THE EFFECT OF DIFFERENT SERUM CONCENTRATIONS OF ANTIMICROBIAL AGENTS ON THE LACTOBACILLUS CASEI FOLATE ASSAY

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Abstract

Seventeen antimicrobial agents were added to serum in vitro and tested for their ability to influence the assay of folic acid activity by the L. casei method. Eight drugs were found to cause significant inhibition. The lowest serum concentrations needed to reduce the estimated folic acid activity by as much as 25 per cent were 0.5–1.0 μg/ml trimethoprim, just below 2.5 μg/ml carbenicillin, 2.5–5 μg/ml lincomycin or phenoxymethylpenicillin, 5–10 μg/ml erythromycin, 10–25 μg/ml benzylpenicillin, just above 25 μg/ml ampicillin and 25–125 μg/ml chloramphenicol.

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