Strains of Vibrio cholerae serogroup O1 biotype El Tor that are susceptible to Mukerjee cholera bacteriophage group IV (S. Mukerjee, Bull. W.H.O. 28:333-336, 1963) were found. Cholera vibrios isolated from epidemics in northeast Thailand were characterized, and 57 of 60 strains isolated in 1986 were susceptible to cholera phage IV. However, all 113 strains isolated in 1988 were not susceptible to the phage. All isolates in both epidemics revealed behaviors typical of El Tor vibrios, except phage IV susceptibility in the 57 strains. Although the plaques of phage IV were generally translucent, plaques on some isolates looked transparent, just like those on classical vibrios. The organisms grown in the plaques were lysogenized. If this kind of strain is frequently isolated, the biotype of V. cholerae O1 should be reconsidered.
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