Multiple mechanisms of trimethoprim resistance in strains of Escherichia coli from a patient treated with long-term co-trimoxazole

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Abstract
A series of strains of Escherichia coli, of different sero- or biotype, isolated from one patient after long-term treatment with co-trimoxazole, showed trimethoprim resistance that was determined in at least three different ways. In one type it was chromosomally-determined, the biochemical mechanism being unknown. A trimethoprim R plasmid (characterized by M.wt and incompatibility group) was present in strains of 4 different types, indicating in vivo transfer. The biochemical mechanism was the production of plasmid-coded, trimethoprim resistant dihydrofolate reductase. A second unrelated trimethoprim R plasmid was also found in one of the latter types.