Abstract
The reliability of the challenge test depends, among other parameters, on the spatial distribution of microorganisms in the matrix. The present study aims to quickly identify factors that are susceptible to impair a uniform distribution of inoculated bacteria in cosmetic matrices in this context. We used mosaic confocal laser scanning microscopy (M-CLSM) to obtain rapid assessment of the impact of the composition and viscosity of cosmetic matrices on S. aureus spatial distribution. Several models of cosmetic matrices were formulated with different concentrations of two thickeners and were inoculated with three S. aureus strains having different levels of hydrophobicity. The spatial distribution of S. aureus in each matrix was evaluated according to the frequency distribution of the fluorescence values of at least 1350 CLSM images. We showed that, whatever the thickener used, an increasingly concentration of thickener results in increasingly bacterial clustered distribution. Moreover, higher bacterial hydrophobicity also resulted in a more clustered spatial distribution. In conclusion, CLSM-based method allows a rapid characterization of bacterial spatial distribution in complex emulsified systems. Both matrix viscosity and bacterial surface hydrophobicity affect the bacterial spatial distribution which can have an impact on the reliability of bacterial enumeration during challenge test.
Highlights
The three 3.S.Results aureus strains were chosen according to their hydrophobicity to evaluate the impact of bacterial surface properties on bacterial distribution in matrices
We have tested the behavior of fluorescent polystyrene three S. aureus strains were chosen according to their hydrophobicity to evaluate the impact microspheres. ofThese have similar morphology aureus of 1 μm in bacterialinert surfacebeads properties on bacterial distribution in matrices.toS. that aureus of
The study of the spatial distribution for three S. aureus strains with different levels of hydrophobicity in several allowed understand determinants of theirof
Summary
Controlling the contamination of cosmetic products is essential to avoid alterations of their organoleptic characteristics (color, odor, composition, etc.) and maintain their innocuity for consumers.These products can be contaminated during the processing or by consumer’s repetitive usage.Among other pathogens, Staphylococcus aureus has been found in various cosmetic products [1,2].This Gram-positive bacterium is present on the skin and mucous membranes of 30% of the population [3]and causes a wide range of skin infections such as abscesses, furuncles, impetigo, etc. [4,5].Each cosmetic product is characterized by a level of microbiological risk according to the standardISO 29621:2017, which depends on several parameters such as the formula composition (preservative, ethanol, water activity, pH) or the type of packaging (unidose, airless pump, pots) [6,7].The preservation efficiency of a cosmetic product is evaluated by a challenge test, as defined in the European standard EN ISO 11930:2019. Controlling the contamination of cosmetic products is essential to avoid alterations of their organoleptic characteristics (color, odor, composition, etc.) and maintain their innocuity for consumers. These products can be contaminated during the processing or by consumer’s repetitive usage. The preservation efficiency of a cosmetic product is evaluated by a challenge test, as defined in the European standard EN ISO 11930:2019. This procedure consists in artificially inoculating a product with a target microorganism at a defined concentration that is between 105 and 106 CFU mL−1
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