Emergence of sulfamethoxazole-trimethoprim resistant Shigella flexneri in Northeastern Brazil.

In contrast to prior experience in this setting, three of four Shigella flexneri strains recently isolated from patients in Northeastern Brazil with acute inflammatory diarrhea were found to be resistant to sulfamethoxazole, trimethoprim and the combination in vitro. We performed mating studies to determine if the resistance was transferable, and then isolated and characterized plasmid DNA from the resistant Shigella isolates, other resistant Enterobacteriaceae isolated simultaneously from the stools of these individuals, and transconjugant strains. Each of the resistant Shigella strains contained a large plasmid. These plasmids were of different molecular weights ranging from 30 to 50 Mdal in size. Two of these plasmids were transferred with sulfamethoxazole-trimethoprim resistance to E. coli K-12 recipient strains. These findings of transferable resistance to sulfamethoxazole-trimethoprim associated with plasmids in Shigella and in other Enterobacteriaceae raises concerns about the potential limitations of this widely used antimicrobial combination.