A Colorimetric Assay for Antimicrobial Agents Based on Inhibition of Enzyme Production in Bacteria

Abstract

A colorimetric assay for residual antimicrobial agents was devised by examination of succinate dehydrogenase andβ-galactosidase produced byBacillus subtilisATCC 6633 andEscherichia 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 forE. 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/ml and 0.1 to 1.5μg/ml usingB. subtilis (2h-incubation) andE. coli (6h-incubation), respectively. B. subtilis was particularly useful for the assay since its spores were available.