Distribution of insertion sequence IS1 in multiple-antibiotic resistant clinical Enterobacteriaceae strains

The presence of insertion sequence IS1 in 70 multiple-antibiotic resistant clinical strains was determined. This 70-strain collection comprised 46 Escherichia coli, 18 Salmonella and 6 Shigella strains. The presence of IS1 was detected in the chromosome and plasmids of 73% and 63% of the strains, respectively, and 51% of the strains carried IS1 in both. The frequency of IS1 was higher in Salmonella than in E. coli and Shigella strains. A total of 31 strains carried large plasmids with IS1; 10 of these strains (32.3%) were able to transfer all or some of the antibiotic resistance markers to E. coli K12 or S. typhimurium recipient strains. Resistance markers of all clinical strains were maintained stably after several generations of growth. The presence of IS1 in a relatively high percentage of plasmids of multiple-antibiotic resistant clinical isolates, suggests a role for this sequence in the dissemination of genes which code for antibiotic resistance.