Abundance and Diversity of Tetracycline Resistance Genes in Soils Adjacent to Representative Swine Feedlots in China

Tetracyclines are commonly used antibiotics in the swine industry for disease treatment and growth promotion. Tetracycline resistance was determined in soils sampled from farmlands in the vicinity of nine swine farms located in three cities in China. Totally, 15 tetracycline resistance (tet) genes were commonly detected in soil samples, including seven efflux pump genes (tetA, tetC, tetE, tetG, tetK, tetL, tetA/P), seven ribosomal protection proteins (RPPs) genes (tetM, tetO, tetQ, tetS, tetT, tetW, tetB/P), and one enzymatic modification gene (tetX). The quantitative real-time PCR was further used to quantify five RPPs genes (tetM, tetO, tetQ, tetW, tetT) and 16S rRNA gene abundances. The concentrations of total tetracyclines (5 typical tetracyclines and 10 of their degradation products) in these soils were measured using liquid chromatography-electrospray tandem mass spectrometry (LC-MS/MS) and were found to range from 5.4 to 377.8 μg·kg(-1) dry soil. Bivariate correlation analysis confirmed that absolute tet gene copies (sum of tetM, tetO, tetQ, tetW genes) were strongly correlated with the concentrations of tetracycline residues (r(2) = 0.45, P < 0.05), ambient bacterial 16S-rRNA gene copies in each soil sample (r(2) = 0.65, P < 0.01), and organic matter in soil (r(2) = 0.46, P < 0.05), respectively. Finally, the phylogenetic analysis on tetM combined with culture-independent molecular techniques revealed at least five genotypes of tetM in nine soil samples.