Between January, 1981 and December, 1984, 419 strains of enterobacteria isolated from patients at the Hôpital Saint-Joseph were studied for (1) the level of resistance to trimethoprim (Tp) by determination of minimal inhibitory concentration (MIC), (2) transferability of this resistance by conjugation into Escherichia coli, (3) plasmid content of wild-type strains and transconjugants by agarose gel electrophoresis of crude bacterial lysates and by incompatibility grouping, and (4) type of dihydrofolate reductase (DHFR) by colony hybridization with probes specific for DHFR types I and II. Tp resistance was defined as MIC greater than or equal to 4 micrograms/ml and high-level resistance by MIC greater than or equal to 1000 micrograms/ml. Amongst the strains studied, 90% were resistant to high levels of Tp, while 10% had low-level resistance to Tp was detected in 180 strains corresponding to 185 plasmids. In the vast majority of the plasmids, resistance to Tp was associated with resistance to sulphonamide (94%), streptomycin (75-90%), ampicillin (75-90%) and chloramphenicol (65-80%). Plasmids conferring resistance to Tp were often large, most (84%) ranging in size from 90 to 175 Kb. They belonged to six different incompatibility groups and Inc6-C was the most prevalent (34 to 75%). The study of the distribution of the dfr genes by colony hybridization in 183 transconjugants and 89 strains with non-transferable Tp resistance revealed the presence of dfrI genes in most of these strains (48 and 53%, respectively). DHFR of types I and II were found in only 3% of the transconjugants, but in 15% of the strains with non-transferable resistance. DHFR of other types were found equally (15%) in strains with transferable and non-transferable resistance. The high incidence of the type I enzyme among the Tp-resistant strains probably results from the integration of transposon Tn7 into the chromosome or into a non-transferable plasmid.
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