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 bacteriumis 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 arethe 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 butthat 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 infectionsdue 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
This review was prepared byG.B. Nair, National Institute of Cholera and Enteric Diseases, Calcutta, India, with contributions from J. Bartram, Water, Sanitation and Health, World Health Organization, Geneva, Switzerland; A.H. Havelaar, Microbiological Laboratory of Health Protection, Rijksinstituut voor Milieuhygiene en Voldsgezondheid, Bilthoven, Netherlands: J. Hueb, Water, Sanitation and Health, World Health Organization,Geneva, Switzerland; and J. Jacob, Institute für Wasser-, Boden- and Lufthygiene, Bad Elster, Germany.
GUIDELINES FOR DRINKING-WATER QUALITY
the A and C antigens. A third serotype (Hikojima) expresses all three antigens but is rare and unstable. Between 1817 and 1961, six pandemics of cholera were recorded. The classical biotype was responsible for the ﬁfth and sixth pandemics andis believed to have been associated with the earlier pandemics as well, although there is no hard evidence. The causative agent of the seventh and current cholera pandemic, which began in 1961, is the El Tor biotype. The classical biotype has been completely displaced worldwide, except in Bangladesh where it reappeared in epidemic proportions in 1982 (Samadi et al., 1983), remained prominent therefor a few years, and now seems to have become extinct again (Siddique et al., 1991). The simple distinction between V. cholerae O1 and V. cholerae non-O1 became obsolete in early 1993 with the ﬁrst reports of a new epidemic of severe, cholera-like disease in Bangladesh (Albert et al., 1993) and India (Ramamurthy et al., 1993b). At ﬁrst, the responsible organism was referred to as non-O1 V.cholerae because it did not agglutinate with O1 antiserum. However, further investigations revealed that the organism did not belong to any of the O serogroups previously described for V. cholerae but to a new serogroup, which was given the designation O139 Bengal after the area where the strains were ﬁrst isolated (Shimada et al., 1993). Since recognition of the O139 serogroup, the designation non-O1...