Evaluation of Manuka honey as an adjuvant antimicrobial preservative in a O/W emulsion

Cosmetics and personal care products need to be protected against microorganism contamination to prevent microbial spoilage and consumers’ health hazards. The use of parabens, the most common preservatives added to cosmetic products, is associated with allergies, and some studies suggested that they could cause hormone disruption; therefore, the need of efficient and safer alternatives to parabens is increasing. The present study investigates the preservative efficacy of Manuka honey, a New-Zealand honey well known for its pronounced antimicrobial activity, in a O/W emulsion, in comparison with the traditional preservative methylparaben. An emulsion containing 10% Manuka honey was tested against bacterial (Escherichia coli ATCC 8739, Staphylococcus aureus ATCC 6538, Pseudomonas aeruginosa ATCC 9027) and fungal strains (Candida albicans ATCC 10231, Aspergillus brasiliensis ATCC 16404) by a challenge test, a procedure in which the efficacy of a preservative system is challenged by contamination with specified bacterial and fungal strains, according to the European Pharmacopeia procedures. The unpreserved emulsion and the emulsion with 0.2% methylparaben were reference samples. Manuka honey showed a good ability to protect the emulsion from contamination with standard bacterial inocula; in particular, it has been proven more effective than methylparaben against Pseudomonas aeruginosa, and showed the same efficacy as methylparaben against E.coli. Conversely, there was no evidence of a protective effect activity of Manuka honey against fungal contamination in our experimental conditions. Manuka honey, if added to a formulation for its humectant and moisturizing activity, also exerts an antibacterial activity, and can therefore be considered as an effective candidate for alternative preservation systems intended for use in cosmetic and personal care products.