Genetic determinants of trimethoprim resistance in a strain of Staphylococcus aureus.

Trimethoprim resistance was transduced from a resistant to a sensitive strain of Staphylococcus aureus by means of a cell-free phage lysate of the resistant strain. The resistance acquired by the sensitive strain was stable and could not be eliminated by growth at an elevated temperature or in the presence of acridine or ethidium bromide. This stability and the kinetics of transduction by ultraviolet-irradiated phage suggest that resistance to trimethoprim in Staph. aureus is determined by a chromosomal rather than a plasmid gene.