Effect of Zero Valent Iron on the Horizontal Gene Transfer of Tetracycline Resistance Genes During Anaerobic Sludge Digestion Process

Abstract

The problem of bacterial resistance has become an important issue in the area of global ecological safety and human health. Waste sludge is an important reservoir and discharge source for antibiotic resistance genes (ARGs). In this study, the quantities of seven tetracycline resistance genes (TC-ARGs), including tetA, tetC, tetG, tetM, tetO, tetW, and tetX, as well as those of class 1 integron (intI1) genes, during anaerobic sludge digestion process were comprehensively quantified by quantitative PCR (qPCR). The effects of different doses of zero valent iron (Fe0) on the decrease and increase in the quantities of TC-ARGs and intI1 genes were investigated. The influence of plasmid conjugation on the horizontal gene transfer (HGT) of the target TC-ARGs was preliminarily analyzed. The correlations between the quantities of TC-ARGs and intI1 gene have been discussed. The results showed that the quantities of TC-ARGs and intI1 genes decreased in different degrees during anaerobic sludge digestion, and the abundance of tetX gene was reduced by 2.4 orders of magnitude. When Fe0 was added, no significant reduction in the quantities of TC-ARGs and intI1 genes was observed. However, as the addition of Fe0 increased, the quantities of TC-ARGs and intI1 genes increased correspondingly, as compared to those in the control group. The results obtained from the quantities of TC-ARGs carried by plasmid DNA showed that plasmid conjugation probably promoted the HGT of TC-ARGs. A positive significant correlation was found between the quantities of tetG and intI1 genes, indicating that intI1 might play an important role in the evolution of tetG during sludge anaerobic digestion process.