Microbiological purity testing of semisolid topical preparations.

The reliability of the poured plate method for purity testing of three different creams and an anhydrous ointment was studied. The ability of some nonionic surfactants to elute the microbes from the semisolid preparations to water phase and possible detrimental effects of these surfactants on the microbes was especially investigated. The method was used to test the effect of preparation temperature and of storage on the number of microbes in creams. Nonionic surfactants studied did not generally harm the bacteria. Variation was, however, found between different bacterial species and even bacterial strains. The surfactants studied eluted bacteria quantitatively from the water-containing creams but not from the water-free ointment. The concentration needed for quantitative elution of microbes was dependent on the surfactant used, being lower for Brij 58 than for Tween 80. Heating of the phases up to 60 or 70 degrees C for a short time during the preparation of the creams does not ensure the microbial purity of the preparation. In creams made at 80 degrees C, however, all the bacteria had died, only the spores of B. subtilis survived this temperature. After two months storage at 5 degrees C, all microbes tested were still detected in creams. There were, however, differences in survival of different microbes. None of the bacteria multiplied during the storage, but decreased slightly in number.