Distribution and content of class 1 integrons in different Vibrio choleraeO-serotype strains isolated in Thailand

In this study, 176 clinical and environmental Vibrio cholerae strains of di fferent O serotypes isolated in Thailand from 1982 to 1995 were selected an d studied for the presence of class 1 integrons, a new group of genetic ele ments which carry antibiotic resistance genes. Using PCR and DNA sequencing , we found that 44 isolates contained class 1 integrons harboring the aadB, aadA2, blaP1, dfrA1, and dfrA15 gene cassettes, which encode resistance to gentamicin, kanamycin, and tobramycin; streptomycin and spectinomycin; bet a-lactams; and trimethoprim, respectively. Each cassette array contained on ly a single antibiotic resistance gene. Although resistance genes in class 1 integrons were found in strains from the same epidemic, as well as in unr elated non-O1, non-O139 strains isolated from children with diarrhea, they were found to encode only some of the antibiotic resistance expressed by th e strains. Serotype O139 strains did not contain class 1 integrons. However , the appearance and disappearance of the O139 serotype in the coastal city Samutsakorn in 1992 and 1993 were associated with the emergence of a disti nct V. cholerae O1 strain which contained the aad-V resistance gene cassett e. A 150-kb self-transmissible plasmid found in three O1 strains isolated i n 1982 contained the aadB gene cassette. Surprisingly, several strains harb ored two integrons containing different cassettes. Thus, class 1 integrons containing various resistance gene cassettes are distributed among differen t V. cholerae O serotypes of mainly clinical origin in Thailand.