Effect of Low-temperature Stock on the Growth of Bacteria.

It is necessary to know the appropriate bacterial culture temperature and stock temperature to carry out high-precision preservative efficacy tests. In this study, we investigated whether the temperature conditions within the culture or stock affect the survivability or growth activity of Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. We found that the cold stocking condition affected the survivability of P. aeruginosa, but not that of E. coli and S. aureus. Furthermore, stocking temperature affects the growth activity of E. coli, P. aeruginosa and S. aureus, and the effect on P. aeruginosa was greater than that on the others. However, the attenuating effect on P. aeruginosa was reversed by culturing into fresh medium for up to six weeks after stocking. These results suggest that cold stress during stocking may affect the growth activity of P. aeruginosa. The results of this study should provide knowledge for the precise evaluation of preservative efficacy in cosmetics.

Comparison of Different Temperature Conditions for Microbial Enumeration of Aspergillus brasiliensis in the Preservative Efficacy Test.

The preservative efficacy test is an important method for assessing the antimicrobial effect of cosmetic products. In this study, the optimum conditions for the efficient microbial enumeration of Aspergillus brasiliensis were investigated. Cosmetic products, inoculated with A. brasiliensis spore suspensions, were cultivated at 22.5°C, 32.5°C, or 40°C and the detection rate and the number of colonies were determined using the pour culture method. There was no difference in the viable counts of visible colonies among different temperature conditions. However, the viable counts after 3 days of culture were significantly greater for the cultures maintained at 32.5°C or 40°C compared with those maintained at 22.5°C. This effect was attenuated in products containing fatty acids, which could inhibit fungal growth. Overall, these results demonstrate that cultivating A. brasiliensis at 32.5°C reduces the time required for enumeration in the preservative efficacy test. Thus, the results of this study are expected to help improve and expedite microbiological quality control in the cosmetic industry.

Factors Affecting Reduction in Preservative Efficacy in Nonwoven Fabrics.

Insufficient preservative efficacy leads to microbial contamination. Cosmetic-impregnated products composed of nonwoven fabrics, such as wipes and masks, can be contaminated with microbes owing to their special form. However, the reduction of preservative efficacy in cosmetic-impregnated products remains unverified. This study aimed to investigate whether preservative efficacy is reduced in nonwoven fabrics impregnated with a cosmetic liquid and the factors affecting this reduction. First, we evaluated the preservative efficacy of face wipes and confirmed that the preservative efficacy was reduced after impregnation of cosmetic liquids into nonwoven fabrics. We thus hypothesized that the adsorption of the antimicrobial components onto nonwoven fabrics decreases the preservative efficacy. Unexpectedly, the antimicrobial components were scarcely adsorbed onto the fabrics, while microbial growth activity was significantly increased on the fabrics, as determined through microbial calorimetry. Furthermore, the antibacterial effects were reduced in the nonwoven fabrics. These results indicate that the nonwoven fabrics enhanced microbial growth, thus decreasing the preservative efficacy. Our results provide novel insights into the microbial control of products composed of nonwoven fabrics.