Location: Agroecosystem Management ResearchTitle: Selected soil antibiotic resistance gene profile changes following manure application and rainfall
|Castleberry, Bobbie - Lana|
|Millmier, Schmidt - University Of Nebraska|
Submitted to: Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2019
Publication Date: N/A
Interpretive Summary: It is common to apply beef cattle manure to fields. Since all animal feces contain antibiotic resistant bacteria and antibiotic resistance genes, application of beef cattle manure to soil has been identified as a potential source of antibiotic resistance in the environment, with subsequent risk for contamination of ground and surface waters. This study measured the number of resistant bacteria and resistance genes in soils with different levels of liquid manure accumulation, and at different depths, and compared them to a control field with no history of liquid manure application. Soil dwelling and fecal indicator bacteria (Enterococcus and Escherichia coli) resistant to three antibiotics (cefotaxime, erythromycin, and tetracycline), and two classes of antibiotic resistant genes (erm and tet) were quantified by soil depth to 1.8 m at six locations per field representing areas of varying manure accumulation. Abundance of soil dwelling bacteria, erythromycin resistant bacteria, fecal indicator bacteria, erm(C), and tet(Q) were greater on the field receiving liquid manure. Antibiotic resistant bacteria and antibiotic resistance genes decreased with depth from the soil surface to 1.8m. All three categories of antibiotic resistant bacteria decreased significantly with depth in the soil, indicating that the soil serves as a filter for these bacteria. Results for antibiotic resistance genes did not mirror those of the antibiotic resistant bacteria, indicating that the genes may persist in the soil, even when the bacteria are no longer culturable. Long-term application of liquid manure increased some targets, but not others, as compared to the control plots. These data demonstrate that individual resistance types have unique ecologies and suggest that assessment of multiple targets is advisable for monitoring resistance in applied field settings.
Technical Abstract: Land application of manure introduces gastrointestinal microbes, including antibiotic resistant bacteria and antibiotic resistance genes (ARB/G), into the environment, where they mix with soil microbes. Here, prevalence of 27 ARG targets was assessed in 20 replicate long-term, no-till plots that had not previously received manure inputs. Swine manure was land applied, followed by simulated rainfall. Manure, pre-application, and 7-day post-application samples were evaluated for tetracycline, ß-lactamase, macrolide, and sulfonamide resistance genes, and an integrase gene. For tetracycline resistance (TETr), only manure and post-application samples were positive, with a mean number of TETr genes detected 9, and 6.45, respectively. All samples were negative for the ten ß-lactamase genes assayed. An increase in the mean number of sulfonamides, macrolide, and integrase genes in the post-application soils was observed, compared to the source manure, attributed in part to warm temperatures and moisture from rain. However, on the plot level only 1/20, 5/20, and 11/20 plots showed an increase in erm(B), sul(I) and intI1, respectively, in the post-application samples. Results confirm the potential for temporary blooms of ARGs following application of manure, likely linked to soil moisture levels, and highlight both the extreme variability between different ARG targets and the uneven distribution of individual targets, even in the same soil type, and on the same farm. This heterogeneity presents a challenge for sorting the true effect from noise in field situations and needs to be considered when designing studies to evaluate impact of best management practices on reducing ARGs, and for environmental surveillance efforts.