706 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Some preservatives recommended by pharmacopoeias Pharmacopoeia French Pharm. Preservative B.P. Codex U.S.P. Helv. Phenol 0.5 • 0.5 •o 0.5 • -- Cresol 0.3 % 0.3 % 0.5 % 0.3 •o Chlorocresol 0.1 •o 0.3 % -- 0.2 % Organomercurial 0.001% -- 0.1% -- Methylparaben -- -- -- O. 1% or 20 times that of the weaker. For chlorocresol, the concentration range is 0.1-0.3• and the most concentrated will have an activity about 30 or 725 times that of the weakest. What is clear is that pharmacopoeias reflect the uncertainty of precisely what constitutes an adequate preservative. If the activities of the various preservatives recommended by any single pharmacopoeia is examined an equally wide variation is obvious (Table Ii), those of the British Pharmacopoeia ranging from about 10 min to 10 h. Table II The activities of preservatives recommended by the British Pharmacopoeia Preservative %w/v Time to kill 10 a ml-1 E. coli Chlorocresol 0.1 10 min Cresol 0.3 30 min Phenylmercuric NO a 0.001 2 h* Phenol 0.5 10 h *Using thioglycolate recovery medium. The only point common to all pharmacopoeias and all preservatives recommended by them is that they are bactericidal rather than bacterio- static and even markedly bactericidal with the possible exception of 0.1•o methyl parahydroxybenzoate. It is interesting to note that a working party report (12) recently published by the Society of Cosmetic Chemists of Great Britain recommends that cosmetic products should be self sterilizing. It is my belief that the most satisfactory requirement to date is that of the U.S.P. XVIII (13) which requires that a satisfactorily preserved ophthalmic solution will reduce the viable count of any contamination to 0.1}/o of the original and not permit any subsequent multiplication of the survivors during a 7 day period of the 28 day test period. The latter clause is all important. Dr Walters and I (14) found, as long ag• a• 1)55,

PRESERVATIVES FOR PHARMACEUTICALS 707 that the last survivors in any initially bactericidal solution may multiply, utilizing as nutrient the purines and pyrimidines released by dead and/or dying cells. Several reports of similar observations have since been pub- lished (15-17) showing that the multiplication is not peculiar to the use of any one preservative and suggesting that it may occur generally with pre- servatives having high intrinsic activity and therefore used in low con- centration. The phenomenon has, to my knowledge, never been associated with preservatives used at concentrations of 0.l•o or higher, unless the product provided considerable nutrient. Although I believe the U.S.P. requirement to be basically satisfactory ! would be even happier if the required mortality level was increased to 100•o (admittedly a difficult condition to demonstrate in practice) and the latter requirement associated with a maximum time limit--perhaps 3 h because this represents the approximate interval at which glaucoma patients apply their ophthalmic drops. Again, pharmacopoeial preser- vatives would often meet this requirement and it therefore represents the type of standard which could be imposed. ACTIVITY AND CAPACITY OF PRESERVATIVES Having postulated possible requirements for preservatives a selection has to be made of a possible compound. A useful guide seems to be that if a saturated aqueous solution of the candidate-compound fails to sterilize an inoculum of about 108 ml -• vegetative organisms within 10 min, it is unlikely to prove suitable. Phenol, cresol, chlorocresol benzalkonium, chloride and chlorhexidine acetate amongst others would meet this re- quirement, chloroxylenol, benzylchlorophenol and the parabens would not. The first group have relatively high water solubility and relatively low intrinsic activity. Again, the acceptable compounds are recognized by the pharmaceutical compendia, the unacceptable usually are not. The concentration of the chosen compound to produce the required activity is next determined, particular consideration being given to the probability of the preserved solution being able to maintain its activity against a heavy infection or even a succession of infections. The destruction of organisms by preservatives involves a chemical or physical reaction between the micro-organism and the preservative resulting in a loss of preservative to the cells from the solution, the heavier the infection the greater the loss of preservative from the solution. The more active pre- servatives are used in lowest concentration and it is these that suffer the