Distinct recent lineages of the strA- strB streptomycin-resistance genes in clinical and environmental bacteria.

We report the linkage of the strA-strB streptomycin-resistance genes with Class 1 integron sequences on pSTR1, a 75-kb multiple antibiotic-resistance plasmid from Shigella flexneri. strA-strB had previously been detected only within Tn 5393, a Tn 3-family transposon, and on small nonconjugative broad-host-range plasmids such as RSF1010. The geographic range of Tn 5393 was also extended to Pseudomonas spp. isolated from apple trees in New Zealand and soil in the USA. Comparative sequence analyses indicated that strA-strB from Tn 5393 and nonconjugative plasmids constitute distinct recent lineages with strA-strB from pSTR1 intermediate between the other two. The carriage of strA-strB within an integron, a transposon, and on broad-host-range plasmids has facilitated the world-wide dissemination of this determinant among at least 21 bacterial genera.