Seasonal and spatial variations of antibiotic resistance genes and bacterial biodiversity in biofilms covering the equipment at successive stages of drinking water purification.

The presence of ARGs (antibiotic resistance genes) in the aquatic environment is a serious threat to public health especially in environmental biofilms as natural reservoirs of ARGs in water treatment plants (WTP). It has been shown that the treatment technology and source of water have a significant impact on the abundance and type of genes determining antibiotic resistance. The following indicator genes were proposed that should absolutely be controlled in environmental biofilms: intl1, sul2, sul1, tetA, blaOXA, and blaTEM. In both studied WTPs, the highest number of copies was determined for the intI1 gene. Among the tested ARGs, the highest values were obtained for genes sul1 and tetA. The qPCR analysis also revealed that the amounts of determined ARGs decreased in the following order: sulphonamides>carbapenems >tetracyclines > β-lactams >macrolides. The dominant bacterial types in all analysed samples were Proteobacteria and Bacteroidetes. Both ARGs and bacterial biodiversity was determined rather by sampling site (spatial variation) than seasonality. The obtained results show that biofilms are reservoirs of ARGs. This may affect the microbiological quality of water entering the water system. It is therefore necessary to include their analysis in the classical studies of water quality.