Determination of antibiotic chemicals using microbiological tests: evaluation of the limits of sensitivity

Sensitivity of Bacillus subtilis BGA and Bacillus stearothermophilus var. calidolactis disc assays to 53 chemio-antibiotics was tested. Test-microorganisms were sown in two different mediums: PM Indicator Agar, Difco U.S.A., and Standard II Nutrient Agar, Merck Germany, modified according to Nouws. The mediums were used with or without addition of Trimethoprim (at a concentration of 0.12 or 0.024 mcg/ml of medium for B. subtilis and for B. stearothermophilus respectively). B. stearothermophilus did not grow in Standard II. However, the B. subtilis assay gave the best results with Standard II, apart for aminoglycosides. The B. stearothermophilus disc assay was the most sensitive to penicillins (Minimum Inhibiting Concentration in mcg/ml, MIC, between 0.001 and 0.004), cephalosporins (MIC between 0.003 and 0.09, apart from Ceftazidime, 0.3) and aminoglycosides (MIC between 0.03 and 0.6). The B.subtilis disc assay showed better sensitivity to quinolines (MIC between 0.05 and 4) and to some tetracyclines (oxytetracycline and chlortetracycline, MIC 0.03). Trimethoprim, where added, determined a higher sensitivity to sulfonamides (MIC between 0.025 and 0.25).