Vibrio cholerae, the causative agent of cholera belongs to a group of organisms whose natural habitats are the aquatic ecosystems. The strains that cause cholera epidemics have evolved from non-pathogenic progenitor strains by acquisition of virulence genes, and
V. cholerae represents a paradigm for this evolutionary process.
Genomics of Vibrio cholerae and its Evolution The 4.0 Mbp genome of N16961, an O1 serogroup, El Tor biotype, 7th pandemic strain of
V. cholerae, is comprised of two circular chromosomes of unequal size that are predicted to encode a total of 3,885 genes. The genomic sequence of this representative strain has facilitated global experimental approaches that have furthered our understanding of the genetic and phenotypic diversity found within the species
V. cholerae. Sequence data have been used to identify horizontally acquired sequences, dissect complex regulatory and signaling pathways, and develop computational approaches to predict patterns of gene expression and the presence of metabolic pathway components. In addition, these data have served as a basis for the construction of microarrays to study the evolution of the organism through comparative genomic analyses. Genomic sequencing of additional strains, subtractive hybridization studies and the introduction of new model systems have also contributed to the identification of novel sequences and pathogenic mechanisms associated with other strains. The sequence of strain N16961 has therefore resulted in an expanded view of the genetic repertoire of
V. cholerae and focused our attention on the progressive evolution of this marine bacterium that can also be a human pathogen.
Population Genetics of Vibrio cholerae The influence of evolutionary forces on the genetic diversity of natural populations of living organisms is the subject matter of population genetics. In the case of
Vibrio cholerae, data obtained from detailed molecular studies of large populations of these bacteria have allowed for a better understanding of the epidemiology of diseases due to their presence in humans. The species has a high genetic diversity and a complex image of its population structure. There is also evidence of linkage disequilibrium and frequent intragenic and assortative recombination events in their housekeeping genes. Horizontal transfer of genes in
V. cholerae is higher than those reported for
Escherichia coli and
Salmonella enterica. In spite of the frequent horizontal gene transfer, clonal lineages of
Vibrio cholerae might persist for decades. The best example of this is the presence and survival of epidemic and pandemic clones over long periods of time. To date, there are four major genetic lines of toxigenic
V. cholerae O1 biotype El Tor: an Australian clone (ET 1); the U.S. Gulf Coast clone (ET 2); the seventh pandemic clone isolated in the South East Asia together with the O139 "Bengal" clone (ET 3); and the clone that caused cholera in Latin America in the 1990's (ET 4). There are also isolated clones that have appeared over time under special conditions, e.g., serogroup O37 that was shown to have limited epidemic potential in the 1960's. Given the close evolutionary relationship between
V. cholerae O1 and other non-O1 virulent serotypes and the fact that virulence genes can be transferred horizontally, new pathogenic strains of
V. cholerae could arise in the future through the modification of existing clones that have the capacity to spread rapidly, and thus cause outbreaks of disease.
Genetics of Vibrio cholerae Colonization and Motility Survival of
Vibrio cholerae either in the aquatic environment or in the human host is mediated by appropriate expression of factors that control motility, colonization, production of virulence factors, as well as sensing the cell density (quorum sensing). Successful transition of the organism between the aquatic and the host intestinal environments thus depends on the coordinated activity of a number of genes and regulatory circuits.
Genetics of O-antigens, Capsules, and the Rugose Variant of Vibrio cholerae The human pathogen
Vibrio cholerae produces three major cell-surface associated polysaccharides, including (i) lipopolysaccharide (LPS), (ii) capsule, and (iii) rugose exopolysaccharide. While LPS and capsule primarily help the bacterium to evade host defense mechanisms, the rugose exopolysaccharide may aid the bacterium in persisting in the nutrient-deficient aquatic environments.
Genetics and Microbiology of Biofilm Formation by Vibrio cholerae In nature, most bacteria grow as matrix-enclosed, surface-associated communities known as biofilms.
Vibrio cholerae, the causative agent of the disease cholera, forms biofilms on diverse surfaces. This ability to form biofilms appears to be critical for the environmental survival and the transmission of
V. cholerae. The molecular mechanisms utilized by
V. cholerae to form and maintain biofilms have been investigated by molecular genetic and microscopic approaches and these studies should prove useful in the development of future strategies for predicting and controlling cholera epidemics.
Molecular Ecology of Vibrio cholerae Although
Vibrio cholerae causes human disease, aquatic ecosystems are major habitats of
V. cholerae, and all
V. cholerae are not pathogenic for humans.
V. cholerae represents a paradigm for origination of pathogenic bacteria from environmental nonpathogenic progenitor strains by horizontal transfer of genes. Besides environmental factors which are not precisely defined, bacteriophages, and horizontally transmissible genetic elements have a significant role in the epidemiology and evolution of the pathogen. Recent studies are beginning to reveal the mechanisms associated with the occurrence of seasonal epidemics in endemic areas, waterborne spread of cholera, and the factors that enable the organisms to survive unfavorable conditions in the aquatic environment. The emergence of new epidemic strains, and their enrichment during epidemics of cholera appear to constitute a natural system for the evolution of
V. cholerae and genetic elements that mediate horizontal transfer of genes among bacterial strains.
Coordinated Regulation of Gene Expression in Vibrio cholerae Vibrio cholerae, the causative agent of the severe diarrhoeal disease cholera, has evolved with intricate signal transduction and gene regulatory systems to survive and grow under various environmental conditions. The virulence regulon of
V. cholerae, which involves multiple genes working in a coordinated manner, represents a regulatory paradigm for extracellular bacterial pathogens. Availability of the whole genome sequence has allowed microarray based transcriptome analyses of
V. cholerae cells isolated directly from cholera patients. Such studies indicate that quite a large number of genes are involved in the disease process and their expression pattern changes as the infection progresses. Further understanding of the process came with the recent discoveries of small noncoding RNAs and intracellular signal molecule c-di-GMP as modulators of gene expression in
V. cholerae. Transcriptome analysis has also shed light on synchronized gene expression related to chitin utilization and development of natural competence when the organism exists in the natural aquatic environment. Thus, the survival, evolution and pathogenesis of
V. cholerae appear to be controlled by several intricate overlapping regulatory circuits.
Evolutionary Relationships of Pathogenic Clones of Vibrio cholerae Evolution refers to the differentiation of an ancestral genome into recognizably distinct genomes. Understanding the evolutionary history of an organism can provide insight into how it can be expected to evolve in the future and provide predictions that serve as the basis of where to best focus effort to prevent the emergence of new pathogenic variants. In order to accurately understand the evolutionary history, the methods used for interpreting the genetic variation need to reflect the mechanisms of genetic change. The critical mechanism for deciding how to interpret the genetic relatedness is the amount of recombination. If recombination is rare, then the traditional phylogenetic analysis based on bifurcating trees works well. If recombination is common, then a method that incorporates recombination must be used. Evolutionary relationships among pathogenic clones based on these assessments have been presented and discussed.
Emerging Hybrid Variants of Vibrio cholerae O1 Rapid emergence of genetic variants among toxigenic epidemic strains of
Vibrio cholerae, contributes to the intricate epidemiological pattern of cholera. A remarkable event in recent years has been the emergence of strains of
V. cholerae O1 which possess traits of both the classical and El Tor biotypes.
Antibiotic Resistance in Vibrio cholerae Antimicrobial resistance has become a major medical and public health problem as it has direct link with the disease management.
Vibrio cholerae, the cholera causing pathogen is increasingly developing resistance towards many antimicrobials used for the treatment of diarrhoea. However, the pattern of resistance differs from country to country. The well-known factor responsible for development and spread of resistance is injudicious use of antimicrobial agents, which is directly related to the stimulation of several mechanisms of resistance. In
V. cholerae, several resistance mechanisms such as plasmid encoded resistance, mutation in the quinolones resistance determining regions, integrons, efflux pumps and SXT constins have been established. Considering the importance of drug resistance, quick diagnostic assay methods are available for the identification of multidrug resistant (MDR)
V. cholerae. Many new generation antimicrobials were discovered, which are effective against
V. cholerae in the
in vitro studies. The resistance pattern of
V. cholerae to several antimicrobials are not always uniform as it depends on the source of isolation. Vibrios can act as reservoirs of antimicrobial resistance as cross-spread is common in
in vitro studies. Promotion of indigenous drugs should be considered in the future and studied in detail for their efficacy.
Further reading:
Vibrio cholerae: Genomics and Molecular BiologyLabels: antibiotic resistance, bacteriology, bacterium, biofilm, evolution, population genetics, regulation, vibrio