Comparative behaviour of classical and El Tor biotypes of Vibrio cholerae 01 isolated in Bangladesh during 1982.
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The change from classical to the El Tor biotype of Vibrio cholerae during the early seventies in Bangladesh remains to be elucidated. The shift in 1982 from El Tor back to the classical was not predicted, but provided an opportunity to study both the biotypes occurring simultaneously in nature, compared with earlier isolates. Comparative studies showed TCBS to be as good as TTGA for isolation of the 2 biotypes. Replacement of the El Tor by the classical biotype as the dominant epidemic strain, occurred over 4 months. Isolates of both biotypes from 1982 were found to be slow in mannitol fermentation. The isolates of El Tor were frankly haemolytic and belonged to phage type 4. The classical strains were of phage type 3. V. cholerae strains isolated in the late sixties and early seventies were similar in these markers. This suggested that the epidemic was caused by strains indigenous to Bangladesh. A classical strain of V. cholerae isolated in 1969 was overgrown by an El Tor strain of the same year when grown together in peptone water. Classical strains of 1982, however, grew competitively with 1969 and 1982 strains of El Tor. 1 classical isolate of 1982 survived for 50 days when grown with an El Tor strain of 1969. These findings suggest that the classical V. cholerae strains of 1982 successfully compete with the El Tor strain. They are more toxigenic than the prevailing El Tor biotype. author's modified