Co-resistance is the dominant mechanism of co-selection and dissemination of antibiotic resistome in nano zinc oxide polluted soil

The broader soil bacterial community responses at ecotoxicologically relevant levels of nano ZnO (nZnO) focussing on co-selection of antibiotic resistance (AR) were investigated. nZnO imposed a stronger influence than the bulk counterpart (bZnO) on antibiotic resistance genes (ARGs) with multidrug resistance (MDR) systems being predominant (63 % of total ARGs). Proliferation of biomarker ARGs especially for last resort antibiotic like vancomycin was observed and Streptomyces hosted multiple ARGs. nZnO was the major driver of the resistome with efflux systems dominating the AR mechanism. Environmental risk associated with nZnO was mediated through metal driven co-selection of ARGs and their probable transfer to eukaryotic hosts through horizontal gene transfer (HGT) via mobile genetic elements (MGEs). Novel resistance genes tetA, mdtA, int and tnpA validated in our study can be used as biomarkers for rapid detection of nZnO toxicity in soils. qRT-PCR validation of resistome in the rhizosphere soil microbiome of turmeric indicated that Zn levels decreased by 16 % compared to bulk soil with 80 % bioaccumulation in rhizomes at 1000 mg Zn kg^-1 and subsequent down regulation of ARGs. Expression of key biosynthetic genes for curcumin in turmeric rhizomes showed an increase up to 500 mg Zn kg^-1 as nZnO. Validation of co-selection phenomenon in microcosm with 10 mg kg^-1 tetracycline without added Zn indicated 20 % upregulation of Zn resistance genes (ZRGs) like czcA, yiip and zntA.