Plasmids imparting sulfonamide resistance in Escherichia coli: implications for persistence

Abstract

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.

FIG. 1.

Organization of genes adjacent to sul2. (A) BamHI fragments cloned from conjugative sul2-containing plasmids. The sequenced regions are indicated by bold lines and are filled in gray, with the positions of the sequencing primers indicated by small arrows. Homology to other genes was determined by the amplification of overlapping PCR products of the predicted length (repA, repC, sul2, strA, strB) and by partial restriction site conservation (trbA-trbC). Dotted lines represent uncharacterized DNA. The scale is in bp. B, BamHI restriction sites. (B) Representative GenBank submissions with different sul2 genes in different contexts. The positions of the diagnostic PCR products used to distinguish the upstream regions are indicated above the sequences. For pO113, the serrated line indicates the region that is replaced by a sequence that includes sul2 in plasmids of group (Grp) 2D.