An evaluation of methods to assess the effect of antimicrobial residues on the human gut flora.

UNLABELLED 1. Barrier effect. Relevant models should include an anaerobic dominant flora that antagonizes minor bacterial populations such as drug resistant E. coli. 2. Anaerobes vs. aerobes. Aerobe counts are more precise and much less time consuming than anaerobe counts. Minor populations of drug resistant aerobes are sensitive markers of the ecosystem balance, and are directly relevant to the potential risk of antimicrobial residues. 3. MIC vs. plate counts. The determination of minimum inhibitory concentrations (MIC) of selected clones in time consuming, does not detect subdominant resistance (less than 1%), and the MIC shift is difficult to test statistically. In contrast, direct counts of bacteria on drug supplemented media allows a rapid measure of minor resistant populations. 4. STATISTICS Most published designs do not include adequate statistical evaluation. This is critical for trials made in conventional humans and animals, where data are highly variable. 5. Human trials: The lowest concentration of antibiotic tested in human volunteers (2 mg oxytetracycline/d for 7d in 6 subjects) significantly increased the proportion of resistant fecal enterobacteria (P = 0.05). However, the huge day-to-day and interindividual variations of human floras make this evidence rather weak. 6. Gnotobiotic mice inoculated with human flora are living isolated models in which the effect of any antimicrobial on the human gut flora can be tested. This in vivo model does include the barrier effect of dominant anaerobes. Interindividual and day-to-day variations of bacterial populations are lower in those mice than in humans. 7. Most resistant enterobacteria in the human gut of untreated people come from bacterial contamination of raw foods. The relative contribution of residues in selecting antibiotic resistance seems to be low when compared to bacterial contamination.