A bioassay was developed to measure the minimum concentration of an antimicrobial drug that disrupts the colonization resistance mediated by model human intestinal microbiota against Salmonella invasion of Caco-2 intestinal cells. The bioassay was used to measure the minimum disruptive concentrations (MDCs) of drugs used in animal agriculture. The MDCs varied from 0.125 μg/ml for some broad-spectrum antimicrobial drugs (e.g., streptomycin) to 16 μg/ml for drugs with limited spectra of antimicrobial activity (e.g., lincomycin). The acceptable daily intake (ADI) residue concentration calculated on the basis of the MDCs were higher for erythromycin, lincomycin, and tylosin than the ADI residue concentrations calculated on the basis of the MICs. The MDC-based ADI values for apramycin, bacitracin, neomycin, novobiocin, penicillin G, streptomycin, tetracycline, and vancomycin were lower than the reported MIC-based ADI values. The effects of antimicrobial drugs at their MDCs on the bacterial composition of the microbiota were observed by denaturing gradient gel electrophoresis of 16S rRNA sequences amplified by PCR. Changes in the population composition of the model colonization resistance microbiota occurred simultaneously with reduced colonization resistance. The results of this study suggest that direct assessment of the effects of antimicrobial drugs on colonization resistance in an in vitro model can be useful in determining ADI values
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