Modulate response of the microbiome and resistome of natural bacterial communities exposed to treated wastewater effluents

Wastewater treatment plants (WWTPs) are a hot-spot for the spread of antibiotic resistance genes (ARGs) into the environment. Several studies targeted the selection and persistence of ARGs in WWTPs applying different disinfection technologies. Still, little is known on the fate of resistances once they are spread into the environment. I present the results of two studies on bacteria and ARGs released by WWTPs once interacting with the resident microbial communities of the receiving water bodies. In the first experiment, mixed communities were designed mimicking the impact of different conventional WWTP effluents on the receiver (lake, river) communities. Bacterial variables were measured by flow-cytometry and microscopy. Bacterial community composition and resistome were assessed by full-genome metasequencing. In the second study, a pristine community was exposed in chemostats to wastewater effluents at agricultural reuse grade, with and without the addition of low-dose antibiotic (cefotaxime). Bacterial, integron1, and antibiotic abundances were assessed, while the communities were described by 16SrDNA metagenetics. The fate of the WWTPs bacteria depended by the disinfection applied but even more on the ecological stability of the microbial community of the receiving system. ARGs released into the environment could establish in the newly formed communities and those already present into the resident community surprisingly increased especially with limited introduction of WWTP effluents. The magnitude of the stress seems to promote the number of integron1 genes, suggesting it as a good indicator of anthropic impact. These studies demonstrate how the impact of WWTP effluents is determined by the ecological conditions of the resident community.