Susceptibility of representative skin-resident microorganisms to cosmetic preservatives and in vitro assessment of consumer-relevant toxicity.

Abstract

Aims: To evaluate the antimicrobial susceptibility profiles of established and emerging cosmetic preservatives against selected skin-resident microorganisms and to assess their irritation and phototoxicity potential using in vitro methods.

Methods and results: Methylparaben, propylparaben, phenoxyethanol, ethylhexylglycerin (EHG), and selected commercial preservative blends were tested against two bacterial species, Cutibacterium acnes and Staphylococcus epidermidis, and two fungal species, Malassezia furfur and Candida parapsilosis. Minimum inhibitory and bactericidal or fungicidal concentrations were determined. Irritation potential was assessed using the Short Time Exposure assay (STE; OECD TG 491) and the Hen's egg test-chorioallantoic membrane (HET-CAM), while phototoxicity was evaluated using the 3T3 Neutral Red Uptake assay (3T3-NRU; OECD TG 432). EHG showed the broadest antimicrobial activity, including against C. acnes, but also exhibited moderate potential for irritation, possibly owing to its surfactant properties. Phenoxyethanol was highly active against fungi and exerted a bacteriostatic effect on S. epidermidis. Parabens displayed selective antifungal activity but no antibacterial activity at the concentrations tested. All preservatives were classified as non-phototoxic, and no IC50 was reached in 3T3 cells under the tested conditions (concentrations ≤ 100 μg/mL).

Conclusions: Most preservatives showed relatively favorable profiles against the selected skin-resident microorganisms, although species-specific antimicrobial activity and irritation potential were observed. Early comparative screening may support the selection of effective preservatives with lower biological aggressiveness before evaluation in finished formulations, challenge testing, and clinical trials.