Vibrio cholerae Description Taxonomy and serological classification

Vibrio cholerae, a member of the family Vibrionaceae, is a facultatively anaerobic, Gram-negative, non-spore-forming curved rod, about 1.4–2.6 mm long, capable of respiratory and fermentative metabolism; it is well defined on the basis of biochemical tests and DNA homology studies (Baumann, Furniss & Lee, 1984). The bacterium is oxidase-positive, reduces nitrate, and is motile by means of a single, sheathed, polar flagellum. Growth of V. cholerae is stimulated by addition of 1% sodium chloride (NaCl). However, an important distinction from other Vibrio spp is the ability of V. cholerae to grow in nutrient broth without added NaCl. Differences in the sugar composition of the heat-stable surface somatic “O” antigen are the basis of the serological classification of V. cholerae first described by Gardner & Venkatraman (1935); currently the organism is classified into 206 “O” serogroups (Shimada et al., 1994; Yamai et al., 1997). Until recently, epidemic cholera was exclusively associated with V. cholerae strains of the O1 serogroup. All strains that were identified as V. cholerae on the basis of biochemical tests but that did not agglutinate with “O” antiserum were collectively referred to as non-O1 V. cholerae. The non-O1 strains are occasionally isolated from cases of diarrhoea (Ramamurthy et al., 1993a) and from a variety of extraintestinal infections, from wounds, and from the ear, sputum, urine, and cerebrospinal fluid (Morris & Black, 1985). They are ubiquitous in estuarine environments, and infections due to these strains are commonly of environmental origin (Morris, 1990). The O1 serogroup exists as two biotypes, classical and El Tor; antigenic factors allow further differentiation into two major serotypes— Ogawa and Inaba. Strains of the Ogawa serotype are said to express the A and B antigens and a small amount of C antigen, whereas Inaba strains express only