Control of biofilm forming clinically important bacteria by green synthesized ZnO nanoparticles and its ecotoxicity on Ceriodaphnia cornuta.

Zinc oxide nanoparticles were synthesized using the aqueous leaf extracts of Plectranthus barbatus (Plb-ZnO NPs) and characterized by UV-visible spectroscopy, XRD, FTIR, SEM and EDS. UV-Visible spectra recorded the absorbance peak of Plb-ZnO NPs at 343 nm. SEM analyses showed the spherical shape of Plb-ZnO NPs with the particle size between 30 and 60 nm. Plb-ZnO NPs exhibited antibacterial and antibiofilm activity against Gram positive Bacillus subtilis at all tested concentrations. In contrast, Plb-ZnO NPs showed antibacterial and antibiofilm activity against Gram negative Vibrio parahaemolyticus and Proteus vulgaris only at 100 μg/ml. The Atomic absorption spectrometry (AAS) revealed that Zn dissolution was 1.87 and 8.8 μg/L at 10 and 160 μg/L of Plb-ZnO NPs respectively. The body accumulation of Zn was increased from 0.8 μg/g body weight to 3.5 μg/g body weight when C. cornuta exposed to 10 μg/L and 160 μg/L respectively. Plb-ZnO NPs were toxic to Ceriodaphnia cornuta neonates (LC: 28 μg/L). Plb-ZnO NPs caused 100% mortality of C. cornuta at 160 μg/L after 24 h. However, zinc acetate does not cause any mortality of C. cornuta upto 350 μg/L. The light and confocal laser scanning microscopic images evidenced the uptake and accumulation of Plb-ZnO NPs on the internal gut regions of C. cornuta at 160 μg/L after 5, 10, 15, 20 and 24 h. Abnormalities in the swimming behaviour such as erratic swimming (ERR), migration to bottom (BOT) and migration to water surface (SUR) of C. cornuta were noticed after treatment with different concentrations of Plb-ZnO NPs.