The Genomics of Cholera

Abstract

Vibrio cholerae, a bacterium autochthonous to the aquatic environment and introduced into the human intestine through contaminated water or food, is the etiological agent of the acute secretory diarrheal disease described as cholera. The pathogen has a free-living planktonic existence in aquatic bodies and has the ability to transmit into humans and cause disease. The process of completing an annual cycle in the environment and the transmission from contaminated water or food to humans is described as the ecology and epidemiology of the pathogen. This species contains a wide variety of both pathogenic and nonpathogenic strains. At the subspecies level, the organism is classified into more than 200 serogroups (Li et al. 2002). The differentiation of V. cholerae into serogroups is based on the differences in the sugar composition and therefore antigenicity of the heat-stable surface somatic “O” antigen. Only strains of serogroups O1 and O139 that produce cholera toxin defined as toxigenic strains have been recognized as agents of sporadic, endemic, epidemic, and pandemic cholera (Fig. 3.1) (Kaper et al. 1995; Sack et al. 2004). Most other serogroups of V. cholerae are not pathogenic or rarely cause local outbreaks, or mild gastroenteritis. V. cholerae strains belonging to serogroup O1 are further differentiated into two biotypes, classical and El Tor. The differentiation into biotypes is based on a combination of phenotypic, biochemical, and genetic traits, that include susceptibility to polymixin B, hemagglutination of chicken erythrocytes, hemolysis of sheep erythrocytes, the Voges–Proskauer test, susceptibility to phages, and nucleotide sequences of specific genes (Kaper et al. 1995). The other serogroups of V. cholerae, collectively called non-O1, non-O139, are not associated with epidemics and are ubiquitously distributed in the aquatic environment (Faruque et al. 1998).