Cosmetic or pharmaceutical formulations containing hydrophilic polymers of natural or synthetic origin, may be more exposed to successful microbial contamination because of a polymer-preservative interaction. The experimental data reported in this paper relate to the possible interference of Carbopol 940 with methyl parahydroxybenzoate. Results show that this hydrophilic polymer, widely employed in many formulations, exerts, on the contrary, an interesting synergism on microbicidal activity of the preserving agent against E. coli and P. Aeruginosa. A reduction in microbicidal activity against S. aureus and C. albicans is observed for a polymer concentration higher than that needed for anti-Gram-negative synergy.

The use of anodic silver ions as preserving agents in cosmetics was tested by a challenge test in a set of cosmetic dispersions with the addition of known preservative inhibitors or micro-organism growth-promoters such as humectants, hydrosoluble collagen and vegetable extracts. Silver's microbicidal efficacy, compared to that of imidazolidinyl urea or methyl p-hydroxybenzoate, showed a more efficient activity especially in the presence of proteinaceous material. This agent may represent a good and safe protection for finished products both in manufacture and during use.