Longitudinal characterization of antimicrobial resistance genes in feces shed from cattle fed different subtherapeutic antibiotics

Abstract

BackgroundEnvironmental transmission of antimicrobial-resistant bacteria and resistance gene determinants originating from livestock is affected by their persistence in agricultural-related matrices. This study investigated the effects of administering subtherapeutic concentrations of antimicrobials to beef cattle on the abundance and persistence of resistance genes within the microbial community of fecal deposits. Cattle (three pens per treatment, 10 steers per pen) were administered chlortetracycline, chlortetracycline plus sulfamethazine, tylosin, or no antimicrobials (control). Model fecal deposits (n = 3) were prepared by mixing fresh feces from each pen into a single composite sample. Real-time PCR was used to measure concentrations of tet, sul and erm resistance genes in DNA extracted from composites over 175 days of environmental exposure in the field. The microbial communities were analyzed by quantification and denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S-rRNA.ResultsThe concentrations of 16S-rRNA in feces were similar across treatments and increased by day 56, declining thereafter. DGGE profiles of 16S-rRNA differed amongst treatments and with time, illustrating temporal shifts in microbial communities. All measured resistance gene determinants were quantifiable in feces after 175 days. Antimicrobial treatment differentially affected the abundance of certain resistance genes but generally not their persistence. In the first 56 days, concentrations of tet(B), tet(C), sul1, sul2, erm(A) tended to increase, and decline thereafter, whereas tet(M) and tet(W) gradually declined over 175 days. At day 7, the concentration of erm(X) was greatest in feces from cattle fed tylosin, compared to all other treatments.ConclusionThe abundance of genes coding for antimicrobial resistance in bovine feces can be affected by inclusion of antibiotics in the feed. Resistance genes can persist in feces from cattle beyond 175 days with concentrations of some genes increasing with time. Management practices that accelerate DNA degradation such as frequent land application or composting of manure may reduce the extent to which bovine feces serves as a reservoir of antimicrobial resistance.

Fate of antimicrobials and antimicrobial resistance genes in simulated swine manure storage

“…The phenomenon of different ARGs exhibiting different behaviors is not uncommon in manure or soil systems. Alexander and co-workers used qPCR to monitor the abundance of ARGs in cattle fecal deposits and their results showed that some ARGs (i.e., tet(B), tet(C), sul(I), sul(II), and erm(A)) first increased and then declined, while other ARGs (tet(M) and tet(W)) gradually decreased over 175 days (Alexander et al, 2011). Fig.…”

Effect of oxytetracycline as well as an acid-based feed supplement on the prevalence and abundance of selected antibiotic resistance genes in weaned piglets

“…The levels of tet genes were not reduced in faeces of piglets additionally administered oxytetracycline, which was indicated by stable tetA and tetB log 10 gene copy numbers at both time points (d10, d28), compared to declining ones in nonantibiotic treatments. Previous studies reported elevated levels of resistant bacteria or resistance genes under selective pressure, revealing a clear association between antibiotic consumption and antibiotic resistance [31][32][33]. Interestingly, shedding of tetraycline resistant bacteria in faecal samples continued after withdrawal of the antibiotic, indicating a long-lasting effect of antibiotic supplementation.…”

Fate of non O157 Shigatoxigenic Escherichia coli in ovine manure composting

“…Most studies on the persistence of antimicrobial-resistant bacteria in livestock waste have focused on large scale management systems including stored manure (Duriez and Topp, 2007) or manure applied to soil (Schmitt et al, 2006) and have used viable bacteria to describe resistance levels. Alexander et al (2011) investigated the persistence of antimicrobial resistance genes sampled in E. coli strains from cattle feces under ambient field conditions and they reported persistence for 175 days. In this study a high degree of resistance to 10 antimicrobial drugs was found among STEC non-O157 strains and also in non-STEC strains, in agreement with the data reported by Mora et al (2005), who verified that there was no difference in the antimicrobial resistance patterns of STEC strains when compared to non-STEC strains.…”