An investigation into the effects of shape and structure on the antibacterial efficacy of metal nanoparticles
Increasing numbers of microbial organisms are becoming resistant to antibiotics, demonstrating a need for new, effective antibacterial agents. Metal nanoparticles (NPs) such as silver (AgNPs), copper (CuNPs) and gold (AuNPs) exhibit antimicrobial properties against bacteria, including Escherichia coli and Enterococcus faecium. E. coli O157:H7, which is known to be virulent and infectious, causing bowel discomfort, diarrhoea, nausea or vomiting. Enterococci frequently causes gastrointestinal infections, urinary tract infections, hepatobiliary sepsis, endocarditis as well as hospital-acquired infections, e.g. nosocomial bacteraemia and surgical wound infection. As regards E. coli, NHS trusts in England reported 38,132 cases of bacteraemia between 1 April 2015 and 31 March 2016 (NHS, 2017). There was an overall increase in the incidence of bacteraemia caused by Enterococcus spp between 2010 and 2017, from 9.9 to 13.1 per 100,000 of the population in England respectively. The antibacterial activity of metal nanoparticles has been investigated extensively due to their high surface area-to-volume ratio and the generation of reactive oxygen species. Metal nanoparticles have potential as alternatives to current antimicrobials used in the hospital environment to combat Healthcare Associated Infections (HCAIs). Nanoparticles were synthesised using the chemical reduction method, where the size and shape were controlled via the precursor or reduction agent, reaction time, temperature and the molar ration between the precursor and reduction agent. Nanoparticles were characterised using ultra-violet/visible spectroscopy (UV-Vis), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Minimum inhibitory concentration (MIC) was used to assess the antimicrobial efficacy of metal NPs. The antimicrobial activity was then determined using growth inhibition kinetics by 96 well plate reader and viable counts. Visualisation of the interaction between metal NPs and bacteria were assessed using SEM. Different shapes of metal NPs were obtained in vitro such as spherical shape NPs (AgNS, AuNS and CuNS), octahedron (AgNOct and AuNOct) and cube shapes (CuNC). All produced NPs are crystalline in nature, confirmed by selected area electron diffraction. The MICs of AgNOct against E. coli were the lowest at 10μg/ml, followed by MICs of CuNC and AuNOct of 15 and 50μg/ml respectively. The MICs of spherical shape (AuNS) against E. faecium and E. coli were the highest at 250 and 230 μg/ml respectively. Significant (p ≤0.05) reductions of ≥8 log (10) CFU/mL and ≥5 log (10) CFU/mL were observed for E. coli and E. faecium respectively treated with AgNOct. While treatment of CuNC resulted in significant (p ≤0.05) reductions of 7.92 log (10) CFU/mL and 3.5 log (10) CFU/mL against E. coli and E. faecium respectively. Reduction data for spherically shaped NPs (AuNS and CuNS), the lowest inhibition, were reduced by ≥1.9 log(10) CFU/mL against E. faecium. Damage to the bacteria cell wall was observed under SEM after treatment with NPs, while the cellular integrity was lost following exposure to AgNOct, AuNOct and CuNC for 24 hours. The antimicrobial efficacy of NPs have been shown to be shape dependent against E. coli and E. faecium. Truncated octahedral AgNOct, AuNOct and cubic shape CuNC exhibited greater antibacterial activity when compared with spherical shape NPs (AgNS, AuNS and CuNS). AgNOct has the greatest antibacterial activity against E. coli and E. faecium compared with AuNOct and CuNC, being bactericidal against E.coli and bacteriostatic against E. faecium. The difference in shape resulted in differences in efficacy, which could possibly be due to the higher surface area and variations in active facets and surface energies. This higher reactivity may ultimately cause more rapid cell death. Among these metals (silver, gold and copper), silver nanoparticles show strong antibacterial activity against E. coli and E. faecium. It is suggested that this is caused by the generation of reactive oxygen species (ROS) at the cell wall affecting metabolic processes by damaging DNA reproduction.
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