1978 Volume 26 Issue 2 Pages 195-199
Minimal inhibitory concentrations (MIC) of Sulfamethoxazole (SMX), Trimethoprim (TMP) and Sulfamethoxazole-Trimethoprim (SMX-TMP 20 : 1) were determined using 216 strains of Serratia marcescens, 141 strains of Proteus species (Proteus mirabilis 52, Proteus vulgaris 20, Proteus rettgeri 16, Proteus morganii 42 and Proteus inconstans 11) isolated from various clinical materials submitted to our Laboratory during the period 1974 to 1975.
Methods of susceptibility testing for SMX-TMP recommended by the Ad Hoc committee of the Japan Society of Chemotherapy for MIC testing methods for SMX and TMP were applied to the determinations of MIC of TMP and SMX-TMP. MIC of SMX was tested using D. S. T. agar (Oxoid) without adding lysed horse blood. The reults were as follows.
1) MICs of SMX against all strains tested were more than 100μg/ml.
2) Seventy percent of 216 S. marcescens strains was inhibited by TMP at concentrations of less than 3.13μg/ml. Also 70 percent of 141 Proteus species was inhibited at MICs ranging from 3. 13 to 25μg/ml with only slight differences according to species.
3) Synergic action was found in SMX-TMP ; 70 percent of 216 S. marcescens strains was inhibited by an MIC of less than 25μg/ml of SMX-TMP (concentration of TMP was less than 1.2μg/ml). Proteus species were inhibited by MIC ranging from 6. 25 to 25μg/ml (concentration of TMP was from 0.3 to 1.2μg/ml) except that Proteus rettgeri and Pr. inconstans were more resistant to SMX-TMP.
4) MICs of SMX-TMP and TMP alone against pigment producing Serratia marcescens strains were lower than those of pigment negative strains.
5) The resistant strains tended to be isolated frequently from urine specimens.
