Structural basis for virulence regulation in Vibrio cholerae by unsaturated fatty acid components of bile

Sensing for survival: specialised regulatory mechanisms of Type III secretion systems in Gram-negative pathogens

For centuries, Gram-negative pathogens have infected the human population and been responsible for numerous diseases in animals and plants. Despite advancements in therapeutics, Gram-negative pathogens continue to evolve, with some having developed multi-drug resistant phenotypes. For the successful control of infections caused by these bacteria, we need to widen our understanding of the mechanisms of host-pathogen interactions. Gram-negative pathogens utilise an array of effector proteins to hijack the host system to survive within the host environment. These proteins are secreted into the host system via various secretion systems, including the integral Type III secretion system (T3SS). The T3SS spans two bacterial membranes and one host membrane to deliver effector proteins (virulence factors) into the host cell. This multifaceted process has multiple layers of regulation and various checkpoints. In this review, we highlight the multiple strategies adopted by these pathogens to regulate or maintain virulence via the T3SS, encompassing the regulation of small molecules to sense and communicate with the host system, as well as master regulators, gatekeepers, chaperones, and other effectors that recognise successful host contact. Further, we discuss the regulatory links between the T3SS and other systems, like flagella and metabolic pathways including the tricarboxylic acid (TCA) cycle, anaerobic metabolism, and stringent cell response.

HilE represses the activity of the Salmonella virulence regulator HilD via a mechanism distinct from that of intestinal long-chain fatty acids

The expression of virulence factors essential for the invasion of host cells by Salmonella enterica is tightly controlled by a network of transcription regulators. The AraC/XylS transcription factor HilD is the main integration point of environmental signals into this regulatory network, with many factors affecting HilD activity. Long-chain fatty acids, which are highly abundant throughout the host intestine, directly bind to and repress HilD, acting as environmental cues to coordinate virulence gene expression. The regulatory protein HilE also negatively regulates HilD activity, through a protein-protein interaction. Both of these regulators inhibit HilD dimerization, preventing HilD from binding to target DNA. We investigated the structural basis of these mechanisms of HilD repression. Long-chain fatty acids bind to a conserved pocket in HilD, in a comparable manner to that reported for other AraC/XylS regulators, whereas HilE forms a stable heterodimer with HilD by binding to the HilD dimerization interface. Our results highlight two distinct, mutually exclusive mechanisms by which HilD activity is repressed, which could be exploited for the development of new antivirulence leads.

Shigella flexneri utilizes intestinal signals to control its virulence

The enteric pathogens have evolved to utilize elements from their surroundings to optimize their infection strategies. A common mechanism to achieve this is to employ intestinal compounds as signals to control the activity of a master regulator of virulence. Shigella flexneri (S. flexneri) is a highly infectious entero-invasive pathogen which requires very few organisms to cause invasion of the colonic mucosa. The invasion program is controlled by the virulence master regulator VirF. Here, we show that the fatty acids commonly found in the colon can be exploited by S. flexneri to repress its virulence, allowing it to energetically finance its proliferation, thus increasing its pathogenicity. Colonic fatty acids such as oleic, palmitoleic and cis-2-hexadecenoic acid were shown to directly bind to VirF and mediate its prompt degradation. These fatty acids also disrupted the ability of VirF to bind to its target DNA, suppressing the transcription of the downstream virulence genes and significantly reducing the invasion of S. flexneri to colonic epithelial cells. Treatment with colonic fatty acids significantly increased the growth rate of the pathogen only under invasion-inducing conditions, showing that the reduction in the burden of virulence promotes a growth advantage. These results demonstrate the process by which S. flexneri can employ intestinal compounds as signals to increase its numbers at its preferred site of invasion, highlighting the mechanism by which the full spectrum of shigellosis is achieved despite a miniscule infectious dose. This highlights an elegant model of environmental adaption by S. flexneri to maximize the pathogenic benefit.

Insight into novel inhibitors from Sterculia urens against Cholera via pharmacoinformatics and molecular dynamics simulation approaches

The underdeveloped countries with large populations are facing a grave global threat in the form of cholera. Vibrio cholerae, the etiologic agent of Cholera has drawn attention recently due to antimicrobial resistance and resulting outbreaks that necessitates establishment of novel medications to counteract virulence and viability of the pathogen. Sterculia urens Roxb. (Malvaceae) is an ethnomedicinally important tree, which harbors a good number of bioactive phytocompounds. In the present study, 53 phytocompounds of S. urens were screened against the promising target ToxT of V. cholerae employing structure-based drug design approach that revealed three lead compounds, viz., 4,4,5,8-Tetramethylchroman-2-ol (-8.2 kcal/mol), Beta-Bisabolol (-8.2 kcal/mol) and Ledol (-8.7 kcal/mol) with satisfactory ADMET properties. Molecular dynamics simulation for 150 ns unveiled notable compactness and structural stability for the lead compounds considering RMSD, RMSF, Rg, MolSA, PSA and protein-ligand contacts parameters. Molecular mechanics-based MM/GBSA binding energy calculation revealed Beta-Bisabolol (-66.74 kcal/mol) to have better scores than 4,4,5,8-Tetramethylchroman-2-ol (-47.42 kcal/mol) and Ledol (-65.79 kcal/mol). Enzymes were mostly found as drug target class, and Nabilone was found as a structurally similar analog for 4,4,5,8-Tetramethylchroman-2-ol. These discoveries could aid in revealing new antibacterial medications targeting ToxT to combat Cholera.Communicated by Ramaswamy H. Sarma.

Expression of Cholera Toxin (CT) and the Toxin Co-Regulated Pilus (TCP) by Variants of ToxT in Vibrio cholerae Strains

The expression of the two major virulence genes of Vibrio cholerae-tcpA (the major subunit of the toxin co-regulated pilus) and ctxAB (cholera toxin)-is regulated by the ToxR regulon, which is triggered by environmental stimuli during infection within the human small intestine. Special culture methods are required to induce the expression of virulence genes in V. cholerae in the laboratory setting. In the present study, induction of the expression of virulence genes by two point mutations (65th and 139th amino acids) in toxT, which is produced by the ToxR regulon and activates the transcription of the virulence genes in V. cholerae, under laboratory culture conditions has been investigated. Each of the four toxT alleles assessed displayed different transcriptional activator functions in a given V. cholerae strain. Although the ToxR regulon has been known to not be expressed by El Tor biotype V. cholerae strains cultured under standard laboratory conditions, the variant toxT alleles that we assessed in this study enabled the expression virulence genes in El Tor biotype strains grown under simple culture conditions comprising shake culture in LB medium, suggesting that the regulation of virulence gene expression may be regulated more complexly than previously thought and may involve additional factors beyond the production of ToxT by the ToxR regulon.

Fatty Acids Abolish Shigella Virulence by Inhibiting Its Master Regulator, VirF

The pathogenicity of Shigella, the intracellular pathogen responsible for human bacillary dysentery, depends on a coordinated and tightly regulated expression of its virulence determinants. This is the result of a cascade organization of its positive regulators, with VirF, a transcriptional activator belonging to the AraC-XylS family, in a pivotal position. VirF itself is submitted to several well-known regulations at the transcriptional level. In this work, we present evidence for a novel posttranslational regulatory mechanism of VirF mediated by the inhibitory interaction with specific fatty acids. By homology modeling and molecular docking analyses, we identify a jelly roll motif in the structure of ViF capable of interacting with medium-chain saturated and long-chain unsaturated fatty acids. In vitro and in vivo assays show that capric, lauric, myristoleic, palmitoleic, and sapienic acids interact effectively with the VirF protein, abolishing its transcription-promoting activity. This silences the virulence system of Shigella, leading to a drastic reduction in its ability to invade epithelial cells and proliferate in their cytoplasm. IMPORTANCE In the absence of a valid vaccine, the main therapeutic approach currently used to treat shigellosis is based on the use of antibiotics. The emergence of antibiotic resistance jeopardizes the future effectiveness of this approach. The importance of the present work resides both in the identification of a new level of posttranslational regulation of the Shigella virulence system and in the characterization of a mechanism offering new opportunities for the design of antivirulence compounds, which may change the treatment paradigm of Shigella infections by limiting the emergence of antibiotic-resistant bacteria.

Salmonella Invasion Is Controlled by Competition among Intestinal Chemical Signals

The intestine is a complex, ever-changing environment replete with an array of signaling molecules. To colonize such a complex organ, pathogens have adapted to utilize specific cues from the local environment to intricately regulate the expression of their virulence determinants. Salmonella preferentially colonizes the distal ileum, a niche enriched in the metabolite formic acid. Here, we show that the relatively higher concentration of this metabolite in the distal ileum prevents other signals from repressing Salmonella invasion in that region. We show that imported and unmetabolized formic acid functions as a cytoplasmic signal that competitively binds to HilD, the master transcriptional regulator of Salmonella invasion, thus preventing repressive fatty acids from binding to the protein. This results in an increased lifetime of HilD and subsequent derepression of invasion genes. This study demonstrates an important mechanism by which Salmonella utilizes competition among signals in the gut to its advantage as a pathogen. IMPORTANCE Enteric pathogens acutely sense their environment for signals to regulate their virulence functions. We demonstrate here that the enteric pathogen Salmonella utilizes the competition among certain regional intestinal constituents to modulate its virulence determinants in that region. We show that the high concentration of formic acid in the ileum outcompetes other signals and triggers the activation of virulence genes in the ileum. This study shows a delicate spatial and temporal mechanism by which enteric pathogens may utilize the competition among environmental cues to optimize their pathogenicity.

Elucidation of the DNA-Binding Activity of VirF from Shigella flexneri for the icsA and rnaG Promoters and Characterization of the N-Terminal Domain To Identify Residues Crucial for Dimerization

A novel approach to treat the highly virulent and infectious enteric pathogen Shigella flexneri, with the potential for reduced resistance development, is to target virulence pathways. One promising such target is the AraC-family transcription factor VirF, which activates downstream virulence factors. VirF harbors a conserved C-terminal DNA-binding domain (DBD) and an N-terminal dimerization domain (NTD). Previously, we studied the wild type (WT) and seven alanine DBD mutants of VirF binding to the virB promoter (N. J. Ragazzone, G. T. Dow, and A. Garcia, J Bacteriol 204:e00143-22, 2022, https://doi.org/10.1128/jb.00143-22). Here, we report studies of VirF binding to the icsA and rnaG promoters. Gel shift assays (electrophoretic mobility shift assays [EMSAs]) of WT VirF binding to these promoters revealed multiple bands at higher apparent molecular weights, indicating the likelihood of VirF dimerization when bound to DNA. For three of the mutants, we observed consistent effects on binding to the three promoters. For the four other mutants, we observed differential effects on promoter binding. Results of a cell-based, LexA monohybrid β-galactosidase reporter assay [D. A. Daines, M. Granger-Schnarr, M. Dimitrova, and R. P. Silver, Methods Enzymol 358:153-161, 2002, https://doi.org/10.1016/s0076-6879(02)58087-3] indicated that WT VirF dimerizes in vivo and that alanine mutations to Y132, L137, and L147 significantly reduced dimerization. However, these mutations negatively impacted protein stability. We did purify enough of the Y132A mutant to determine that it binds in vitro to the virB and rnaG promoters, albeit with weaker affinities. Full-length VirF model structures, generated with I-TASSER, predict that these three amino acids are in a "dimerization" helix in the NTD, consistent with our results. IMPORTANCE Antimicrobial-resistant (AMR) infections continue to rise dramatically, and the lack of new approved antibiotics is not ameliorating this crisis. A promising route to reduce AMR is by targeting virulence. The Shigella flexneri virulence pathway is a valuable source for potential therapeutic targets useful to treat this infection. VirF, an AraC-family virulence transcription factor, is responsible for activating necessary downstream virulence genes that allow the bacteria to invade and spread within the human colon. Previous studies have identified how VirF interacts with the virB promoter and have even developed a lead DNA-binding inhibitor, but not much is known about VirF dimerization or binding to the icsA and rnaG promoters. Fully characterizing VirF can be a valuable resource for inhibitor discovery/design.

Structural Insights into Regulation of Vibrio Virulence Gene Networks

One of the best studied aspects of pathogenic Vibrios are the virulence cascades that lead to the production of virulence factors and, ultimately, clinical outcomes. In this chapter, we will examine the regulation of Vibrio virulence gene networks from a structural and biochemical perspective. We will discuss the recent research into the numerous proteins that contribute to regulating virulence in Vibrio spp such as quorum sensing regulator HapR, the transcription factors AphA and AphB, or the virulence regulators ToxR and ToxT. We highlight how insights gained from these studies are already illuminating the basic molecular mechanisms by which the virulence cascade of pathogenic Vibrios unfold and contend that understanding how protein interactions contribute to the host-pathogen communications will enable the development of new antivirulence compounds that can effectively target these pathogens.

Anti-infective bile acids bind and inactivate a Salmonella virulence regulator

Bile acids are prominent host and microbiota metabolites that modulate host immunity and microbial pathogenesis. However, the mechanisms by which bile acids suppress microbial virulence are not clear. To identify the direct protein targets of bile acids in bacterial pathogens, we performed activity-guided chemical proteomic studies. In Salmonella enterica serovar Typhimurium, chenodeoxycholic acid (CDCA) most effectively inhibited the expression of virulence genes and invasion of epithelial cells and interacted with many proteins. Notably, we discovered that CDCA can directly bind and inhibit the function of HilD, an important transcriptional regulator of S. Typhimurium virulence and pathogenesis. Our characterization of bile acid-resistant HilD mutants in vitro and in S. Typhimurium infection models suggests that HilD is one of the key protein targets of anti-infective bile acids. This study highlights the utility of chemical proteomics to identify the direct protein targets of microbiota metabolites for mechanistic studies in bacterial pathogens.

LuxR family transcriptional repressor YjjQ modulates the biofilm formation and motility of avian pathogenic Escherichia coli

Avian pathogenic Escherichia coli (APEC) can cause the acute and sudden death of poultry, which leads to serious economic losses in the poultry industry. Biofilm formation contributes to the persistence of bacterial infection, drug resistance, and resistance to diverse environmental stress. Many transcription regulators in APEC play an essential role in the formation of biofilm and could provide further insights into APEC pathogenesis. YjjQ has an important role in the pathogenicity of bacteria by regulating the expression of virulence factors, such as flagellar and iron uptake. However, YjjQ regulates other virulence factors, and their role in the overall regulatory network is unclear. Here, we further evaluate the function of YjjQ on APEC biofilm formation and motility. In this study, we successfully constructed mutant (AE27∆yjjQ) and complement (AE27ΔyjjQ-comp) strains of the wild-type strain AE27. Inactivation of the yjjQ gene significantly increased biofilm-forming ability in APEC. Scanning electron microscopy showed that the biofilm formation of the AE27 was single-layered and flat, whereas that of the AE27∆yjjQ had a porous three-dimensional structure. Moreover, the deletion of the yjjQ gene inhibited the motility of APEC. RNA-sequencing was used to further investigate the regulatory mechanism of YjjQ in APEC. The results indicate that YjjQ regulates biofilm formation and flagellar genes in AE27∆yjjQ. RT-qPCR shows that YjjQ affects the transcriptional levels of genes, including flagella genes (flhD, flhC and flgE), and biofilm formation genes (pstA, uhpC, nikD, and ygcS). These results confirm that the transcription regulator YjjQ is involved in APEC biofilm formation and motility, and provide new evidence for the prevention and control of APEC.

Long Chain Fatty Acids and Virulence Repression in Intestinal Bacterial Pathogens

When bacterial pathogens enter the gut, they encounter a complex milieu of signaling molecules and metabolites produced by host and microbial cells or derived from external sources such as the diet. This metabolomic landscape varies throughout the gut, thus establishing a biogeographical gradient of signals that may be sensed by pathogens and resident bacteria alike. Enteric bacterial pathogens have evolved elaborate mechanisms to appropriately regulate their virulence programs, which involves sensing and responding to many of these gut metabolites to facilitate successful gut colonization. Long chain fatty acids (LCFAs) represent major constituents of the gut metabolome that can impact bacterial functions. LCFAs serve as important nutrient sources for all cellular organisms and can function as signaling molecules that regulate bacterial metabolism, physiology, and behaviors. Moreover, in several enteric pathogens, including Salmonella enterica, Listeria monocytogenes, Vibrio cholerae, and enterohemorrhagic Escherichia coli, LCFA sensing results in the transcriptional repression of virulence through two general mechanisms. First, some LCFAs function as allosteric inhibitors that decrease the DNA binding affinities of transcriptional activators of virulence genes. Second, some LCFAs also modulate the activation of histidine kinase receptors, which alters downstream intracellular signaling networks to repress virulence. This mini-review will summarize recent studies that have investigated the molecular mechanisms by which different LCFA derivatives modulate the virulence of enteric pathogens, while also highlighting important gaps in the field regarding the roles of LCFAs as determinants of infection and disease.

Helicobacter pylori and Alzheimer's Disease-Related Metabolic Dysfunction: Activation of TLR4/Myd88 Inflammation Pathway from p53 Perspective and a Case Study of Low-Dose Radiation Intervention

Gut dysbiosis is observed in Alzheimer's disease (AD) and is frequently associated with AD-induced metabolic dysfunction. However, the extent and specific underlying molecular mechanisms triggered by alterations of gut microbiota composition and function mediating AD-induced metabolic dysfunction in AD remain incompletely uncovered. Here, we indicate that Helicobacter pylori (H. pylori) is abundant in AD patients with relative metabolic dysfunction. Fecal microbiota transplantation from the AD patients promoted metabolic dysfunction in mice and increased gut permeability. H. pylori increased gut permeability through activation of the TLR4/Myd88 inflammation pathway in a p53-dependent manner, leading to metabolic dysfunction. Moreover, p53 deficiency reduced bile acid concentration, leading to an increased abundance of H. pylori colonization. Overall, these data identify H. pylori as a key promoter of AD-induced metabolic dysfunction.

FabR senses long-chain unsaturated fatty acids to control virulence in pathogen Edwardsiella piscicida

Long-chain unsaturated fatty acids (UFAs) can serve as nutrient sources or building blocks for bacterial membranes. However, little is known about how UFAs may be incorporated into the virulence programs of pathogens. A previous investigation identified FabR as a positive regulator of virulence gene expression in Edwardsiella piscicida. Here, chromatin immunoprecipitation-sequencing coupled with RNA-seq analyses revealed that 10 genes were under the direct control of FabR, including fabA, fabB, and cfa, which modulate the composition of UFAs. The binding of FabR to its target DNA was facilitated by oleoyl-CoA and inhibited by stearoyl-CoA. In addition, analyses of enzyme mobility shift assay and DNase I footprinting with wild-type and a null mutant (F131A) of FabR demonstrated crucial roles of FabR in binding to the promoters of fabA, fabB, and cfa. Moreover, FabR also binds to the promoter region of the virulence regulator esrB for its activation, facilitating the expression of the type III secretion system (T3SS) in response to UFAs. Furthermore, FabR coordinated with RpoS to modulate the expression of T3SS. Collectively, our results elucidate the molecular machinery of FabR regulating bacterial fatty acid composition and virulence in enteric pathogens, further expanding our knowledge of its crucial role in host-pathogen interactions.

Fatty acid metabolism and acyl-CoA synthetases in the liver-gut axis

Fatty acids are energy substrates and cell components which participate in regulating signal transduction, transcription factor activity and secretion of bioactive lipid mediators. The acyl-CoA synthetases (ACSs) family containing 26 family members exhibits tissue-specific distribution, distinct fatty acid substrate preferences and diverse biological functions. Increasing evidence indicates that dysregulation of fatty acid metabolism in the liver-gut axis, designated as the bidirectional relationship between the gut, microbiome and liver, is closely associated with a range of human diseases including metabolic disorders, inflammatory disease and carcinoma in the gastrointestinal tract and liver. In this review, we depict the role of ACSs in fatty acid metabolism, possible molecular mechanisms through which they exert functions, and their involvement in hepatocellular and colorectal carcinoma, with particular attention paid to long-chain fatty acids and small-chain fatty acids. Additionally, the liver-gut communication and the liver and gut intersection with the microbiome as well as diseases related to microbiota imbalance in the liver-gut axis are addressed. Moreover, the development of potentially therapeutic small molecules, proteins and compounds targeting ACSs in cancer treatment is summarized.

Long-chain fatty acids alter transcription of Helicobacter pylori virulence and regulatory genes

Infection with Helicobacter pylori is one of the most important risk factors for developing gastric cancer (GC). The type IV secretion system (T4SS) encoded in the cag pathogenicity island is the main virulence factor of H. pylori associated with GC. Additionally, other virulence factors have been shown to play a role in the H. pylori virulence, such as vacuolizing cytotoxin (VacA), urease, flagella, and adhesins. Long-chain fatty acids (LCFAs) are signaling molecules that affect the transcription of virulence genes in several pathogenic bacteria such as Salmonella enterica, Vibrio cholerae, Pseudomonas aeruginosa and Mycobacterium tuberculosis. However, the effect of LCFAs on the transcription of H. pylori virulence and regulatory genes remains unknown. Here we analyzed whether the transcription of virulence genes that encode T4SS and cellular envelope components, flagellins, adhesins, toxins, urease, as well as the transcription of different regulatory genes of the H. pylori strain 26695, are altered by the presence of five distinct LCFAs: palmitic, stearic, oleic, linoleic, and linolenic acids. Palmitic and oleic acids up-regulated the transcription of most of the virulence genes tested, including cagL, cagM, flaB, sabA, mraY and vacA, as well as that of the genes encoding the transcriptional regulators NikR, Fur, CheY, ArsR, FlgR, HspR, HsrA, Hup, and CrdR. In contrast, the other LCFAs differentially affected the transcription of the virulence and regulatory genes assessed. Our data show that LCFAs can act as signaling molecules that control the transcription of the H. pylori virulome.

Optimised Heterologous Expression and Functional Analysis of the Yersinia pestis F1-Capsular Antigen Regulator Caf1R

The bacterial pathogen, Yersinia pestis, has caused three historic pandemics and continues to cause small outbreaks worldwide. During infection, Y. pestis assembles a capsule-like protective coat of thin fibres of Caf1 subunits. This F1 capsular antigen has attracted much attention due to its clinical value in plague diagnostics and anti-plague vaccine development. Expression of F1 is tightly regulated by a transcriptional activator, Caf1R, of the AraC/XylS family, proteins notoriously prone to aggregation. Here, we have optimised the recombinant expression of soluble Caf1R. Expression from the native and synthetic codon-optimised caf1R cloned in three different expression plasmids was examined in a library of E. coli host strains. The functionality of His-tagged Caf1R was demonstrated in vivo, but insolubility was a problem with overproduction. High levels of soluble MBP-Caf1R were produced from codon optimised caf1R. Transcriptional-lacZ reporter fusions defined the P_M promoter and Caf1R binding site responsible for transcription of the cafMA1 operon. Use of the identified Caf1R binding caf DNA sequence in an electrophoretic mobility shift assay (EMSA) confirmed correct folding and functionality of the Caf1R DNA-binding domain in recombinant MBP-Caf1R. Availability of functional recombinant Caf1R will be a valuable tool to elucidate control of expression of F1 and Caf1R-regulated pathophysiology of Y. pestis.

Structure of the master regulator Rns reveals an inhibitor of enterotoxigenic Escherichia coli virulence regulons

Enteric infections caused by the gram-negative bacteria enterotoxigenic Escherichia coli (ETEC), Vibrio cholerae, Shigella flexneri, and Salmonella enterica are among the most common and affect billions of people each year. These bacteria control expression of virulence factors using a network of transcriptional regulators, some of which are modulated by small molecules as has been shown for ToxT, an AraC family member from V. cholerae. In ETEC the expression of many types of adhesive pili is dependent upon the AraC family member Rns. We present here the 3 Å crystal structure of Rns and show it closely resembles ToxT. Rns crystallized as a dimer via an interface similar to that observed in other dimeric AraC's. Furthermore, the structure of Rns revealed the presence of a ligand, decanoic acid, that inhibits its activity in a manner similar to the fatty acid mediated inhibition observed for ToxT and the S. enterica homologue HilD. Together, these results support our hypothesis that fatty acids regulate virulence controlling AraC family members in a common manner across a number of enteric pathogens. Furthermore, for the first time this work identifies a small molecule capable of inhibiting the ETEC Rns regulon, providing a basis for development of therapeutics against this deadly human pathogen.

A diffusible signal factor of the intestine dictates Salmonella invasion through its direct control of the virulence activator HilD

Successful intestinal infection by Salmonella requires optimized invasion of the gut epithelium, a function that is energetically costly. Salmonella have therefore evolved to intricately regulate the expression of their virulence determinants by utilizing specific environmental cues. Here we show that a powerful repressor of Salmonella invasion, a cis-2 unsaturated long chain fatty acid, is present in the murine large intestine. Originally identified in Xylella fastidiosa as a diffusible signal factor for quorum sensing, this fatty acid directly interacts with HilD, the master transcriptional regulator of Salmonella, and prevents hilA activation, thus inhibiting Salmonella invasion. We further identify the fatty acid binding region of HilD and show it to be selective and biased in favour of signal factors with a cis-2 unsaturation over other intestinal fatty acids. Single mutation of specific HilD amino acids to alanine prevented fatty acid binding, thereby alleviating their repressive effect on invasion. Together, these results highlight an exceedingly sensitive mechanism used by Salmonella to colonize its host by detecting and exploiting specific molecules present within the complex intestinal environment.

The Canonical Long-Chain Fatty Acid Sensing Machinery Processes Arachidonic Acid To Inhibit Virulence in Enterohemorrhagic Escherichia coli

Polyunsaturated fatty acids (PUFAs) play important roles in host immunity. Manipulation of lipid content in host tissues through diet or pharmacological interventions is associated with altered severity of various inflammatory diseases.

The mammalian gastrointestinal tract is a complex biochemical organ that generates a diverse milieu of host- and microbe-derived metabolites. In this environment, bacterial pathogens sense and respond to specific stimuli, which are integrated into the regulation of their virulence programs. Previously, we identified the transcription factor FadR, a long-chain fatty acid (LCFA) acyl coenzyme A (acyl-CoA) sensor, as a novel virulence regulator in the human foodborne pathogen enterohemorrhagic Escherichia coli (EHEC). Here, we demonstrate that exogenous LCFAs directly inhibit the locus of enterocyte effacement (LEE) pathogenicity island in EHEC through sensing by FadR. Moreover, in addition to LCFAs that are 18 carbons in length or shorter, we introduce host-derived arachidonic acid (C_20:4) as an additional LCFA that is recognized by the FadR system in EHEC. We show that arachidonic acid is processed by the acyl-CoA synthetase FadD, which permits binding to FadR and decreases FadR affinity for its target DNA sequences. This interaction enables the transcriptional regulation of FadR-responsive operons by arachidonic acid in EHEC, including the LEE. Finally, we show that arachidonic acid inhibits hallmarks of EHEC disease in a FadR-dependent manner, including EHEC attachment to epithelial cells and the formation of attaching and effacing lesions. Together, our findings delineate a molecular mechanism demonstrating how LCFAs can directly inhibit the virulence of an enteric bacterial pathogen. More broadly, our findings expand the repertoire of ligands sensed by the canonical LFCA sensing machinery in EHEC to include arachidonic acid, an important bioactive lipid that is ubiquitous within host environments.

Pathogenicity and virulence regulation of Vibrio cholerae at the interface of host-gut microbiome interactions

The Gram-negative bacterium Vibrio cholerae is responsible for the severe diarrheal pandemic disease cholera, representing a major global public health concern. This pathogen transitions from aquatic reservoirs into epidemics in human populations, and has evolved numerous mechanisms to sense this transition in order to appropriately regulate its gene expression for infection. At the intersection of pathogen and host in the gastrointestinal tract lies the community of native gut microbes, the gut microbiome. It is increasingly clear that the diversity of species and biochemical activities within the gut microbiome represents a driver of infection outcome, through their ability to manipulate the signals used by V. cholerae to regulate virulence and fitness in vivo. A better mechanistic understanding of how commensal microbial action interacts with V. cholerae pathogenesis may lead to novel prophylactic and therapeutic interventions for cholera. Here, we review a subset of this burgeoning field of research.

In Silico-Guided Identification of New Potent Inhibitors of Carbonic Anhydrases Expressed in Vibrio cholerae

Carbonic anhydrases from Vibrio cholerae (VchCAs) play a significant role in bacterial pathophysiological processes. Therefore, their inhibition leads to a reduction of gene expression virulence and bacterial growth impairment. Herein, we report the first ligand-based pharmacophore model as a computational tool to study selective inhibitors of the β-class of VchCA. By a virtual screening on a collection of sulfonamides, we retrieved 9 compounds that were synthesized and evaluated for their inhibitory effects against VchCAβ as well as α- and γ-classes of VchCAs and selectivity over human ubiquitous isoforms hCA I and II. Notably, all tested compounds were active inhibitors of VchCAs. The N-(4-sulfamoylbenzyl)-[1,1'-biphenyl]-4-carboxamide (20e) stood out as the most exciting inhibitor toward the β-class (K _i = 95.6 nM), also showing a low affinity against the tested human isoforms. By applying docking procedures, we described the binding mode of the inhibitor 20e within the catalytic cavity of the modeled open conformation of VchCAβ.

Virulence Regulation and Innate Host Response in the Pathogenicity of Vibrio cholerae

The human pathogen Vibrio cholerae is the causative agent of severe diarrheal disease known as cholera. Of the more than 200 "O" serogroups of this pathogen, O1 and O139 cause cholera outbreaks and epidemics. The rest of the serogroups, collectively known as non-O1/non-O139 cause sporadic moderate or mild diarrhea and also systemic infections. Pathogenic V. cholerae circulates between nutrient-rich human gut and nutrient-deprived aquatic environment. As an autochthonous bacterium in the environment and as a human pathogen, V. cholerae maintains its survival and proliferation in these two niches. Growth in the gastrointestinal tract involves expression of several genes that provide bacterial resistance against host factors. An intricate regulatory program involving extracellular signaling inputs is also controlling this function. On the other hand, the ability to store carbon as glycogen facilitates bacterial fitness in the aquatic environment. To initiate the infection, V. cholerae must colonize the small intestine after successfully passing through the acid barrier in the stomach and survive in the presence of bile and antimicrobial peptides in the intestinal lumen and mucus, respectively. In V. cholerae, virulence is a multilocus phenomenon with a large functionally associated network. More than 200 proteins have been identified that are functionally linked to the virulence-associated genes of the pathogen. Several of these genes have a role to play in virulence and/or in functions that have importance in the human host or the environment. A total of 524 genes are differentially expressed in classical and El Tor strains, the two biotypes of V. cholerae serogroup O1. Within the host, many immune and biological factors are able to induce genes that are responsible for survival, colonization, and virulence. The innate host immune response to V. cholerae infection includes activation of several immune protein complexes, receptor-mediated signaling pathways, and other bactericidal proteins. This article presents an overview of regulation of important virulence factors in V. cholerae and host response in the context of pathogenesis.