Tracking antibiotic resistance genes in soil irrigated with dairy wastewater

Authors: Dungan, Robert - Rob; McKinney, Chad; Leytem, April

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2018
Publication Date: 4/20/2018
Citation: Dungan, R.S., Mckinney, C.W., Leytem, A.B. 2018. Tracking antibiotic resistance genes in soil irrigated with dairy wastewater. Science of the Total Environment. 635:1477-1483. https://doi.org/10.1016/j.scitotenv.2018.04.020.
DOI: https://doi.org/10.1016/j.scitotenv.2018.04.020

Interpretive Summary: Dairies accumulate manure wastewater in storage ponds that must be pumped out to accommodate new influxes of wastewater. The wastewater is often applied to agricultural soils to irrigate forage crops, recycling nutrients, and reuse the wastewater. Because the wastewaters also contains antibiotic resistance genes, there are concerns that these genes can be transferred to pathogens, that will in turn resist drug treatment. The objective of this study was to track the abundance and occurrence of clinically important antibiotic resistance genes in soil irrigated with dairy wastewater on a monthly basis for six months. The genes were rarely present in the soils before treatment, but were present at high levels in the dairy wastewater. As a result, the gene levels also increased dramatically in soils receiving the wastewater. However, it is not known if these genes were intracellular, associated with pathogens, or even expressed. This study prompts the need for additional research to determine the potential contribution of dairy wastewater irrigation to the growing threat of drug-resistant bacteria.

Technical Abstract: In southern Idaho, the application of dairy wastewater to agricultural soils is a widely used practice to irrigate crops and recycle nutrients. In this study, small-scale field plots were irrigated monthly (6 times) with dairy wastewater (100%), wastewater diluted to 50% with irrigation (canal) water, and diluted wastewater spiked with copper sulfate (50 mg Cu/L), while control plots were irrigated with canal water. In addition, half of all plots were either planted with wheat or were left as bare soil. Biweekly soil samples were collected during this period and processed to determine the occurrence and abundance of antibiotic resistance genes [blaCTX-M-1, erm(B), sul1, tet(B), tet(M), and tet(X)] and a class 1 integron-integrase gene (intI1) via quantitative real-time PCR (qPCR). Only sul1 and tet(X) were detected in soil (3 out of 32 samples) before the wastewater treatments were applied. However, the occurrence and relative abundance (normalized to 16S rRNA gene copies) of most genes [erm(B), intI1, sul1, and tet(M)] increased dramatically after wastewater irrigation and levels were maintained during the entire study period. blaCTX-M-1 was the only gene not detected in wastewater-treated soils, which is likely related to its absence in the dairy wastewater. Relative gene levels in soil were found to be statistically similar among the treatments in most cases, regardless of percentage of wastewater applied, presence or absence of plants, and level of copper in the wastewater. The key result from this study is that dairy wastewater irrigation significantly enlarges the reservoir of ARGs and intI1 in soils, while detection of these genes rarely occurred in soil irrigated only with canal water.