Transfer of plasmid-mediated CTX-M-9 from Salmonella enterica serotype Virchow to Enterobacteriaceae in human flora-associated rats treated with cefixime.

Food animals are a potential source of CTX-M resistance genes for humans. We evaluated the transfer of the bla(CTX-M-9) gene from an animal strain of Salmonella enterica serotype Virchow to Enterobacteriaceae of the human intestinal flora by using human flora-associated (HFA) rats with and without cefixime treatment. In the absence of antibiotic, no transconjugant enterobacteria were found in the feces of HFA rats. However, the transfer rate was high if Escherichia coli J5 recipient strains were coinoculated orally with Salmonella. S. enterica serotype Virchow persisted in the rat fecal flora both during and after treatment with therapeutic doses of cefixime. The drug did not increase the transfer rate, and E. coli J5 transconjugants were eliminated from the flora before the end of cefixime treatment. No cefixime was recovered in the rat feces. In the presence of recipient strains, the bla(CTX-M-9) resistance gene was transferred from a strain of animal origin to the human intestinal flora, although transconjugant colonization was transient. Antibiotic use enhanced the persistence of donor strains, increasing the resistance gene pool and the risk of its spread.

Use of avilamycin for growth promotion and avilamycin-resistance among Enterococcus faecium from broilers in a matched case-control study in France.

We assessed the putative link between avilamycin-resistant Enterococcus faecium carriage and avilamycin consumption in broilers. As part of the French programme of monitoring for antimicrobial resistance, broilers sampled at slaughterhouse in 1999 and 2000 and carrying avilamycin-resistant E. faecium were matched by slaughterhouse, slaughter month and production type (free-range, standard, light) with control broilers carrying avilamycin-susceptible strains. History of antibiotics consumption (either for growth promotion or therapeutic purpose) in the broiler flocks sampled was collected from the monitoring programme and consumption of each antibiotic class was screened as a potential risk factor. Avilamycin was a risk factor for avilamycin-resistant E. faecium carriage: OR=2.3.

Evaluation of residual and therapeutic doses of tetracycline in the human-flora-associated (HFA) mice model.

In order to evaluate an in vivo model system for assessing the effect of therapeutic and residue levels of tetracycline on human intestinal microflora, tetracycline was administered via drinking water (1, 10, and 100 mg/liter) to human-flora-associated (HFA) male and female mice. The effects of the antibiotic on fecal aerobic and anaerobic populations, selection of bacteria resistant to tetracycline, metabolic parameters of the microflora, and maintenance of the intestinal barrier against exogenous Salmonella (resistance to colonization) were recorded. In both sexes of mice, tetracycline exposure at 10 and 100 mg/liter induced the selection of several resistant bacterial species (Gram-positive anaerobes, Bacteroides fragilis, enterobacteria, and enterococci). This effect was also observed at the lowest dose (1 mg/liter) in female mice and indicates the potential sensitivity of this endpoint for evaluating the microbiological risk of tetracycline residues. The resistance to colonization was impaired at 100 mg/liter, a concentration corresponding to about half of the therapeutic doses in humans and animals. Metabolic parameters of the microflora were not affected by tetracycline at all levels. In this study, the no-observed-effect level (NOEL) of tetracycline on intestinal flora in this study was less than 1 mg of tetracycline per liter of drinking water. This concentration in the mouse corresponds to 0.125 mg of tetracycline per kilogram of body weight per day. Within the constraints of the experimental design employed here, the HFA mice model proved to be acceptable for studying dose-related effects of tetracycline on human intestinal microflora.