[Antibiotic residues and digestive microflora].

Can antibiotic residues modify the human gut flora, and select drug resistant bacteria? In volunteers given ampicillin (1.5 mg/d) or oxytetracyclin (2 mg/d), the fecal excretion of resistant enterobacteria was not changed significantly. However a possible effect would be hidden in human beings by the huge day to day fluctuations in the resistant bacterial populations. Heteroxenic mice (i.e. germfree mice associated with the flora of a human donor) and dixenic mice (i.e. mice harboring 2 isogenic strains, one of which carries an R-plasmid) are possible models to study the gut flora in vivo without contaminations and interfering factors. Minimum selecting doses in these models are between 0.5 and 10 mg antibiotic per liter of drinking water. These doses are smaller than what a consumer could ingest in food. Last, fecal resistant enterobacteria directly come from contamined food, as shown by their clearance from stools of volunteers eating a sterile diet. Hence antibiotic residues do not modify meaningfully the gut flora.