Method for evaluation of the efficacy of antimicrobial preservatives in cosmetic wet wipes

Many cosmetic formulations are now available in the form of wet wipes packaged in sealed sachets or packets. Like the majority of cosmetic products having an aqueous phase, wipes are susceptible to microbial contamination and require the addition of preservatives. The efficacy of such preservatives can be evaluated using a standard challenge test performed on the wetting liquid but this test cannot be regarded as representative for this new type of formulation. The method presented here evaluates the efficacy of preservatives used in wet wipes kept in their original packaging. Dried inoculums were prepared by membrane filtration followed by drying in an incubator. The method is applicable to bacteria (Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus and Enterococcus faecalis), Bacillus cereus spores and fungi (Candida albicans and Aspergillus niger). These inoculated carriers were inserted between two wipes in the original package, which was then re-sealed immediately. The test requires one dry inoculum per packet and one packet for each control or test. After incubation at 22.5 degrees C for 1, 2, 7, 14 or 28 days and, for the control, immediately after insertion of the membrane (time 0), microorganism counts were performed on the germ-carrier membranes as well as on adjacent wipes, after transfer into a suitable neutralizing agent. The membranes were shaken in the presence of glass beads and microorganisms were dissociated from the wipes by means of a Stomacher. The supernatants recovered after being left to stand for 20 min are counted by pour plate method or membrane filtration. The feasibility of the method was demonstrated for each of the seven above-mentioned strains. The repeatability and reproducibility of the results obtained is similar to that obtained for preservative efficacy tests in the Pharmacopoeias. The lethal rate of microorganisms during the preparation of dry inoculums ranges from 50 to 90% depending on the strain and the test (generally, a spontaneous reduction of about 1 log up to a maximum of 2 log). The recovery rate for microorganisms from dry inoculums (on membranes) at time 0 (control = T(0)) is around 90%, regardless of the strain or the test. The number of microorganisms recovered from the wipes (W(0)) is between 2 and 10% of the number recovered from membranes (T(0)) and may be considered negligible. Application of this method to different types of wipes demonstrates that the efficacy of preservatives, expressed as the logarithmic reduction in the number of microorganisms at each time point, depends on the type of wipe and on the strain tested. The results obtained are considerably different from those found with the standard challenge tests applied to wetting liquids for wipes. The differences found confirm the need for a specific method applicable to wipes.