Current status of models for testing antibiotic residues

Models for testing the effect of antibiotic residues on the human gut flora should be (i) feasible (quality of data, statistics, price, duration), (ii) sensitive (baseline resistance, background variability, response to low doses), and (iii relevant (composition and ecology of the flora, predictive value). An evaluation of existing models led to the following conclusions: 1. Flora. Relevant models must include anaerobic flora that antagonize colonization by minor bacterial populations. In vitro tests on pure strains, batch or continuous flow cultures, do not take this barrier effect into account. 2. Drug treatment. Treatment should be long enough to allow the detection of indirect selection due to a weakening of the barrier effect (i.e., greater than 15 days). For dose response analysis, trials should include at least 3 groups: a control group, an experimental group given the antibiotic at residue levels, and a positive control group given a high dose of the same drug. 3. Target strain(s). The target bacteria should be identified, resident in the gut, accurately countable, drug susceptible but with drug resistant variants, and related to a pathogen. Some gut aerobes meet these requirements: sensitive to the ecosystem balance, easy to count accurately, opportunistic pathogens, and R-plasmid carriers. Total anaerobes are not suitable. An anaerobe species may serve as a target strain if it is particularly susceptible to, or selected by, the test antibiotic. Thus, each in vivo protocol should be based on in vitro data. 4. Measure of resistance. The determination of the minimum inhibitory concentration (MIC) of selected clones from the model flora is time consuming, does not detect subdominant resistance (less than 1%); and a MIC shift is difficult to test statistically. In contrast, direct counting of bacteria on drug supplemented media allows for the rapid measure of minor resistant populations. 5. Statistics. Most published designs do not include a power calculation, nor yield enough data points for accurate statistical analysis. Sufficient statistical power is critical for studies using conventional humans and animals, because the data are highly variable and not normally distributed. 6. Human trials. The lowest level of antibiotic tested in human volunteers (2 mg oxytetracycline/day for 7 days in 6 subjects) increased the proportion of resistant fecal enterobacteria (p = 0.05). However, the large day-to-day and inter individual variation in the human flora limit the significance of this change. Confirmation of this finding would require a large number of volunteers.(ABSTRACT TRUNCATED AT 400 WORDS)