The use of impedance for preservative efficacy testing of pharmaceuticals and cosmetic products.

Impedance was investigated for its applicability to preservative efficacy testing of pharmaceuticals and cosmetics. A good correlation between impedance detection time (Td) and total colony counts (colony-forming units (cfu)) was obtained for untreated suspensions of Staphylococcus aureus, Candida albicans, Aspergillus niger and Pseudomonas aeruginosa in phosphate-buffered saline (PBS). A good correlation between Td and the number of cfu was also obtained for suspensions of test organisms treated for varying contact periods with selected concentrations of chlorhexidine, methyl paraben and phenoxyethanol in PBS, and methyl paraben in cetomacrogol cream, but these correlations were significantly different from those for untreated suspensions. It was found that for any given number of cfu the Td for preservative treated cells was extended. It is concluded that impedance represents a valid method for preservative efficacy testing of pharmaceuticals and cosmetics which could be used to achieve more comprehensive but economic screening of formulations against a wider range of preservative systems and concentrations than is the current approach where only a limited range of systems are tested because of the workload involved.