Side effects of free nitrous acid on the sewer resistome and mobilome

Abstract

The dosage of free nitrous acid (FNA) is an effective treatment to control emissions of hydrogen sulfide and methane from sewers. It is unclear, however, whether its application could affect the sewer resistome and mobilome (the pool of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs), respectively). Here, a lab-scale assay for FNA treatment (60 ppm for 6 h) was conducted to explore its effects on the abundance of ARGs and MEGs among sewer compartments (i.e., suspended cells and biofilms) using shotgun metagenomics. FNA treatment caused a significant reduction in viable cells (≈25%, p < 0.01), a 75% and a 83% reduction in the abundance of Aeromonadaceae and Moraxellaceae, respectively, and a 50% increase for Campylobacteraceae in the suspended cell fraction whereas bacterial communities in sewer biofilms remained unaltered. Comparative analyses before and after the FNA treatment also showed a significant reduction (p < 0.01) in the relative abundance of ARGs in the suspended cell fraction, particularly for genes conferring resistance to multidrug, macrolide-lincosamide-streptogramin B, quinolones and bacitracin. Besides, the relative abundance of mobile ISCR8 element significantly decreased (from 0.014% to 0.010%) whereas genes conferring resistance to glycopeptides significantly increased (from 0.0008% to 0.0016%) after 6 h of treatment. A heatmap analysis also showed a clear reduction in the relative abundance of genes conferring resistance to β-lactam antibiotics and multidrug resistance. The observed reduction in the relative abundance of ARGs and MGEs in suspended cells is a beneficial side effect of a treatment originally intended to mitigate emissions of noxious gases from sewers and may be helpful to guide future policy recommendations to lessen the spread of antibiotic resistance across the urban water cycle.