Removal of antibiotic resistance genes and co-selectors in a full-scale sewage treatment plant during droughts and floods

We investigated the effect of droughts and floods on the removal of antibiotic resistance genes and co-selectors in a 60 MLD sewage treatment plant (STP) in Perungudi town, Chennai city. Samples were collected once a month for 13 months and analysed for 7 antibiotic resistance genes (sul1, ermF, tetW, mcr5, sul2, parC, and bla(OXA-1)), the most used antibiotic in the region (ciprofloxacin), a non-pharmaceutical antibiotic (triclosan), and 3 co-selective heavy metals (copper, chromium, and lead). It was found that the total bacterial count decreased significantly in the STP (p-value = 0.03, Wilcoxon rank sum test), but the relative abundance of ARGs was not reduced in the STP. In fact, the relative abundances of tetW and bla(OXA-1) increased in the maturation pond and were higher in the effluent than in the influent (p-value = 0.03, Wilcoxon rank sum test). On the other hand, the relative abundances of most ARGs were reduced significantly in the anaerobic digester (p-value < 0.05, Wilcoxon rank sum test). Overall relative abundances of mcr5, tetW, ermF, intI1, and sul2 in the raw sewage were higher in the dry summer than in the monsoon. The relative abundance of intI1 was correlated (Spearman correlation) with the relative abundances of mcr5 (rho = 0.54), ermF (rho = 0.52), and tetW (rho = 0.61). Except for chromium, all the other targeted co-selectors were significantly removed from the STP throughout the year (p > 0.05, Wilcoxon rank sum test). Overall, the targeted ARGs were not removed in the STP, and this underperformance was unaffected by periods of droughts and floods.