Low trimethoprim susceptibility of anaerobic bacteria due to insensitive dihydrofolate reductases.

All the 28 Bacteroides fragilis strains investigated were susceptible to sulfamethoxazole (minimal inhibitory concentration < 16 mug/ml) and resistant to trimethoprim (TMP; minimal inhibitory concentration > 4 mug/ml). Synergism between sulfamethoxazole and TMP was present in all strains at a ratio of 1:1. The few clostridia investigated proved more resistant to both compounds. Dihydrofolate reductases from B. fragilis, C. perfringens, and some other anaerobic species were isolated. Inhibition profiles with six structurally different inhibitors revealed major differences in all enzymes. For 50% inhibition, the enzyme from B. fragilis and all clostridia required concentrations of TMP which were between several hundredfold and 1,000-fold higher than those required for the enzyme of Escherichia coli, whereas the enzyme from Propionibacterium acnes only needed a threefold higher concentration. In vitro activities of TMP were seen to correspond to the activity at the enzymatic level in B. fragilis and P. acnes, but correspond to a much lesser extent to the activity at the enzymatic level in clostridia, where a poor penetration is assumed to be involved. Dihydrofolate reductase inhibitors other than TMP were found to be as active as TMP both at the enzyme and in vitro. In B. fragilis, higher concentrations of exogenous thymidine were required for increasing the minimal inhibitory concentration of TMP than in E. coli and probably also in C. perfringens.