Distribution of Antibiotic Resistance Genes in Beef Cattle Feedlot Pens.

A serious concern for modern animal production is the fear that feed antimicrobials, such as monensin, increase the potential for high levels of antibiotic resistant (AR) gene prevalence in the manure, which may subsequently be shared with soil communities and eventually be taken up by human pathogens.  The objective of this study was to determine how AR gene prevalence changes in feedlot soils. In each of ten cattle feedlot pens, twelve sample sites were selected based upon a range of electromagnetic induction readings, which correlates with manure content.  Surface soil samples were collected at two time-periods from pens where animals were fed either a dry-rolled corn-based or wet-distillers grains with solubles (WDGS) diet, both containing monensin, an antimicrobial feed additive.  Soil DNA extracts were screened for two common AR genes using PCR analysis. The bla_SHV gene resulted in no positive samples, while the ermB gene was found in 78.3% of samples (n=240). No difference was shown in the presence of  ermB  in  feedlot  material from animals fed  WDGS versus the control.  At the initiation of the study, samples collected from the edge were more likely to test positive for the ermB gene than samples collected from the mound (P = 0.008).  After two years without pen cleaning, location had no statistically significant effect on ermB gene prevalence (P = 0.566).  Similarly, the prevalence of ermB over time did not change as manure accumulated.  A comparison of AR-positive and AR-negative soil parameters indicated that ermB positive soils were more manure-like (wetter, higher N, S, P, and higher volatile solids content).  Although we expected a greater prevalence of AR genes in manure-accumulating areas over time, diet and time had no effect.  These results suggest that fecal AR genes do not accumulate in cattle feedlot pens even though the fecal solids are accumulating.