Abstract:
What is known and objective: Preservative efficacy in medicines is typically investigated using challenge tests. In such tests the product is artificially contaminated with a high concentration of standard bacterial and fungal test strains such as Candida albicans and Pseudomonas aeruginosa. The rate and extent of reductions in inoculum viability over a specified period forms the basis for acceptance/rejection of preservative efficacy. The water activity and nutrient content of medicines can vary greatly, for example cough mixtures and eye drops, and medicines are often stored cold. None of the strains named for inclusion in the challenge test outlined in the Europoean Pharmacopoeia are associated with the contamination of high quality water used in pharmaceutical production. Alpha- and betaproteobacteria are easily the most common microbes in waters intended for pharmaceutical production. In addition, none of the test strain panel prefer low nutrient, dilute conditions, or grow at or around refrigeration temperatures. We investigate the importance of these factors when testing preservative efficacy by including other strains in challenge tests. Methods: Psychrotolerant, low-nutrient preferring beta- and alphaproteobacteria and a yeast were isolated from pristine waters and compared in challenge tests with Candida albicans and Pseudomonas aeruginosa. Pharmaceutical products differing widely in water content, pH and preservative systems were tested. Results and discussion: Regardless of the type of medicine tested Candida albicans always showed superior survival characteristics to the yeast isolate. One of three screened bacterial strains (a Sphingomonas sp.) survived significantly better than Pseudomonas aeruginosa in all but one product tested. However, the results for all products taken together cannot easily be explained by reference to this strain’s psychrotolerancy or its preference for dilute, low-nutrient environments. The present study supports previous work indicating that the inclusion of wild-type test strains, in this instance strains which are suited to survival in high quality waters, improve preservative efficacy tests. What is new and Conclusion: Use of a Sphingomonas sp. isolated from a pristine water as a challenge test strain, gave a more stringent indication of preservative efficacy in a wide-range of pharmaceutical than did Pseudomonas aeruginosa.
