FUNDAMENTALS OF MICROBIOLOGY IN RELATION TO CLEANSING 47 should contain a bacteriostatic substance (or condition) capable of pre- venting their growth. For pathogens the requirement is different and more exact. Either the product must be free from pathogens or it must contain a substance (or condition) that kills them. This is because a surviving pathogen may not be able to grow in the preparation itself but may be able to proliferate when applied to the skin or mucous membranes, or when inhaled or ingested. The expression 'free from pathogens' needs definition, and as a stan- dard we suggest failure to recover recognized pathogens, particularly Staph. aureus, Ps. aeruginosa and Salmonella from 100 g using the common standard methods. Coliforms constitute a convenient index group as a measure of quality of raw materials, plant sanitation and hygiene in hand- ling, processing and packaging. The unpredictability of micro-organisms The classical textbooks divided bacteria into pathogens and harmless organisms, but unfortunately the true position cannot be so lightly dis- missed. During the last 25 years many bacteria previously regarded as hamless commensals or possibly as useful indicator organisms, such as E. coli, have been shown to possess pathogenic powers and be capable of causing illness and even death. Whether this is due to the frequent uncritical and perhaps irresponsible use of antibiotics since 1945 by the medical and veterinary professions must be a matter of opinion, but even some members of these professions are beginning to have doubts about the wisdom of the present day widespread use of antibiotics (1). Whatever the reason, today organisms such as Pseudomonas aeruginosa, E. coli, Aerobacter, Flavobacterium, Serratia have been responsible for killing diseases, sometimes in epidemic form. These are all Gram-negative, resistant to disinfectants and antibiotics, and flourish under watery con- ditions. The irresponsible use of disinfectants, preservatives and anti- biotics may even fayour the establishment of these types by repressing the Gram-positive organisms, against which they are very effective. Another aspect of the greatest importance is the considerable degree of variability in any genus, and even in any one species of micro-organism. Textbooks may define the conditions permitting growth and thermal death points, etc., but there will always be exceptions, 6r in"•6ther words, freak organisms. Such atypical strains are often responsible for disease and

48 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS defects in products such as cosmetics, pharmaceutical preparations and foods. It follows that assumptions must never be made, risks must never be taken, and a substantial margin of safety always allowed in specifying any heat-treatment, conditions of using a disinfectant and concentration for any preservative until certainty of result has been clearly established by a large number of tests. The chemist and bacteriological phenomena The chemist who has not received any biological training often has difficulty in appreciating the wide range of variation in biological behaviour, and the relatively enormous and unavoidable errors in micro-biological testing. For example, the ordinary 'total count', generally the most commonly performed test, has a large error, especially when made on solid materials. In solid and semi-solid materials micro-organisms occur in colonies containing possibly millions of cells. Maceration of the product breaks up the colonies with unpredictable scattering, leading to a large error in the resultant count or very poor reproducibility. Chemical analysis can usually give repeated results agreeing within about 0.1%, but in such tests as the total count it is better to think in terms of logarithmic values, e.g. to allow a difference of X 10 before asserting that one result is really different from another, or that one sample is better than another. Assuming reliable sampling, two chemical laboratories can usually get reasonable agreement with their analyses. In bacteriological work wide differences may be found, because the method of sampling, handling of the sample, temper- ature of transport and storage, and technique differences between labor- atories may be of tremendous significance. The chemical condition of a substance is usually static whereas the microbiological condition may be quite mobile. FACTORS CONTROLLING BACTERIAL GROWTH The most important conditions controlling micro-organisms are (1) availability of food for growth and as a source of energy, (2) warmth or a certain range of temperature, ($) moisture, water activity or relative humidity, and (4) absence of lethal factors. Food Micro-organisms can flourish with such minute amounts of food that