794 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS may rapidly acquire a massive count without obvious sign that any change is occurring. An energetic programme of microbiological surveillance at all stages of manufacture, packaging and warehousing is the only safeguard against disastrous consequences, such as the irrecoverable spoilage of complete batches of product.

Appendix E 1. STOCK CULTURES OF SPOILAGE MICRO-ORGANISMS Several references are made in this monograph to the use of spoilage micro-organisms in testing preservative systems. For this purpose, it is helpful to maintain cultures of various species and strains, isolated at various times, but some care is necessary to obtain consistent results in preservative studies. Requirements of different laboratories vary as regards the size and type of collection. This may often be limited to a few essential strains, for which full records are kept. Test strains may be isolated from contaminated materials and either freeze-dried or maintained on agar. Freeze-dried cultures may also be purchased from various National Collections (see Appendix E.2.). 1.1 Freeze-dried cultures Freeze-dried (lyophylized) cultures remain true to type and are pre- ferred to stock cultures maintained continuously on growth media, since freeze-dried cultures are less liable to undergo changes in preservative resistance and other characteristics. For the preparation of freeze-dried cultures, appropriate publications should be consulted {1). Slope cultures for current use are prepared from freeze-dried cultures and stored at 5-10ø the sub-culture should be checked for purity and used preferably within a month. After making four serial sub-cultures a new freeze-dried tube should be opened and the old culture discarded. 1.2 Other stock cultures When freeze-drying is not carried out, cultures may be maintained on a suitable nutrient agar medium by sub-culturing at appropriate intervals. The cultures should be stored in tightly-capped bottles in the dark at 5-10 ø. Cultures maintained continuously on agar should be specially checked for purity and to see that the required degree of preservation resistance is being maintained. To guard against accidental contamination or the de- velopment of an atypical strain, a duplicate set of master cultures should always be kept. Suitable culture media and suggestions for minimal frequency of sub- culture needed to ensure survival of the organisms are listed in Table •.2.•. ½). 795