Molecular epidemiology of the SRL pathogenicity island

Abstract

The Shigella resistance locus (SRL), which is carried on the SRL pathogenicity island (PAI) in Shigella flexneri 2a YSH6000, mediates resistance to the antibiotics streptomycin, ampicillin, chloramphenicol, and tetracycline. In the present study, we investigated the distribution and structural variation of the SRL and the SRL PAI in 71 Shigella isolates and 28 other enteric pathogens by PCR and Southern analysis. The SRL and SRL-related loci, although absent from the other enteric pathogens evaluated in this study, were found to be present in a number of Shigella isolates. SRL PAI markers were also present in the majority of strains carrying the SRL and SRL-related loci. PCR linkage studies with six of these strains demonstrated that the SRL is carried on elements similar in structure and organization to the YSH6000 SRL PAI, consistent with the hypothesis that the SRL PAI may be involved in the spread of multiple-antibiotic resistance in these strains.

FIG. 1.

Schematic representation of the genetic organization of the SRL PAI in strain YSH6000. (A) The SRL PAI is represented as a thin black line, with the positions and directions of relevant genes and open reading frames represented as arrows. The SRL is indicated by the large shaded box. Thick lines represent the regions amplified in linkage analysis of the SRL, with the corresponding numbers indicating their sizes (in kilobases) in the YSH6000 SRL PAI. Small black boxes represent the positions of the markers that were probed by Southern analysis or PCR amplification: box A, int; box B, fecA; and box C, orf58. (B) Thick black lines represent the regions amplified in linkage analyses of SBA1299 (S. flexneri 1a), SBA1303 (S. sonnei form I), SBA1304 (S. dysenteriae 3), SBA1308 (S. boydii 4), SBA1386 (S. boydii 8), and SBA1391 (S. flexneri 4a), with the corresponding numbers indicating their sizes (in kilobases).