Activity of trimethoprim (TMP), sulphamethoxazole (SMX) and the combination against Bacteroides fragilis group isolates.

The in-vitro efficacy of trimethoprim/sulphamethoxazole (TMP/SMX) was tested against 59 isolates belonging to the Bacteroides fragilis group of bacteria and was shown to be dependent upon the inoculum size. With an inoculum of 3 X 10(5) colony forming units (cfu), 98% of these isolates were susceptible to the combination, whereas with a higher inoculum of 10(6) cfu, 88% were susceptible. None of the isolates were susceptible to less than 2 mg/l of TMP, whereas 57 (97%) were susceptible to SMX at the lower inoculum and 21 (36%) at the higher inoculum. These data indicate that TMP/SMX has moderate activity against organisms of the Bact. fragilis group when tested at an inoculum of 10(6) cfu.