Detection of the class 1 integrons and SGI1 among Salmonella enterica Serovar Typhimurium DT104, U302, DT120, DT193, and nontypable human isolates.

Salmonella enterica serovar Typhimurium strains of particular phage types, such as DT104, U302, DT120, DT193, and nontypable strains, are often characterized by resistance to multiple antibiotics. This antibiotic resistance can be caused by the presence of the integrons, transposons, Salmonella genomic island 1 (SGI1), or conjugative plasmids. In this study we were interested in the relative contribution of integrons and SGI1 to the antibiotic resistance of the four mentioned phage types and nontypable S. Typhimurium human strains. Altogether 193 isolates were characterized for antibiotic susceptibility, presence of class 1 integrons, and the left junction of SGI1. Based on the presence of class 1 integrons and the left junction of SGI1, all strains could be clustered into three groups. The first group consisted of 69 strains positive for both the class 1 integrons and the left junction of SGI1. The strains of this group belonged mainly to DT104, U302, and DT120 phage types with resistance phenotype ACSSuT or ACSSuTNA. The second group comprised 9 strains which were positive only for the presence of class 1 integrons. In this group were some strains of multiple-antibiotic-resistant phage types: DT120, DT193, U302; and nontypable. The third group consisted of 115 strains in which neither the class 1 integrons nor the left junction of SGI1 were detected. Although the isolates were resistant to 2-8 antibiotics, the most frequent resistance type of these strains were ASSuT and SSu. By nucleotide sequencing of class 1 integrons PCR amplicons, the following embedded gene cassettes were determined: aadA1, aadA2, aadA5, aadA7, bla(PSE-1), sat1, dfrA1; and dfrA14. Our study shows a high prevalence and diversity of class 1 integrons embedded antimicrobial gene cassettes and their strong association with SGI1.