Reduction of antibiotic resistome and integron-integrase genes in laboratory-scale photobioreactors treating municipal wastewater.

Wastewater treatment systems receiving municipal wastewater are major dissemination nodes of antibiotic resistance genes (ARGs) between anthropogenic and natural environments. This study examined the fate of antibiotic resistome and class 1-3 integron-integrase genes in photobioreactors that were treating municipal wastewater diluted (70/30) with lake or tap water for the algal biomass production. A combined approach of metagenomic and quantitative (qPCR) analysis was undertaken. Municipal wastewater treatment in the photobioreactors led to reduced antibiotic resistome proportion, number of ARG subtypes, and abundances of individual ARGs in the bacterial community. The ARGs and intI1 gene abundances and relative abundances in the discharges of the photobioreactors were either comparable or lower than the respective values in the effluents of conventional wastewater treatment plants. The reduction of the resistome proved to be strongly related to the changes in the bacterial community composition during the wastewater treatment process as it was responding to rising pH levels caused by intense algal growth. Several bacterial genera (e.g., Azoarcus, Dechloromonas, and Sulfuritalea) were recognized as potential hosts of multiple antibiotic resistance types. Although the lake water contributed a diverse and abundant resistome and intI genes profile to the treatment system, it proved to be considerably more beneficial for wastewater dilution than the tap water. The diversity (number of detected resistance types and subtypes) and proportion of the antibiotic resistome, the amount of plasmid borne integron-integrase gene reads, and the abundances and relative abundances of the majority of quantified ARGs (aadA, sul1, tetQ, tetW, qnrS, ermB, blaOXA2-type) and intI1 gene as well as the amount of multi-resistance determinants were significantly lower in the discharges of photobioreactors where lake water was used to dilute wastewater.