Recent Study of The Fate of Antibiotic Resistance Genes in Four Wastewater Treatment Plants

Antibiotic resistance is becoming an increasingly concern because of the overuse and misuse of the antibiotics in agricultural areas and human activities. There are in total 12 families of bacteria that pose the greatest threat to human health by World Health Organization because of their developing resistant ability to current antibiotics in the market. Wastewater treatment plants (WWTPs) not only receive wastewater containing large levels of antibiotic resistance genes (ARGs) and antibiotic resistant bacteria (ARB) but may also discharge ARGs and ARB into the environment. In order to understand the fate of antibiotic resistance genes in WWTPs, four plants were selected near Los Angeles, California, two of which are long solids retention time (SRT) plants and the other two short SRT plants, to study the fate of ARGs in the activated sludge processes and the impacts of SRT. Four genes (sul1, sul2, tetW, and 16S rRNA) were selected and quantified by quantitative Polymerase Chain Reaction (qPCR) from the primary effluent (secondary influent) and secondary effluent samples at the four WWTPs. The two long SRT plants significantly reduced the absolute abundance of sul1, sul2, and tetW up to 4.5 log gene copies/mL while the two short SRT plants removed the three ARGs up to 1.92 log gene copies/mL. Plant 3 (a short SRT plant) was also observed to slightly increase the absolute abundance of these three ARGs by 0.17 log gene copies/mL for sul2. However, the relative abundance (RA) removals of the three ARGs vary more differently at the four plants. In addition, we found significant mean log removals of absolute abundance at the long SRT plants were 3.2 and 2.5 times greater than at the long SRT plants for sul2 and tetW genes, and nearly 6 times greater mean log removal of relative abundance of tetW at the two long SRT plants than at the short SRT plants. Furthermore, moderate to strong positive (r ≥ 0.65) correlations between the abundance of 16S rRNA and the concentration of volatile suspended solids (VSS) were observed at the four plants, indicating that higher concentrations of VSS may result in more abundance of 16S rRNA and more resistance to antibiotics.