Comparison of the compact dry TC method with the standard method ISO 21149:2006 for determining aerobic colony counts in cosmetic emulsion

Abstract

Objective

Compact Dry TC, a rapid method kit for determining aerobic colony counts, has been developed by Nissui Pharmaceutical Co. for food application. These plates are presterilized and contain culture medium, a cold-soluble gelling agent and a color redox indicator for rapid enumeration. In the present study, the alternative method is compared with the standard method ISO 21149:2006 – Cosmetic - Microbiology - Enumeration and detection of aerobic mesophilic bacteria, for cosmetic emulsions application.

Methods

An oil-in-water (o/w) cosmetic emulsion was contaminated with a pool of bacterial strains (Escherichia coli ATCC 8739, Staphylococcus aureus ATCC 6538, Pseudomonas aeruginosa ATCC 9027). 1 mL samples were spread on agar as described in ISO 21149. The colonies were enumerated after 3 days of incubation. At the same time, 1.2 mL samples were spread on Compact Dry TC kits. The kit was incubated at 35 °C ± 1 °C for 48 h and the colonies were enumerated.

Accuracy determination was carried out using 6 replicates at 4 levels of concentrations (10, 50, 100, and 250 CFU/ml). The repeatability study was carried out using 12 replicates at 4 levels of concentrations (10, 50, 100, and 250 CFU/ml). Variations relative to the analyst and to the batch of emulsion have been investigated.

Results

The linear correlation coefficients of Compact Dry TC Kit enumeration with standard method ISO 21149:2006 was 0.9999. In comparison study, no apparent differences were noted between the Compact Dry TC Kit and the reference method ISO 21149, for the detection level of aerobic microorganisms. Relative accuracy, repeatability, and intermediate precision studies were acceptable.

In the repeatability study, the Shapiro-Wilk test has confirmed the normally distribution of the twelve assays. No significant variations in Compact Dry TC count results were observed with different analysts and different batches of emulsion.

Conclusion

The results showed that the two compared methods " Compact Dry TC" versus "Conventional pour plate" performed equally well. Demonstration was achieved that the Compact Dry TC method may constitute a useful alternative tool for rapid enumeration of aerobic mesophilic bacteria in cosmetic emulsions.

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