J. Soc. Cosmet. Chem. 24 229-238 (1973) ¸ 1973 Society of Cosmetic Chemis• •f Great Britain Assessment of the preservative capacity of shampoos PHYLLIS C. FLAWN, S. A. MALCOLM and R. C. S. WOODROFFE* Based on a lecture delivered on the 29th September 1971 in London, at the Symposium on 'Microbial Control', organized by the Pharmaceutical Society of Great Britain and the Society of Cosmetic Chemists of Great Britain. Synopsis--Some factors involved in the development of a TEST for PRESERVATIVE CAPA- CITY (challenge test) for SHAMPOOS are discussed. A microbiological survey of public WATER supplies revealed the presence of Gram-negative bacteria, including PSEUDOMONAS sp capable of utilizing ANIONIC DETERGENTS as a sole carbon and sulphur source by virtue of their inducible SULPHATASE ENZYMES. Tap water ttterefore provides a suitable source of challenge test bacteria which may be isolated on Ionagar containing 0.1% detergent. The amount of slime produced by these organisms is increased during growth in detergent. Utilizing these data, a challenge test for shampoos is described. INTRODUCTION Toilet products, as is well known, may contain micro-organisms. Therefore, it is necessary that they should be adequately preserved. Some products are completely preserved by the formulation and do not require added preservative. Yet others are in a specific and therefore only partially effective state of preservation. In these cases some micro-organisms are completely inhibited while others may multiply in the product. The inclusion of antimicrobial agents as preservatives in products is an attempt to shift the preservation type from specific to non-specific. There- fore, any test to challenge preservative capacity must be designed to deter- * Unilever Research Laboratory, 455 London Road, Isleworth, Middlesex. 229

230 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS mine the degree of specificity of preservation of the product being tested. The chances of contamination occurring will be lowest when the number of different organisms capable of growth in the product is small and when these organisms are rare or absent in the manufacturing and use environ- ments. Currently available tests for preservative capacity Tests for preservative capacity currently in use can be divided into three categories: (i) Inoculation of the finished product followed by incubation and examination at intervals for signs of spoilage. (ii) An inoculation, incubation, sampling cycle which is repeated until samples show evidence of viable organisms. (iii) A single inoculation followed by incubation and periodic sampling for viable organisms. Tests in the first category require a long time period for completion and only show whether a product is spoiled rather than whether it is con- taminated. It is in fact quite possible to find products containing many millions of organisms per gram but with unaltered appearance. The second type of test attempts to reproduce in-use conditions. How- ever, because of the large number of organisms introduced with successive inoculations, and the progressive dilution of the product, it is biased against preservatives readily inactivated by organic matter and those having high concentration exponents, i.e. those in which a small change in concentration is accompanied by a large change in activity. The test in the third category not only detects the growth of organisms in the product but also the survival of non-multiplying organisms, which may be an indication of possible eventual multiplication should the pre- servative system be unstable. Laboratory challenge tests have been known to indicate that a product is sufficiently well preserved even when this same product is later shown to be contaminated. The most obvious reason is that the wrong test organism was used in the challenge test. However, cases have also arisen in which organisms isolated from a contaminated product and cultured in laboratory media did not survive when re-applied to the original product, giving the impression that the formulation could not support bacterial growth. Apparently the organisms adapt to their new surroundings and the adaptive mechanisms are lost during sub-culture in laboratory media so