Integrating small molecule signalling and H-NS antagonism in Vibrio cholerae, a bacterium with two chromosomes

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.

Regulatory Hierarchies Controlling Virulence Gene Expression in Shigella flexneri and Vibrio cholerae

Gram-negative enteropathogenic bacteria use a variety of strategies to cause disease in the human host and gene regulation in some form is typically a part of the strategy. This article will compare the toxin-based infection strategy used by the non-invasive pathogen Vibrio cholerae, the etiological agent in human cholera, with the invasive approach used by Shigella flexneri, the cause of bacillary dysentery. Despite the differences in the mechanisms by which the two pathogens cause disease, they use environmentally-responsive regulatory hierarchies to control the expression of genes that have some features, and even some components, in common. The involvement of AraC-like transcription factors, the integration host factor, the Factor for inversion stimulation, small regulatory RNAs, the RNA chaperone Hfq, horizontal gene transfer, variable DNA topology and the need to overcome the pervasive silencing of transcription by H-NS of horizontally acquired genes are all shared features. A comparison of the regulatory hierarchies in these two pathogens illustrates some striking cross-species similarities and differences among mechanisms coordinating virulence gene expression. S. flexneri, with its low infectious dose, appears to use a strategy that is centered on the individual bacterial cell, whereas V. cholerae, with a community-based, quorum-dependent approach and an infectious dose that is several orders of magnitude higher, seems to rely more on the actions of a bacterial collective.

Control of virulence gene transcription by indirect readout in Vibrio cholerae and Salmonella enterica serovar Typhimurium

Indirect readout mechanisms of transcription control rely on the recognition of DNA shape by transcription factors (TFs). TFs may also employ a direct readout mechanism that involves the reading of the base sequence in the DNA major groove at the binding site. TFs with winged helix-turn-helix (wHTH) motifs use an alpha helix to read the base sequence in the major groove while inserting a beta sheet 'wing' into the adjacent minor groove. Such wHTH proteins are important regulators of virulence gene transcription in many pathogens; they also control housekeeping genes. This article considers the cases of the non-invasive Gram-negative pathogen Vibrio cholerae and the invasive pathogen Salmonella enterica serovar Typhimurium. Both possess clusters of A + T-rich horizontally acquired virulence genes that are silenced by the nucleoid-associated protein H-NS and regulated positively or negatively by wHTH TFs: for example, ToxR and LeuO in V. cholerae; HilA, LeuO, SlyA and OmpR in S. Typhimurium. Because of their relatively relaxed base sequence requirements for target recognition, indirect readout mechanisms have the potential to engage regulatory proteins with many more targets than might be the case using direct readout, making indirect readout an important, yet often ignored, contributor to the expression of pathogenic phenotypes.

Bacterial pathogen gene regulation: a DNA-structure-centred view of a protein-dominated domain

The mechanisms used by bacterial pathogens to regulate the expression of their genes, especially their virulence genes, have been the subject of intense investigation for several decades. Whole genome sequencing projects, together with more targeted studies, have identified hundreds of DNA-binding proteins that contribute to the patterns of gene expression observed during infection as well as providing important insights into the nature of the gene products whose expression is being controlled by these proteins. Themes that have emerged include the importance of horizontal gene transfer to the evolution of pathogens, the need to impose regulatory discipline upon these imported genes and the important roles played by factors normally associated with the organization of genome architecture as regulatory principles in the control of virulence gene expression. Among these architectural elements is the structure of DNA itself, its variable nature at a topological rather than just at a base-sequence level and its ability to play an active (as well as a passive) part in the gene regulation process.

H-NS: an overarching regulator of the Vibrio cholerae life cycle

Vibrio cholerae has become a model organism for studies connecting virulence, pathogen evolution and infectious disease ecology. The coordinate expression of motility, virulence and biofilm enhances its pathogenicity, environmental fitness and fecal-oral transmission. The histone-like nucleoid structuring protein negatively regulates gene expression at multiple phases of the V. cholerae life cycle. Here we discuss: (i) the regulatory and structural implications of H-NS chromatin-binding in the two-chromosome cholera bacterium; (ii) the factors that counteract H-NS repression; and (iii) a model for the regulation of the V. cholerae life cycle that integrates H-NS repression, cyclic diguanylic acid signaling and the general stress response.