Metagenomics indicates abundance of biofilm related genes and horizontal transfer of multidrug resistant genes among bacterial communities in nano zinc oxide polluted soil

The unsafe and reckless disposal of metal oxide nanoparticles like ZnO (nZnO) into the soil could seriously impact bacterial behavioural responses and functions. Under such stress, biofilm formation is considered to be a robust mechanism for bacterial survival in soil. We examined the response of bacterial metagenomes in soils exposed to varying levels of Zn (50, 200, 500 and 1000 mg kg-1) as nano Zn oxide (nZnO) in terms of biofilm genesis and regulation and their co-occurrences with multidrug resistance genes (MDRGs) and mobile genetic elements (MGEs). The size-specific effects of nZnO were verified using its bulk counterpart (bZnO). Both nZnO and bZnO facilitated profusion of biofilm related genes (BGs) especially at higher Zn levels (500 and 1000 mg kg-1 Zn), though maximum abundance was registered at a comparatively lower level under nZnO. In general, nZnO favoured an enhancement of genes involved in exopolysaccharide biosynthesis and attachment, while bZnO favoured genes related to capsule formation, chemotaxis and biofilm dispersion. Co-occurrence network analysis revealed significant positive correlations between abundances of BGs, MDRGs and MGEs, indicating an enhanced probability for horizontal gene transfer of MDRGs in nZnO polluted soils.