Molecular diversification in the quorum-sensing system of Vibrio cholerae: Role of natural selection in the emergence of pandemic strains

ArcAB system promotes biofilm formation through direct repression of hapR transcription in Vibrio cholerae

Vibrio cholerae, the causative agent of cholera, can efficiently adapt its metabolic processes, including biofilm formation, in response to varying respiratory conditions- such as aerobic, microaerobic, and anaerobic- through the ArcAB system. In this study, we elucidate the activation mechanism of V. cholerae ArcB and ArcA and identify ArcB residues H292, D577, and H722, along with ArcA residue D54 as key phosphorylation sites. Furthermore, we demonstrate that the ArcAB system plays a crucial role in regulating biofilm formation under both aerobic and anaerobic conditions. Our findings reveal that the positive regulation of biofilm formation by the ArcAB systems involves the high cell density (HCD) quorum sensing (QS) regulator HapR. Specifically, phosphorylated ArcA represses hapR transcription, thereby promoting biofilm formation under anaerobic condition. This study also highlights an epistatic relationship between ArcA and HapR in biofilm regulation. Overall, our results underscore the critical role of the ArcAB system in the biofilm formation of pathogenic V. cholerae under oxygen-limiting conditions.

Sequence Polymorphisms in Vibrio cholerae HapR Affect Biofilm Formation under Aerobic and Anaerobic Conditions

We investigated the influence of hapR sequence mutations on the biofilm formation of Vibrio cholerae. In this study, hapR sequences from 85 V. cholerae strains belonging to both pandemic and nonpandemic serogroup were investigated through phylogenetic and sequence analyses. Biofilm formation assays under aerobic and anaerobic conditions were also performed. Sequence variations include single point mutations and insertions/deletions (indels) leading to either truncated or frameshifted HapR. Population structure analysis revealed two major hapR haplogroups, hapR1 and hapR2. Phylogenetic reconstruction displayed a hypothetical ancestral hapR sequence located within the hapR1 haplogroup. Higher numbers of single nucleotide polymorphisms and genetic diversity indices were observed in hapR1, while indels occurred dominantly in hapR2. Aerobic conditions supported more robust biofilms compared to anaerobic conditions. Strains with frameshifted HapR produced the largest amount of biofilm under both oxygen conditions. Quantitative real-time PCR assay confirmed that strains with truncated and frameshifted HapR resulted in a nonfunctional regulator as exhibited by the significantly low hapA gene expression. The present study shows that HapR mutations had a strong influence on biofilm formation and that sequence polymorphisms leading to the disruption of DNA-binding sites or dimerization of the HapR will result in more-robust V. cholerae biofilms. IMPORTANCE Our study revealed an ancestral hapR sequence from a phylogenetic reconstruction that displayed the evolutionary lineage of the nonpandemic to the pandemic strains. Here, we established hapR1 and hapR2 as major hapR haplogroups. The association of the O1 and O139 serogroups with the hapR2 haplogroup demonstrated the distinction of hapR2 in causing cholera infection. Moreover, mutations in this regulator that could lead to the disruption of transcription factor-binding sites or dimerization of the HapR can significantly affect the biofilm formation of V. cholerae. These observations on the relationship of the hapR polymorphism and V. cholerae biofilm formation will provide additional considerations for future biofilm studies and insights into the epidemiology of the pathogen that could ultimately help in the surveillance and mitigation of future cholera disease outbreaks.

Quorum sensing provides a molecular mechanism for evolution to tune and maintain investment in cooperation

As selection frequently favors noncooperating defectors in mixed populations with cooperators, mechanisms that promote cooperation stability clearly exist. One potential mechanism is bacterial cell-to-cell communication, quorum sensing (QS), which can allow cooperators to prevent invasion by defectors. However, the impact of QS on widespread maintenance of cooperation in well-mixed conditions has not been experimentally demonstrated over extended evolutionary timescales. Here, we use wild-type (WT) Vibrio campbellii that regulates cooperation with QS and an unconditional cooperating (UC) mutant to examine the evolutionary origins and dynamics of novel defectors during a long-term evolution experiment. We found that UC lineages were completely outcompeted by defectors, whereas functioning QS enabled the maintenance of cooperative variants in most WT populations. Sequencing evolved populations revealed multiple luxR mutations that swept the UC lineages. However, the evolution of mutant lineages with reduced levels of bioluminescence (dims) occurred in many WT lineages. These dim variants also decreased other cooperative phenotypes regulated by QS, including protease production, indicating they result from changes to QS regulation. This diminished investment phenotype optimizes a tradeoff between cooperative input and growth output and suggests that decreasing the cost of QS could be a favorable strategy for maintaining the cooperative behaviors it regulates.

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.

Crystal structure of an inactive variant of the quorum-sensing master regulator HapR from the protease-deficient non-O1, non-O139 Vibrio cholerae strain V2

HapR is a TetR-family transcriptional regulator that controls quorum sensing in Vibrio cholerae, the causative agent of cholera. HapR regulates the expression of hemagglutinin protease, virulence and biofilm genes. The crystal structure of wild-type HapR from V. cholerae strain O1 El Tor C6706 has previously been solved. In this study, the structure of a DNA-binding-deficient variant of HapR (HapRV2) derived from the protease-deficient V. cholerae serotype O37 strain V2 is reported. The structure reveals no structural differences compared with wild-type HapR. However, structural alignment of HapRV2 with the TetR-family member QacR in complex with its operator DNA suggests that the aspartate residue located between the regulatory and DNA-binding domains may clash with and electrostatically repel the phosphate backbone of DNA to prevent binding.

"Quorum Non-Sensing": Social Cheating and Deception in Vibrio cholerae

Quorum sensing (QS) is widely used by bacteria to coordinate behavior in response to external stimuli. In Vibrio cholerae, this process is important for environmental survival and pathogenesis, though, intriguingly, a large percentage of natural isolates are QS deficient. Here, we show that QS-deficient mutants can spread as social cheaters by ceasing production of extracellular proteases under conditions requiring their growth. We further show that mutants stimulate biofilm formation and are over-represented in biofilms compared to planktonic communities; on this basis, we suggest that QS-deficient mutants may have the side effect of enhancing environmental tolerance of natural populations due to the inherent resistance properties of biofilms. Interestingly, high frequencies of QS-deficient individuals did not impact production of QS signaling molecules despite mutants being unable to respond to these inducers, indicating that these variants actively cheat by false signaling under conditions requiring QS. Taken together, our results suggest that social cheating may drive QS deficiency emergence within V. cholerae natural populations.

Differential RNA-seq of Vibrio cholerae identifies the VqmR small RNA as a regulator of biofilm formation

Quorum sensing (QS) is a process of cell-to-cell communication that enables bacteria to transition between individual and collective lifestyles. QS controls virulence and biofilm formation in Vibrio cholerae, the causative agent of cholera disease. Differential RNA sequencing (RNA-seq) of wild-type V. cholerae and a locked low-cell-density QS-mutant strain identified 7,240 transcriptional start sites with ∼ 47% initiated in the antisense direction. A total of 107 of the transcripts do not appear to encode proteins, suggesting they specify regulatory RNAs. We focused on one such transcript that we name VqmR. vqmR is located upstream of the vqmA gene encoding a DNA-binding transcription factor. Mutagenesis and microarray analyses demonstrate that VqmA activates vqmR transcription, that vqmR encodes a regulatory RNA, and VqmR directly controls at least eight mRNA targets including the rtx (repeats in toxin) toxin genes and the vpsT transcriptional regulator of biofilm production. We show that VqmR inhibits biofilm formation through repression of vpsT. Together, these data provide to our knowledege the first global annotation of the transcriptional start sites in V. cholerae and highlight the importance of posttranscriptional regulation for collective behaviors in this human pathogen.

The Prevalence of Functional Quorum-Sensing Systems in Recently Emerged Vibrio cholerae Toxigenic Strains

Vibrio cholerae live in aquatic environments and cause cholera disease. Like many other bacteria, V. cholerae use quorum-sensing (QS) systems to control various cellular functions, such as pathogenesis and biofilm formation. However, some V. cholerae strains are naturally QS-defective, including defective mutations in the quorum sensing master regulator HapR. Here we examined the QS functionality of 602 V. cholerae clinical and environmental strains isolated in China from 1960-2007, by measuring QS-regulated gene expression. We found that a greater percentage of the toxigenic strains (ctxAB(+)) had functional QS as compared to the non-toxigenic strains (ctxAB(-)), and that this trend increased significantly over time. We hypothesize that QS provides adaptive value in V. cholerae pathogenic settings.