THE USE OF IMPEDANCE FOR PRESERVATIVE EFFICACY TESTING OF PHARMACEUTICALS AND COSMETIC PRODUCTS

Impedance was investigated for its applicability to preservative effic acy testing of pharmaceuticals and cosmetics. A good correlation betwe en impedance detection time (Td) and total colony counts (colony-formi ng units (cfu)) was obtained for untreated suspensions of Staphylococc us aureus, Candida albicans, Aspergillus niger and Pseudomonas aerugin osa in phosphate-buffered saline (PBS). A good correlation between Td and the number of cfu was also obtained for suspensions of test organi sms treated for varying contact periods with selected concentrations o f chlorhexidine, methyl paraben and phenoxyethanol in PBS, and methyl paraben in cetomacrogol cream, but these correlations were significant ly different from those for untreated suspensions. It was found that f or any given number of cfu the Td for preservative treated cells was e xtended. It is concluded that impedance represents a valid method for preservative efficacy testing of pharmaceuticals and cosmetics which c ould be used to achieve more comprehensive but economic screening of f ormulations against a wider range of preservative systems and concentr ations than is the current approach where only a limited range of syst ems are tested because of the workload involved.