Antimicrobial activity of essential oils and metals and their interaction with layered silicates

Essential Oils (EOs) and metal ions have been utilised and investigated for antimicrobial applications for centuries; antimicrobial resistance has led to renewed interest in such antimicrobials. This study assesses their use as broad-spectrum antimicrobial agents. The essential oils (EOs) investigated were; Rosewood oil (RO), Clove Leaf Oil (CLO), Orange Oil (OO), Myrtle Oil (MO) and Manuka Oil (MNO). The metal ions studied were; Silver (Ag), Palladium (Pd) and Platinum (Pt). The ultimate aim was to encapsulate the best performing antimicrobial agents in a layered silicate controlled release substrate for use in various polymer-based formulations. The first stage of the study was targeted at assessment of the antimicrobial efficacy of these agents, both individually and in combination (to establish synergistic effects), against Staphylococcus aureus (gram-positive) and Pseudomonas aeruginosa (gram-negative). Antimicrobial efficacy was monitored using Zone of Inhibition, micro-dilution and checkerboard micro-broth dilution methods. Zone of Inhibition (ZoI) proved to be a reliable qualitative method for assessment of the antimicrobial activity of metal ions. Nutrient Agar (NA) was found to be the best growth media for higher metal concentrations (>100 mg/L) and Mueller-Hinton (MH) worked better at lower ion concentrations (