Effects of Orally Administered Tetracycline on the Intestinal Community Structure of Chickens and on tet Determinant Carriage by Commensal Bacteria and Campylobacter jejuni

Abstract

There is a growing concern that antibiotic usage in animal production has selected for resistant food-borne bacteria. Since tetracyclines are common therapeutic antibiotics used in poultry production, we sought to evaluate the effects of oral administration on the resistance of poultry commensal bacteria and the intestinal bacterial community structure. The diversity indices calculated from terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA amplicons did not indicate significant changes in the cecal bacterial community in response to oxytetracycline. To evaluate its effects on cultivable commensals, Enterococcus spp., Escherichia coli, and Campylobacter spp. were isolated from the cecal droppings of broiler chickens. Enterococcus spp. and E. coli expressed tetracycline MICs of >8 g/ml and harbored a variety of tet resistance determinants regardless of the tetracycline exposure history of the birds. The enterococcal isolates possessed tetM (61%), tetL (25.4%), and tetK (1.3%), as well as tetO (52.5%), the determinant known to confer a tetracycline resistance phenotype in Campylobacter jejuni. E. coli isolates harbored tetA (32.2%) or tetB (30.5%). Tetracycline MICs remained at <2 g/ml for Campylobacter isolates before and after tetracycline treatment of the chickens, even though isolates expressing MICs of >16 g/ml were commonly cultured from flocks that did not receive oxytetracycline. The results imply that complex ecological and genetic factors contribute to the prevalence of antibiotic resistance arising from resistance gene transfer in the production environment.

From Structure to Function: the Ecology of Host-Associated Microbial Communities

“…Chickens and pigs, for example, are routinely given antibiotics as a dietary supplement. In one study, it was found that oxytetracycline had no effect on the richness, diversity, composition, or structure of the intestinal community in chickens, as determined by T-RFLP analysis (87). In contrast, other studies have shown that virginiamycin and a cocktail of bacitracin and salinomycin both alter community structure (74,84).…”

Application of Methods for Identifying Broiler Chicken Gut Bacterial Species Linked with Increased Energy Metabolism

“…Thus, the potential of T-RFLP has not been fully utilized. Many studies using T-RFLP have based analysis on the visual interpretation of graphical T-RFLP profile outputs in order to identify the presence or absence of particular peaks indicative of bacterial species (27,29,30,52). Such studies have also typically been based on small numbers of individual samples or even pooled samples per treatment.…”

Food Animals and Antimicrobials: Impacts on Human Health

“…Other findings suggest that complex ecologic and genetic factors may play a role in perpetuating resistance (63). Resistance (particularly to tetracycline, erythromycin, and ampicillin) has been found inherently in some antibiotic-free animals, (10,45,93,130), suggesting that its emergence is related to other factors, such as diet, animal age, specific farm type, cohort variables, and environmental pressures (26).…”