Plasmids imparting sulfonamide resistance in Escherichia coli: implications for persistence

Sulfonamide resistance remains prevalent among clinical isolates of Escherichia coli in the United Kingdom, despite a dramatic (>97%) national decline in the rate of prescription of sulfonamides in the 1990s. To investigate potential mechanisms accounting for this persistence, we characterized plasmids carrying sul2, the most prevalent determinant of sulfonamide resistance. Among 33 conjugative and 5 nonconjugative plasmids carrying sul2, resistance to other antimicrobial agents was common, but the spectrum of resistance profiles was diverse: 82%, 74%, and 45% carried resistance to ampicillin, streptomycin, and trimethoprim, respectively. Resistance to mercury was carried by 33% of the plasmids, but none conferred significant resistance to silver or to any of three disinfectants tested. The potential virulence genes iutA (aerobactin system) and traT (serum survival) were carried by 21% and 36% of the plasmids, respectively. The 33 conjugative plasmids belonged to five different incompatibility groups, FIB, B/O, I1, K/B, and P (42%, 33%, 9%, 3% and 3%, respectively), with 3 plasmids being unassigned, and to 19 similarity groups on the basis of their restriction profiles. The sequences flanking sul2 were diverse and suggested more than one mechanism of genetic mobility. The five nonconjugative plasmids were all related to p9123 (pBP1), which was previously found to confer a fitness advantage on its host. We propose that the persistence of sul2, despite the reduced rate of prescription of sulfonamides, is due to a combination of coselection by antibiotics still in common use, a lack of a selective disadvantage in sul2 carriage, and the genetic mobility of sul2.