Antimicrobial sensitivity of Escherichia coli to alumina nanoparticles

Metal oxide nanoparticles (NPs) are known to possess strong antimicrobial properties. Aluminum oxide NPs have wide-range applications in industrial as well as personal care products. In the absence of prior reports on the antimicrobial properties of alumina NPs for a wide concentration range, the principal objective of the present work was to study the growth-inhibitory effect of alumina NPs over a wide concentration range (10-1000 mu g/mL) on an environmentally relevant gram-negative model microorganism, Escherichia coli. The mean diameter of the NPs was determined to be 179 nm in aqueous dispersion used for this study, and surface area was determined to be 21.23 m(2)/g. The concentration of 1000 mu g/mL was found to be moderately inhibitory for bacteria. Almost negligible dependence of growth rate on the concentration of the NPs was observed. The extracellular protein content was found to be slightly lower in case of cells interacting with 1000 mu g/mL alumina than the uninteracted control cells. Fourier transform-infrared studies established differences in structure between interacted and uninteracted cells. Alumina NPs showed a mild growth-inhibitory effect, only at very high concentrations, which might be due to surface charge interactions between the particles and cells. Free-radical scavenging properties of the particles might have prevented cell wall disruption and drastic antimicrobial action. This laboratory-scale study suggests that alumina NPs may only exhibit mild toxicity toward microorganisms in the environment.