FUNDAMENTALS OF MICROBIOLOGY IN RELATION TO CLEANSING 49 scrupulous cleansing is necessary to free any item of equipment from these food traces. Protein of animal origin may be described as the favoured food for human pathogens, and skin secretions, mucus, etc. afford excellent nutrients for bacteria, assuming conditions are favourable. Temperature Broadly speaking, pathogens flourish best at about blood heat, but most can grow over the range 16-45øC. Where conditions are unavoid- ably favourable for bacterial growth, e.g. a nutrient liquid at pH 7, the best, simplest and least objectionable way to prevent or delay growth is to hold the product at a low temperature, e.g. about 0øC. Cold does not kill bacteria per se, but if organisms cannot grow they tend to die out. It is impossible to generalize about temperature conditions in respect of organisms producing defects they may vary from psychrophils (psychro- trophs) growing well at -5 to +5øC to the obligate thermophils which grow only above 37øC and flourish happily at 55-63øC. Cosmetic prepara- tions are often, or attempt to be, antiseptic in character, i.e. they contain one or more ingredients possessing some disinfectant power. Unfortunately these may have little efficacy against Pseudomonas, etc., so that low temperature storage of cosmetics cannot be relied upon to prevent growth of these types. All pseudomonads and many Gram-negative bacteria, yeasts and moulds can grow down to low temperatures, some moulds down to -23øC. Even extreme cold, e.g. --100øC, cannot be relied upon to kill micro-organisms, although they usually die off slowly. Moisture A certain 'moisture' or water activity (aw) is one of the major con- ditions for bacterial proliferation. On surfaces it is customary to speak in terms of relative humidity, but the essential feature is the water activity (osmotic pressure) of the medium in which the micro-organisms are con- tained. This may be a protein-fat film of minute thickness, perhaps only a few microns. The simplest way of restraining growth of all micro-organisms is to keep the product or equipment dry. Thus in powders, even foods, micro-organ- isms will die out if the moisture content is low, e.g. below 5%, and they will also die out in tanks, fillers, pipelines, homogenizers, containers, etc., if these are kept dry. The one requisite is that they must be absolutely clean

50 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS i.e. entirely devoid of organic matter. If they are not, then organisms will survive but not grow, and so sterilization becomes ineffective. There is one important point to bear in mind in this connection. Bac- teria are classified in two broad groups on the basis of the gram stain. gram-positive organisms, such as Streptococcus, Staphylococcus, Coryne- bacterium, are more susceptible to disinfectants and antibiotics than the gram-negative (Pseudomonas, Salmonella, Shigella, coliforms) but are more resistant to drying, especially if protected by traces of organic matter. Thus the streptococci of scarlet fever, basically very delicate bacteria, can survive for years in particles of skin shed by patients, be hidden in such places as bed equipment, and cause an infection years later. The hygiene 'probability equation' Developing the theme of the factors controlling bacterial growth we may express the situation in highly condensed form as: Pt =probable bacterial count at time t =f (N o, T, a w (or RH),Fqn, Fn, L-l,t) where N O =initial number T =temperature a w =water activity in product RH ----relative humidity in equipment Fqn = amount of nutrients available (e.g. soil in equipment) F n =nature or type of product L =lethality factor (e.g. uv, sunlight, acidity, high osmotic pressure) t =time of observation after production This means simply that the probable count in a product or in a piece of equipment after time t will be a function of the factors discussed above. If all factors except two, e.g. T and t, are standardized it may be possible to estimate Pt with a fair degree of accuracy, using data pre- viously obtained. Acidity In general, micro-organisms flourish best in the pH range 5-8.5. Pathogens are usually more sensitive to extreme values and many grow well only between pH 5.5 and 8. Any product, including even foods, which is below pH 4.5 can be regarded as safe because no ordinary pathogens can