Antibiotic resistance profiles in fallow soil receiving raw, composted or stockpiled beef manure, or inorganic fertilizer

Authors: ZELT, MARA - University Of Nebraska; MILLMIER-SCHMIDT, AMY - University Of Nebraska; STALEY, ZACHARY - University Of Nebraska; LI, XU - University Of Nebraska; WANG, BING - University Of Nebraska; Miller, Daniel

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 3/6/2020
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: The bacterial resistance profile in agricultural soil was monitored over the fallow season (October through April) following manure application in the central United States (Nebraska). Resistance to three common antibiotics – tylosin, azithromycin and tetracycline – was monitored following application of raw, stockpiled, or composted beef feedlot manure, or inorganic fertilizer. Assessment of antibiotic resistant (AR) bacteria and AR genes was accomplished by measuring both the live resistant bacteria [azithromycin (AZR)- and tetracycline (TETR)-resistant Escherichia coli and tylosin (TYR)- and TETR-resistant Enterococci] and AR genes [tetO, tetQ, ermB and intI1]. Twenty plots (4.5 m x 3.0 m) were arranged in 4 blocks along a single elevation field contour to prevent cross-contamination by runoff. Plots within each block were randomly assigned to one of five experimental treatments: fresh, composted, or stockpiled manure (45 Mg/ha); commercial fertilizer (N:P:K at 15-23-10 sufficient to apply 157 kg/ha); and a control. A series of two-proportions Z-tests were conducted to determine differences in prevalence of AR bacteria and genes. Differences in gene quantification were assessed using a repeated-measures analysis of variance (ANOVA) with plot as the experimental unit and sampling event as a repeated measure. Results were considered significant at a = 0.05. Where a significant difference was determined using ANOVA, Tukey’s studentized range test was used to determine differences among factors. Overall, concentrations of all monitored resistant bacteria were below the detection limit for enumeration. Analysis of bacterial prevalence following sample enrichment revealed an observable concentration of native AR bacteria in control plots throughout the study and the highest prevalence among treatments in soil receiving inorganic fertilizer. Higher prevalence of AZR was observed in stockpiled manure-amended soil than soil receiving composted or fresh manure; however, prevalence of bacteria did not vary significantly between the control plots and any of the manure treatments. Of the four genes studied, only intI1 was present in 100% of the samples. In general, the greatest gene prevalence and abundance were observed in samples from weeks 4 and 7, with prevalence decreasing to background levels by week 22. However, with the lowest initial prevalence (10%), tetO prevalence also increased following treatment application and peaked in week 7, but had not yet decreased to background concentrations by week 22. Ultimately, treatment had only one significant effect when compared to control, increasing the total prevalence of ermB in plots receiving composted manure. However, on the final sampling day, there were no significant differences among treatment for any measured ARGs. By all measures, AR increased initially following treatment applications and only inorganic fertilizer application significantly increased AR bacterial prevalence.