Unusual expression of new low-level-trimethoprim-resistance plasmids.

In a survey, 42% of trimethoprim-resistant clinical members of the family Enterobacteriaceae were able to transfer trimethoprim resistance to standard Escherichia coli strains when selection was made on complex bacteriological media. When transfer experiments were performed with minimal medium, another 16% of the clinical strains were shown to have transferred trimethoprim resistance. Twelve transconjugants produced negligible trimethoprim resistance in complex media but were resistant in minimal medium. The methionine, glycine, and purine components of complex media appeared to be responsible for the reduced expression of trimethoprim resistance in these strains.