Sensitivity of Streptococcus pyogenes to sulphamethoxazole, trimethoprim, and cotrimoxazole

When tested on Wellcotest (sensitivity test agar, Wellcome) supplemented with lysed horse blood, 59 clinical isolates of beta-haemolytic streptococci, belonging to several serological groups, are all sensitive to sulphamethoxazole, to trimethoprim, and to cotrimoxazole. The results obtained on diagnostic sensitivity test agar (Oxoid) containing lysed horse blood are comparable. When Mueller Hinton agar (Difco) is used, some of these strains are called resistant: this is especially true in the case of sulphamethoxazole.Furthermore, on Mueller Hinton agar, even supplemented with lysed blood, discrepancies between diameters of inhibition zone and MIC values exist for the three drugs tested, strains with a low MIC being considered resistant by the disc method. Such disagreements are not observed with non-streptococcal control strains.