AN EVALUATION OF METHODS TO ASSESS THE EFFECT OF ANTIMICROBIAL RESIDUES ON THE HUMAN GUT FLORA

1. Barrier effect. Relevant models should include an anaerobic dominan t flora that antagonizes minor bacterial populations such as drug resi stant E. coli. 2. Anaerobes vs. aerobes. Aerobe counts are more precis e and much less time consuming than anaerobe counts. Minor populations of drug resistant aerobes are sensitive markers of the ecosystem bala nce, and are directly relevant to the potential risk of antimicrobial residues. 3. MIC vs. plate counts. The determination of minimum inhibi tory concentrations (MIC) of selected clones is time consuming, does n ot detect subdominant resistance (less than 1 %), and the MIC shift is difficult to test statistically. In contrast, direct counts of bacter ia on drug supplemented media allows a rapid measure of minor resistan t populations. 4. Statistics. Most published designs do not include ad equate statistical evaluation. This is critical for trials made in con ventional humans and animals, where data are highly variable. 5. Human trials. The lowest concentration of antibiotic tested in human volunt eers (2mg oxytetra-cycline/d for 7d in 6 subjects) significantly incre ased the proportion of resistant fecal enterobacteria (P=0.05). Howeve r, the huge day-to-day and interindividual variations of human floras make this evidence rather weak. 6. Gnotobiotic mice inoculated with hu man flora are living isolated models in which the effect of any antimi crobial 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 r elative contribution of residues in selecting antibiotic resistance se ems to be low when compared to bacterial contamination.