J. Soc. Cosmet. Chem. 24 65-78 (1973) (•) 1973 Society of Cosmetic Chemists of Great Britain Methods for counting and testing for micro-organisms in raw materials, topical and oral products H. MeG. SCOTT* Presented on 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--The TEST METHODS used by several workers, or recommended by official com- pendia, for the MICROBIAL CONTROL of pharmaceutical and cosmetic products are reviewed and discussed. Attention has been drawn to important factors which can influence the results of tests--such as product composition, sample preparation, time and temperature of incubation and culture media. Reference is also made to the need for the development of simple and quicker methods for ROUTINE APPLICATION. Comparative data between laboratories on the reproducibility of official test methods is also suggested. INTRODUCTION Not unexpectedly, the Swedish Board of Health's report in 1965 (1) on microbial contamination of pharmaceutical products has stimulated similar investigations to be carried out in many other countries. Some are still in progress others have already led to several publications (2-31) on suggested standards for pharmaceutical and cosmetic preparations, includ- ing raw materials, which were not previously subject to microbial control. All standards, whatever their nature, depend on the availability of suitable methods for assessing the particular quality involved and, for microbial control, standards can have little meaning unless based on effective methods *Pharmaceutical Department, Pharmaceuticals Division, Imperial Chemical Industries Limited, Macclesfield, Cheshire. 65

66 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS for detection or enumeration of the pertinent species or group of undesir- able micro-organisms. Since both micro-organisms and products vary widely in their individual properties, it is inevitable that many problems are highlighted in any discussion of microbial standards and methods of examination of pharma- ceutical products, cosmetics and foodstuffs. As with conventional sterility tests, results can vary with the test method used, sample preparation, time and temperature of incubation and cultural media used (32-36). TEST METHODS According to published data, two main types of method are being used in the examination of products and raw materials. For convenience, these are classified as (a) count methods and (b) 'sterility test type' methods. Count methods For the estimation of the number of viable micro-organisms, the count methods in common use are the pour-plate, roll-tube, surface, drop methods and the multiple-tube method or most probable number or index (MPN). Basically, these techniques consist of serially diluting the test sample, if necessary, in a suitable diluent and then adding aliquots of the dilution into or on suitable culture media. For materials or products which are already in solution or are water soluble or are capable of being solubilized in an inert solvent, the membrane filtration method is also used. Pour plate This method is most widely used in microbiology and is now included in the United States Pharmacopoeia 18th rev. (37) for the microbial examina- tion of products. It has also been used by many workers in various countries in their investigations on contamination of products: Denmark (2-8), Sweden (9, 10), U.S.A. (11-13, 15, 16), Great Britain (18, 19), Belgium (23), Switzerland (24-26), Rumania (29) and Czechoslovakia (30). Briefly, the method consists of mixing a measured amount of sample with molten agar medium in Petri dishes and, after setting, incubating the inoculated dishes at various temperatures. The disadvantages of the method for use with pharmaceutical and related products are that undissolved particles of test