The effect of bambermycin, carbadox, chlortetracycline and olaquindox on antibiotic resistance in intestinal coliforms: a new animal model.

Groups of germ-free mice kept in isolators and associated with faecal microflora from piglets were continuously given either water or a solution of one of the following: chlortetracycline (20 micrograms/ml), carbadox (50 micrograms/ml), olaquindox (50 micrograms/ml), bambermycin (flavomycin) (5 micrograms/ml) or mixtures of these drugs. The proportions of lactose-fermenting bacteria in their faeces which were resistant to chlortetracycline, carbadox or olaquindox were measured by a comparative plate-counting procedure. Compared to occurrence in control mice, the occurrence of antimicrobial drug-resistant bacteria was higher in mice receiving chlortetracycline (P less than 0.001) and lower in mice receiving bambermycins (P less than 0.005). In contrast, olaquindox and carbadox did not change the proportion of resistant coliforms in mice faeces. A control experiment was conducted with five groups of germ-free mice given the same flora and kept without drugs in separate isolators. No difference in the occurrence of resistant coliforms could be found between these groups. The germ-free mouse associated with faecal microflora from a conventional animal seems to be a suitable model for determining in vivo the effect of low doses of antimicrobial drugs on drug resistance in lactose-fermenting enteric flora.