Antibiotics, bacteria, and antibiotic resistance genes: aerial transport from cattle feed yards via particulate matter

Abstract

Background: Emergence and spread of antibiotic resistance has become a global health threat and is often linked with overuse and misuse of clinical and veterinary chemotherapeutic agents. Modern industrial-scale animal feeding operations rely extensively on veterinary pharmaceuticals, including antibiotics, to augment animal growth. Following excretion, antibiotics are transported through the environment via runoff, leaching, and land application of manure; however, airborne transport from feed yards has not been characterized.

Objectives: The goal of this study was to determine the extent to which antibiotics, antibiotic resistance genes (ARG), and ruminant-associated microbes are aerially dispersed via particulate matter (PM) derived from large-scale beef cattle feed yards.

Methods: PM was collected downwind and upwind of 10 beef cattle feed yards. After extraction from PM, five veterinary antibiotics were quantified via high-performance liquid chromatography with tandem mass spectrometry, ARG were quantified via targeted quantitative polymerase chain reaction, and microbial community diversity was analyzed via 16S rRNA amplification and sequencing.

Results: Airborne PM derived from feed yards facilitated dispersal of several veterinary antibiotics, as well as microbial communities containing ARG. Concentrations of several antibiotics in airborne PM immediately downwind of feed yards ranged from 0.5 to 4.6 μg/g of PM. Microbial communities of PM collected downwind of feed yards were enriched with ruminant-associated taxa and were distinct when compared to upwind PM assemblages. Furthermore, genes encoding resistance to tetracycline antibiotics were significantly more abundant in PM collected downwind of feed yards as compared to upwind.

Conclusions: Wind-dispersed PM from feed yards harbors antibiotics, bacteria, and ARGs.

Figure 1

Concentrations (ng/g PM) of five targeted veterinary antibiotics measured in PM collected immediately downwind of feed yards (n = 10). Abbreviations: CTC, chlortetracycline; MON, monensin; OTC, oxytetracycline; TC, tetracycline; TYL, tylosin. Individual data points represent concentrations from each feed yard. Boxes extend from the 25th to the 75th percentile, solid horizontal lines represent the median of each distribution, and dotted horizontal lines represent the mean. Numbers within the graph indicate the specific feed yard. Feed yard 4 had consistently elevated concentrations of antibiotics.

Figure 2

Biplot illustrating Principal Coordinates Analysis (PCoA) of microbial assemblages in airborne PM samples collected upwind and downwind of feed yards located in the Southern High Plains (USA). Each data point represents a sample, and samples from the same feed yard are linked together via a gray line. Two upwind samples were removed from the analysis because of an insufficient number of quality sequencing reads. The proximity of points in this biplot is an approximation of the similarity of samples with respect to their microbiome composition; similarities were measured using the weighted UniFrac measure, which quantifies the phylogenetic distances among samples. Ovals represent the 95% confidence ellipse around the group (upwind vs. downwind) centroids (i.e., multivariate means), and the arrows represent the correlation of individual phyla with PCoA axes. The percent of variability in UniFrac distances accounted for by each axis is indicated.

Figure 3

Mean fold increase in abundance of tetracycline resistance genes (TetB, TetL, TetM, TetO, TetQ, and TetW) in PM collected immediately downwind of feed yards relative to samples collected upwind (n = 10/group). All six targeted tetracycline resistance genes were significantly more abundant in PM downwind of feed yards than upwind, with fold change values ranging from 100 to 1,000. Resistance gene TetM had the highest fold increase in abundance between PM collected downwind and upwind. *p < 0.002.