A study of the use of rapid methods for preservative efficacy testing of pharmaceuticals and cosmetics.

Three rapid microbiological methods, impedance, the direct epifluorescence technique (DEFT-MEM) and ATP bioluminescence (ATP-B) were evaluated for their applicability to preservative efficacy testing (PET) of pharmaceuticals and cosmetics. A good correlation between rapid method response and total colony counts was obtained for untreated suspensions of Staphylococcus aureus, Pseudomonas aeruginosa and Candida albicans with all three methods but, for Aspergillus niger, with impedance only. For chlorhexidine-treated suspensions of Staph. Aureus and C. albicans, a good dose-response curve was obtained with impedance, but ATP-B and DEFT-MEM methods underestimated the kill by the order of 1-6 logs. From the results of this study it is concluded that impedance offers an alternative method to colony counting methods for PET but, at their present level of method development, neither DEFT-MEM nor ATP-B can be considered as satisfactory.