A colorimetric assay for residual antimicrobial agents was devised by examination of succinate dehydrogenase andβ-galactosidase produced by
Bacillus subtilisATCC 6633 and
Escherichia coliK12, respectively. To the organisms grown in Mueller-Hinton broth containing ampicillin, kanamycin, oxytetracycline or sulphadimethoxine, was added either 2, 3, 5-triphenyltetrazolium chloride for B. subtilis or resorufin-β-Dgalactopyranoside for
E. colias the substrate, and the optical density at 550nm was compared with that of controls. Antimicrobial agents were detected at concentrations of 0.05 to 0.4μg/m
l and 0.1 to 1.5μg/m
l using
B. subtilis (2h-incubation) and
E. coli (6h-incubation), respectively. B. subtilis was particularly useful for the assay since its spores were available.
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