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

Abstract

In this study, 176 clinical and environmental Vibrio cholerae strains of different O serotypes isolated in Thailand from 1982 to 1995 were selected and studied for the presence of class 1 integrons, a new group of genetic elements 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; beta-lactams; and trimethoprim, respectively. Each cassette array contained only a single antibiotic resistance gene. Although resistance genes in class 1 integrons were found in strains from the same epidemic, as well as in unrelated non-O1, non-O139 strains isolated from children with diarrhea, they were found to encode only some of the antibiotic resistance expressed by the 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 distinct V. cholerae O1 strain which contained the aadA2 resistance gene cassette. A 150-kb self-transmissible plasmid found in three O1 strains isolated in 1982 contained the aadB gene cassette. Surprisingly, several strains harbored two integrons containing different cassettes. Thus, class 1 integrons containing various resistance gene cassettes are distributed among different V. cholerae O serotypes of mainly clinical origin in Thailand.

FIG. 1

Association between integron structure and PCR products (modified from reference 40). The lines below the integron structure represent amplicons, and the bold line represents the sequenced amplicons. The primers above the PCR products are described in Table 1. attI1 is the attenuation site, and qacEΔ1 and sul1 encode resistance to disinfectants and sulfonamides, respectively. The individual gene cassettes are drawn to scale. The recombination site (59-bp element) is shown as a dark circle.

FIG. 2

Examples of PCR products of V. cholerae O1 strains isolated in Thailand by using the primers in-F and in-B (lanes b to i), in-F and aadA-B (lanes j and k), in-F and blaP1-B (lanes l and m), and qacEΔ1-F and sul1-B (lanes n and o). Lanes: a, 100-bp molecular mass standard; b, strain VO-3417; c, strain 1075/25; d, strain 292/90; e, strain VO-258; f, strain 30/90; g, strain VO-3198; h, S. enterica serotype Typhimurium strain 9720921 (positive control); i, Acinetobacter sp. strain R4-96 (negative control); j, strain 1076/25; k, strain 9720921 (positive control); l, strain SKF-09; m, strain 9720921 (positive control); n, strain VO-557; o, strain 9720921 (positive control); p, PCR mixture, (negative control); q, 100-bp molecular mass standard.