Antimicrobial Effects of Metal Oxide Nanoparticles

and Introduction: In a world of emerging nanotechnology, one of the primary concerns is the potential environmental impact of nanoparticles (NPs). An efficient way to estimate nanotoxicity is to monitor the response of bacteria exposed to these particles [1]. This experiment explored the antimicrobial properties of nickel oxide, cobalt (II,III) oxide, zinc oxide, copper (II) oxide, iron (III) oxide, titanium dioxide, and iron (II,III) oxide against a model microorganism, Escherichia coli. The toxicity of these metal oxide NPs was tested using two methods: culturing in liquid media containing NPs, and electrospraying the NPs directly onto bacterial surface. Aqueous exposure mimics the natural interaction between microbial species as NPs diffuse in the environment [2]. During these tests, there was noticeable aggregation, preventing effective interaction between the particles and the bacteria. The limited growth inhibition observed from this form of exposure to metal oxide NPs was therefore attributed to their ionic species. On the other hand, the electrospray technique allows direct interaction between the NPs and cells. This exposure method grants insight into how “nano” associated properties from metal NPs affect the environment [2]. This method observed a higher death rate when the bacteria were exposed to oxidized nickel, zinc, and cobalt species; but no antimicrobial properties from titanium or iron. The disparity in the results of the two exposure techniques indicates that toxicity is dependant both upon the exposure method and the size of the particle. Experimental Procedure: Escherichia coli (E. coli) were cultivated in M9 minimal media at 37°C. Optical density was measured at 600 nm (OD600) using a UV spectrometer (Genesys, Thermo -Scientific, USA). Experiments began with a 5 mL E. coli culture with OD600 = 0.05 in M9 minimal media. The aqueous exposure method tracked the growth rate of E. coli with 2, 20, and 100 mg/L NPs. OD600 was recorded at 3, 6, 9, 21, 24 hours. The experiment was also performed using equivalent amounts of soluble chloride salt of the metals to test ionic toxicity. For electrospray exposure experiments, the aliquot of E. coli was first filtered onto a polyvinylidene fluoride (PVDF) membrane (0.22 μm pore size, 1.25 cm × 1.25 cm, Millipore, US) to form a biofilm, which was then electrosprayed with NPs. The electrospray system was kept at a flow rate of 5 μL/min and a current of ~ 7 kV to maintain a cone shaped spray; the particles were suspended in 1.0 M sodium phosphate (Na2HPO4, pH 7) buffer. Then the biofilm was washed from membrane using minimal medium and the total living cells after electrospray exposure was measured based on the colony forming unit (CFU) using LB agar plates. Colonies were counted after the plates were incubated in 37°C for ~ 24 hours. Meanwhile, the cells from biofilm will be resuspended in the M9 minimal media and the recovery of growth was monitored by OD600. Scanning electron microscopy (SEM) was used to observe changes in cell morphology after exposure to NPs. Table 1: Summary of results. T= toxic, NT = non-toxic.