Optimisation of an agar overlay assay for the assessment of the antimicrobial activity of topically applied semi-solid antiseptic products including honey-based formulations

The aim of this study was to optimise the so-called agar overlay assay to investigate the antimicrobial activity of some currently available topical antimicrobial products against a range of Gram positive and Gram negative bacteria. During the optimisation process, various assay parameters including base agar volume, overlay agar volume, overlay agar concentration, placement of products into wells or onto coverslip and inoculum concentration were taken into consideration. The optimised assay was found to be a convenient, suitable and effective platform for the assessment of the antimicrobial activity of commercial semi-solid over-the-counter (OTC) products (i.e. non-prescription medicines for topical application used commonly without supervision by a healthcare professional) containing a range of active pharmaceutical ingredients and to be potentially also applicable to complex natural product-based formulations (e.g. honey-based formulation, melaleuca oil-based formulation). The most auspicious aspect of the optimised method was its capability of determining the antimicrobial activity of intact products without further manipulation, unlike other tests that require products to be dissolved or dilute, thus compromising their integrity.