Pollution from azithromycin-manufacturing promotes macrolide-resistance gene propagation and induces spatial and seasonal shifts in receiving river sediments

Effluents from antibiotic manufacturing may contain high concentrations of antibiotics, which are the main driving force behind the selection and spread of antibiotic resistance genes in the environment. However, our knowledge about the impact of such effluent discharges on the antibiotic resistome and bacterial communities is still limited. To gain insight into this impact, we collected effluents from an azithromycin-manufacturing industry discharge site as well as upstream and downstream sediments from the receiving Sava river during both winter and summer season. Chemical analyses of sediment and effluent samples indicated that the effluent discharge significantly increased the amount of macrolide antibiotics, heavy metals and nutrients in the receiving river sediments. Quantitative PCR revealed a significant increase of relative abundances of macrolideresistance genes and class 1 integrons in effluent- impacted sediments. Amplicon sequencing of 16S rRNA genes showed spatial and seasonal bacterial community shifts in the receiving sediments. Redundancy analysis and Mantel test indicated that macrolides and copper together with nutrients significantly correlated with community shift close to the effluent discharge site. The number of taxa that were significantly increased in relative abundance at the discharge site decreased rapidly at the downstream sites, showing the resilience of the indigenous sediment bacterial community. Seasonal changes in the chemical properties of the sediment along with changes in effluent community composition could be responsible for sediment community shifts between winter and summer. Altogether, this study showed that the discharge of pharmaceutical effluents altered physicochemical characteristics and bacterial community of receiving river sediments, which contributed to the enrichment of macrolide-resistance genes and integrons.