Association of IS26-composite transposons and complex In4-type integrons generates novel multidrug resistance loci in Salmonella genomic island 1.

OBJECTIVES: Clinical isolates of Salmonella enterica serovar Haifa and Newport, which displayed extended multidrug resistance phenotypes, were investigated for the presence of Salmonella genomic island 1 (SGI1) and the genetic organization of its antibiotic resistance gene clusters.

METHODS: The S. enterica strains were isolated from humans in France in 2003 and 2004. Antibiotic susceptibility tests and various molecular techniques were used for detection and characterization of SGI1.

RESULTS: We identified SGI1 integrated in the 3' end of the chromosomal thdF gene in six multidrug-resistant serovar Haifa and Newport strains. Two strains, of serovar Haifa and Newport, harboured the previously described SGI1-H variant. A new variant of the novel SGI1-Ks group, named SGI1-K6, revealed IS26-mediated rearrangements of the antibiotic resistance gene cluster in two serovar Newport strains. Two other serovar Newport strains harboured the SGI1-L complex class 1 integron containing the dfrA15 and bla(PSE-1) resistance gene cassettes. In addition, these variants of SGI1 also contained large IS26-composite transposons inserted by a transposition event in the SGI1 backbone. These IS26-composite transposons showed a similar genetic structure to the SGI1-K variants containing an In4-type integron, a mercury resistance operon and parts of Tn1721 and Tn5393. These extended resistance gene clusters containing up to 10 antibiotic resistance genes were named SGI1-L1 and -L2.

CONCLUSIONS: The serovar Haifa represents the 16th S. enterica serovar in which SGI1 has been identified. The genomic island SGI1 appears to be a hotspot of acquisition of antibiotic resistance genes by the transposition of In4-type integrons and large IS26-composite transposons.