Lack of homology of enterococci which have high-level resistance to trimethoprim with the dfrA gene of Staphylococcus aureus.

Multiresistant enterococci were tested for susceptibility to trimethoprim (TMP). Although most enterococci are inhibited by less than or equal to 1.0 microgram/ml, the MICs for 7 of 29 selected multiresistant isolates were greater than or equal to 8 micrograms/ml, including for two beta-lactamase positive (Bla+) strains, for which the MICs of TMP were greater than 1,000 micrograms/ml, and for another Bla+ strain, for which the MIC was 128 micrograms/ml. None of five isolates tested transferred TMP resistance and none of the resistant isolates hybridized to the dfrA gene of Staphylococcus aureus. Whether TMP resistance in enterococci is due to a mutation(s) or to acquisition of a new gene is not known. Acquisition of resistance to TMP is another example of the multiple antimicrobial resistance typically displayed by enterococci.