Antimicrobial resistance and the ecology of Escherichia coli plasmids.

Four hundred and seven clinical isolates of Escherichia coli were examined for the presence of plasmids. These isolates comprised 189 which were collected irrespective of antimicrobial resistance (VP) and 218 which were collected on the basis of high-level trimethoprim resistance (TPR). The VP isolates were divided into drug sensitive (VPS) and drug-resistant (VPR) subpopulations. Plasmids were detected in 88% of VP isolates (81% of VPS and 94% of VPR) and 98% of TPR isolates. The distribution of plasmids in both groups and subpopulations was very similar. However, there were small but statistically significant differences between the plasmid distributions. These showed that more isolates in the resistant groups harboured plasmids than in the sensitive subpopulation (VPS) and that the number of plasmids carried by resistant isolates was greater. Multiple drug resistance was significantly more common among TPR isolates than the VPR subpopulation and this was paralleled by increased numbers of plasmids. Fifty-eight per cent of VPR and 57% of TPR isolates transferred antimicrobial resistance and plasmids to E. coli K12. Of the R+ isolates, 60% carried small plasmids (MW less than 20Md) and 52% of these co-transferred with R-plasmids. These results are discussed.

Sequential acquisition of R-plasmids in vivo by Salmonella typhimurium.

Salmonella typhimurium, resistant only to trimethoprim and sulphamethoxazole, was isolated from the faeces and blood of a chronic alcoholic patient in acute renal failure. The isolates harboured an 18 Md non-conjugative plasmid. He was dialysed peritoneally and treated with ampicillin; four days later there was no clinical improvement and his peritoneal dialysis fluid (PDF) had become infected. Salm. typhimurium was isolated from faeces and PDF. Both isolates were additionally resistant to ampicillin and contained two plasmids (55 Md and 18 Md). Therapy was changed to chloramphenicol and gentamicin was added to the PDF. Two weeks later Salm. typhimurium was again isolated from PDF and faeces. The PDF isolate was unchanged but 4% of the colonies isolated from this faecal specimen were resistant to chloramphenicol and had acquired an additional 62 Md plasmid. From all PDF and faecal specimens two different strains of Escherichia coli and one strain of Klebsiella pneumoniae were isolated which contained plasmids indistinguishable, on the basis of molecular weight and transferable resistance markers, from those acquired by Salm. typhimurium. The transferability of these plasmids in vitro to E. coli K12 and to the patient's initial Salm. typhimurium was studied and the results discussed.

Characterization of spontaneous resistant variants of Serratia marcescens selected in the presence of carbenicillin.

Spontaneous carbenicillin-resistant variants were isolated from pigmented Serratia marcescens GRI 2677 with frequencies as high as 1 in 7.4 x 10(4). One-hundred and thirteen such variants were characterized with respect to pigmentation, antimicrobial susceptibility and beta-lactamase production. Sixty-eight were less pigmented than the parent culture, 84 showed low level resistance to five aminoglycosides and 17 showed increased beta-lactamase activity. Thirty-three variants were unstable and reverted to the parental phenotype at high frequency. One designated GRI 2677-8, was additionally auxotrophic and was further characterized. On the basis of these criteria the variants were assigned to one of 21 sub-groups. These results are discussed in relation to resistant clinical isolates and a possible mechanism that explains the diversity of variants.