To the Editor: The traditional causative agent of cholera, Vibrio chol-erae O1, has 2 biotypes, classical and El Tor. The current seventh pandemic that began in 1961 and has spread to much of the world is caused by the El Tor biotype. This biotype has replaced the classical biotype responsible for the previous pandemics. The classical and El Tor biotypes are differentiated by phenotypic tests (1), and several nucleotide base differences occur at positions 115 and 203 in the ctxB gene (C in both positions in the classical and T in both positions in the El Tor biotype). These differences translate to histidine at amino acid position 39 and threonine at amino acid position 68 for the B subunit of cholera toxin (CT) in the classical biotype and ty-rosine and isoleucine, respectively, for the corresponding amino acids in the El Tor biotype (2).
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