Transferable resistance plasmids as a contributory cause of increasing trimethoprim resistance in general practice

Abstract

The incidence of trimethoprim resistance found in enterobacteria causing urinary tract infection in general practice was surveyed over six-month periods in each year from 1978 to 1981 and was found to be steadily rising. Amongst strains of Escherichia coli, transmissible plasmids accounted directly for about 40% of the resistance observed. An increasing number of plasmid incompatibility groups was detected and it was concluded that increasing resistance in E. coli was linked to an increase in the pool of transmissible resistance genes in the community. In contrast, only two plasmids were transferred from Klebsiella spp. (6% of isolates) and none from isolates of Proteus spp. Resistance in Klebsiella spp. seemed to be chromosomal rather than plasmid-mediated and it was concluded that the increased frequency of resistance in this genus was a consequence of increased use of trimethoprim rather than an increase in the pool of transmissible genes. With the strains of Proteus spp. isolated, the MIC values found were indicative of the presence of plasmid genes, although no transmissible plasmids were detected. This may reflect a general tendency for transmissible plasmids to be unstable in Proteus spp.