EMERGENCE OF A NEW CLONE OF TOXIGENIC VIBRIO-CHOLERAE-01 BIOTYPE EL-TOR DISPLACING VIBRIO-CHOLERAE O139 BENGAL IN BANGLADESH

The emergence of Vibrio cholerae O139 Bengal in 1992, its rapid spread in an epidemic form, in which it replaced existing strains of V. chol erae O1 during 1992 and 1993, and the subsequent reemergence of V. cho lerae O1 of the El Tor biotype in Bangladesh since 1994 have raised qu estions regarding the origin of the reemerged El Tor vibrios, We studi ed 50 El Tor vibrio strains isolated in Bangladesh and four other coun tries in Asia and Africa before the emergence of V. cholerae O139 and 32 strains isolated in Bangladesh during and after the epidemic caused by V. cholerae O139 to determine whether the reemerged El Tor vibrios were genetically different from the El Tor vibrios which existed befo re the emergence of V. cholerae O139, Analysis of restriction fragment length polymorphisms in genes for conserved rRNA, cholera toxin (ctxA ), and zonula occludens toxin (zot) or in DNA sequences Banking these genes showed that the El Tor strains isolated before the emergence of V. cholerae O139 belonged to four different ribotypes and four differe nt ctx genotypes, Of 32 El Tor strains isolated after the emergence of O139 vibrios, 30 strains (93.7%) including all the clinical isolates belonged to a single new ribotype and a distinctly different ctx genot ype, These results provide evidence that the reemerged El Tor strains represent a new clone of El Tor vibrios distinctly different from the earlier clones of El Tor vibrios which were replaced by the O139 vibri os, Further analysis showed that all the strains carried the structura l and regulatory genes for toxin-coregulated pilus (tcpA, tcpI, and to xR). All strains of the new clone produced cholera toxin (CT) in vitro , as assayed by the G(MI)-dependent enzyme-linked immunosorbent assay, and the level of CT production was comparable to that of previous epi demic isolates of El Tor vibrios. Further studies are required to asse ss the epidemic potential of the newly emerged clone of V. cholerae O1 and to understand the mechanism of emergence of new clones of toxigen ic V. cholerae.