ABSTRACT
Successful preservation, especially of a complex formulation, can sometimes be elusive. Many factors contrive to oppose the preservative agent. These include the physicochemical characteristics of the formulation, the breadth, level and type of microbial challenge, possible interactions with formulation ingredients, and preservative instability. One compound increases the effective concentration of the other by inhibiting the microbial system responsible for its inactivation. A semiquantitative modification to this basic technique involves a combination of one antimicrobial agent in agar, possibly in the form of a concentration gradient, and the other impregnated in an overlaid paper disc or strip. It is important to recognize when considering in vitro test methods that correlation between them is not always obtained since they often measure different parameters and employ different test conditions. The potential permutations of preservative combination, concentration ratio, product excipient, and formulation factors can be daunting.
