MOLECULAR CHARACTERIZATION OF VIBRIO-CHOLERAE O1 BIOTYPE EL-TOR STRAINS ISOLATED BETWEEN 1992 AND 1995 IN CALCUTTA, INDIA - EVIDENCE FOR THE EMERGENCE OF A NEW CLONE OF THE EL-TOR BIOTYPE

Sixty-one clinical strains of Vibrio cholerae O1 El Tor isolated in Ca lcutta before, during, and after the V. cholerae O139 Bengal outbreak were examined to see if the O1 strains of the post-O139 period were di fferent from those in existence before. Comparison of the restriction fragment length polymorphism of the rRNA genes (ribotyping) and the CT X genetic element revealed that all ''before'' strains except 1 belong ed to a single known ribotype, whereas all ''after'' strains except 2 belonged to a hitherto undescribed ribotype. Also, 23 of 25 ''before'' strains harbored two or more copies of CTX in tandem and also a ''fre e'' RS1 clement away from CTX, whereas 19 of 21 ''after'' strains had a single copy of CTX and no free RS1 element. CTX occupied different c hromosomal locations in ''before'' and ''after'' strains. These studie s clearly showed that El Tor O1 strains, which displaced V. cholerae O 139 in Calcutta, belonged to a new clone and suggested that there is a continuous genetic reassortment among El Tor strains of V. cholerae O 1.