While viable indicator bacteria provide useful baseline resistance
data, the capacity for bacteria to transfer or acquire antibiotic resistance genes stresses the importance of considering the total level of encoded resistance in a bacterial community . In addition, some bacteria may be intrinsically resistant to a class of antimicrobials, limiting their usefulness in predicting the relevance of resistance expression to dissemination of the trait . DNA-based methods P005091 are increasingly being used to monitor the level of resistance genes in environmental samples and have an advantage in that they allow for analysis of community resistance, including bacteria that are un-culturable in the laboratory. Metagenomic studies have been used to examine the prevalence of tetracycline and erythromycin resistance genes in fecal, soil, lagoon and ground water samples in agricultural environments that use antimicrobials [8–11]. However, in some instances these studies lacked detailed information on antimicrobial exposure or the extent to which these CAL-101 cost determinants persisted over time. In a previous study, we analyzed AR Escherichia coli in artificial fecal deposits originating from animals with a known history of antimicrobial-use . We observed a treatment effect on AR genes encoded by E. coli displaying a similar phenotype and also differences
in survival of AR genotypes within treatments. In the present study, we sought to extend those findings by determining if differential persistence of AR genes (tet, erm, sul) L-NAME HCl within the microbial community occurs as a result of the subtherapeutic use of antimicrobials in beef cattle production. Results Antimicrobial resistance genes in fecal deposits from cattle fed subtherapeutic levels of antimicrobial growth promoters were investigated over a 175-day period. The subtherapeutic antimicrobials were selected based on the commonality of use in the industry and included chlortetracycline (44 ppm, A44), chlortetracycline plus sulfamethazine (both at 44 ppm, AS700), tylosin phosphate
(11 ppm, T11) or no antibiotic supplementation (control). Resistance genes were quantified by real-time PCR. In addition, differences in bacterial populations, represented by 16S-rRNA, were analyzed by real-time PCR and DGGE. A detailed description of the complete feedlot experiment has been previously published . 16S-rRNA genes Copies of 16S-rRNA genes were affected by an interaction between time of fecal pat exposure and treatment (P = 0.0001, Figure 1). Generally, the concentration of 16S-rRNA increased in all treatments by day 56. Concentrations decreased thereafter, but by day 175, were not different from the concentrations on day 7. Figure 1 Quantification of 16S-rRNA in cattle fecal deposits under field selleckchem conditions.