Laboratory study demonstrates standard streptomycin levels do not alter penicillin disk diffusion assays using Sarcina lutea, indicating penicillin can be measured accurately in combined therapy.
Microbiological assay of individual antibiotics in mixtures of antibiotics depends on the use of selective inactivation and/or of test bacteria with differential susceptibility. Controlled experiments revealed that streptomycin in concentrations of 20 and 40 mug/ml did not influence a disk diffusion assay of penicillin with Sarcina lutea (ATCC 9341) as the test organism. In the case of penicillin concentrations less than or equal to 1 IU/ml, addition of 80 mug of streptomycin per ml influenced the penicillin assay significantly. Clinical use of streptomycin resulting in levels above 40 mug/ml usually did not occur; therefore penicillin could be assayed as though streptomycin were not present. We observed additionally that S. lutea was unable to grow on agar plates prepared with semicarbazide hydrochloride.
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Dennis Raahave (1974) studied this question.
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