Dissemination of the strA-strB streptomycin-resistance genes among commensal and pathogenic bacteria from humans, animals, and plants.

Gene transfer within bacterial communities has been recognized as a major contributor in the recent evolution of antibiotic resistance on a global scale. The linked strA-strB genes, which encode streptomycin-inactivating enzymes, are distributed worldwide and confer streptomycin resistance in at least 17 genera of gram-negative bacteria. Nucleotide sequence analyses suggest that strA-strB have been recently disseminated. In bacterial isolates from humans and animals, strA-strB are often linked with the suIII sulfonamide-resistance gene and are encoded on broad-host-range nonconjugative plasmids. In bacterial isolates from plants, strA-strB are encoded on the Tn3-type transposon Tn5393 which is generally borne on conjugative plasmids. The wide distribution of the strA-strB genes in the environment suggests that gene transfer events between human, animal, and plant-associated bacteria have occurred. Although the usage of streptomycin in clinical medicine and animal husbandry has diminished, the persistence of strA-strB in bacterial populations implies that factors other than direct antibiotic selection are involved in maintenance of these genes.