Co-occurrence of integrase 1, antibiotic and heavy metal resistance genes in municipal wastewater treatment plants

The impact of human activities on the spread and on the persistence of antibiotic resistances in the environment is still far from being understood. The natural background of resistances is influenced by human activities, and the wastewater treatment plants (WWTPs) are among the main sources of the release of antibiotic resistance into the environment. The various treatments of WWTPs provide a number of different environmental conditions potentially favoring the selection of antibiotic resistance genes (ARGs) and thereby their well-documented spread in the environment. Although the distribution of different ARGs in WWTPs has been deeply investigated, very little is known on the ecology and on the molecular mechanisms underlying the selection of specific ARGs. This study investigates the fate of diverse ARGs, heavy metal resistance genes (HMRGs) and of a mobile element (the class I integron) in three WWTPs. Abundances of the different genetic markers were correlated to each other and their relation to biotic and abiotic factors (total organic carbon, total nitrogen, prokaryotic cell abundance and its relative distribution in single cells and aggregates) influencing the microbial communities in the different treatment phases in three WWTPs, were investigated. Water samples were analyzed for the abundance of six ARGs (tetA, sulII, blaTEM, blaCTXM, ermB, and qnrS), two HMRGs (czcA and arsB), and of the class I integron (int1). The measured variables clustered in two well-defined groups, the first including tetA, ermB, qnrS and the different biotic and abiotic factors, and a second group around the genes sulII, czcA, arsB and int1. Moreover, the dynamics of sulII, HMRGs, and int1 correlated strongly. Our results suggest a potentially crucial role of HMRGs in the spread, mediated by mobile elements, of some ARGs, i.e. sulII. The possibility of a relation between heavy metal contamination and the spread of ARGs in WWTPs calls for further research to clarify the mechanisms of co-selection and their ecology, in order to implement the removal efficiency of the applied treatments.