties of Xds using purified protein in combination with a real time nuclease activity assay. The data define an optimal buffer composition for Xds activity as 50 mM Tris/HCl pH 7, 100 mM NaCl, 10 mM MgCl2, and 20 mM CaCl2. Moreover, maximal activity was observed using substrate DNA with low GC content and ambient temperatures of 20-25°C. In silico analysis and homology modeling predicted an exonuclease domain in the C-terminal part of the protein. Biochemical analyses with truncated variants and point mutants of Xds confirm that the C-terminal region is sufficient for nuclease activity. We also find that residues D787 and H837 within the predicted exonuclease domain are key to formation of the catalytic center.

Transcriptome changes in response to temperature in the fish pathogen Photobacterium damselae subsp. damselae: Clues to understand the emergence of disease outbreaks at increased seawater temperatures

The marine bacterium Photobacterium damselae subsp. damselae (Pdd) is a generalist and facultative pathogen that causes disease in a wide range of marine animals including fish species of importance in aquaculture. Disease outbreaks in fish farms have been correlated with an increased water temperature during summer months. In this study, we have used RNA sequencing to analyze the transcriptome of Pdd RM-71 cultured at two different temperatures, which simulated temperature conditions experienced during free swimming lifestyle at mid latitudes in winter months (15°C) and during outbreaks in aquaculture in warm summer months (25°C). The enhanced bacterial growth of Pdd observed at 25°C in comparison to 15°C suggests that an elevated seawater temperature contributes to the build-up of a sufficient bacterial population to cause disease. In comparison to growth at 15°C, growth at 25°C resulted in the upregulation of genes involved in DNA synthesis, nutrient uptake, chemotaxis, flagellar motility, secretion systems and antimicrobial resistance. Plasmid-encoded virulence factors, which include a putative adhesin/invasin OmpU, a transferrin receptor and a serum resistance protein, were also upregulated. Transcription factor RpoS, genes involved in cold shock response, modulation of cell envelope and amino acid metabolism, as well as genes of yet unknown function were downregulated at 25°C. Notably, the gene encoding damselysin cytotoxin (Dly) was among the most highly transcribed genes at the two assayed temperatures, at levels comparable to the most highly expressed housekeeping genes. This study contributes to our understanding of the regulatory networks and biology of a generalist marine bacterial pathogen, and provides evidence that temperature regulates multiple physiological and virulence-related functions in Pdd.

NtrC Adds a New Node to the Complex Regulatory Network of Biofilm Formation and vps Expression in Vibrio cholerae

The biofilm growth mode is important in both the intestinal and environmental phases of the Vibrio cholerae life cycle. Regulation of biofilm formation involves several transcriptional regulators and alternative sigma factors. One such factor is the alternative sigma factor RpoN, which positively regulates biofilm formation. RpoN requires bacterial enhancer-binding proteins (bEBPs) to initiate transcription. The V. cholerae genome encodes seven bEBPs (LuxO, VC1522, VC1926 [DctD-1], FlrC, NtrC, VCA0142 [DctD-2], and PgtA) that belong to the NtrC family of response regulators (RRs) of two-component regulatory systems. The contribution of these regulators to biofilm formation is not well understood. In this study, we analyzed biofilm formation and the regulation of vpsL expression by RpoN activators. Mutants lacking NtrC had increased biofilm formation and vpsL expression. NtrC negatively regulates the expression of core regulators of biofilm formation (vpsR, vpsT, and hapR). NtrC from V. cholerae supported growth and activated glnA expression when nitrogen availability was limited. However, the repressive activity of NtrC toward vpsL expression was not affected by the nitrogen sources present. This study unveils the role of NtrC as a regulator of vps expression and biofilm formation in V. choleraeIMPORTANCE Biofilms play an important role in the Vibrio cholerae life cycle, contributing to both environmental survival and transmission to a human host. Identifying key regulators of V. cholerae biofilm formation is necessary to fully understand how this important growth mode is modulated in response to various signals encountered in the environment and the host. In this study, we characterized the role of RRs that function as coactivators of RpoN in regulating biofilm formation and identified new components in the V. cholerae biofilm regulatory circuitry.

AAA+ proteases and their role in distinct stages along the Vibrio cholerae lifecycle

The facultative human pathogen Vibrio cholerae has to adapt to different environmental conditions along its lifecycle by means of transcriptional, translational and post-translational regulation. This study provides a first comprehensive analysis regarding the contribution of the cytoplasmic AAA+ proteases Lon, ClpP and HslV to distinct features of V. cholerae behaviour, including biofilm formation, motility, cholera toxin expression and colonization fitness in the mouse model. While absence of HslV did not yield to any altered phenotype compared to wildtype, absence of Lon or ClpP resulted in significantly reduced colonization in vivo. In addition, a Δlon deletion mutant showed altered biofilm formation and increased motility, which could be correlated with higher expression of V. cholerae flagella gene class IV. Concordantly, we could show by immunoblot analysis, that Lon is the main protease responsible for proteolytic control of FliA, which is required for class IV flagella gene transcription, but also downregulates virulence gene expression. FliA becomes highly sensitive to proteolytic degradation in absence of its anti-sigma factor FlgM, a scenario reported to occur during mucosal penetration due to FlgM secretion through the broken flagellum. Our results confirm that the high stability of FliA in the absence of Lon results in less cholera toxin and toxin corgulated pilus production under virulence gene inducing conditions and in the presence of a damaged flagellum. Thus, the data presented herein provide a molecular explanation on how V. cholerae can achieve full expression of virulence genes during early stages of colonization, despite FliA getting liberated from the anti-sigma factor FlgM.

Multifaceted roles of extracellular DNA in bacterial physiology

In textbooks, DNA is generally defined as the universal storage material for genetic information in all branches of life. Beyond this important intracellular role, DNA can also be present outside of living cells and is an abundant biopolymer in aquatic and terrestrial ecosystems. The origin of extracellular DNA in such ecological niches is diverse: it can be actively secreted or released by prokaryotic and eukaryotic cells by means of autolysis, apoptosis, necrosis, bacterial secretion systems or found in association with extracellular bacterial membrane vesicles. Especially for bacteria, extracellular DNA represents a significant and convenient element that can be enzymatically modulated and utilized for multiple purposes. Herein, we discuss briefly the main origins of extracellular DNA and the most relevant roles for the bacterial physiology, such as biofilm formation, nutrient source, antimicrobial means and horizontal gene transfer.

CytR Is a Global Positive Regulator of Competence, Type VI Secretion, and Chitinases in Vibrio cholerae

The facultative pathogen Vibrio cholerae transitions between its human host and aquatic reservoirs where it colonizes chitinous surfaces. Growth on chitin induces expression of chitin utilization genes, genes involved in DNA uptake by natural transformation, and a type VI secretion system that allows contact-dependent killing of neighboring bacteria. We have previously shown that the transcription factor CytR, thought to primarily regulate the pyrimidine nucleoside scavenging response, is required for natural competence in V. cholerae. Through high-throughput RNA sequencing (RNA-seq), we show that CytR positively regulates the majority of competence genes, the three type VI secretion operons, and the four known or predicted chitinases. We used transcriptional reporters and phenotypic analysis to determine the individual contributions of quorum sensing, which is controlled by the transcription factors HapR and QstR; chitin utilization that is mediated by TfoX; and pyrimidine starvation that is orchestrated by CytR, toward each of these processes. We find that in V. cholerae, CytR is a global regulator of multiple behaviors affecting fitness and adaptability in the environment.