Evaluation of Bacterial and Fungal Growth Media for Ability to Neutralize Cosmetic Preservatives

Abstract

Standard methods agar (SMA) and letheen agar (essentially SMA plus lecithin and Tween 80) were compared for bacterial growth and ability to neutralize cosmetic preservatives. Potato dextrose and malt extract agars (each prepared with and without lecithin and Tween 80) were compared with letheen agar and SMA in similar studies with fungi. Twelve bacterial strains, representing 8 species, and 2 fungal species were used as inocula. Plate counts of bacterial cultures (no preservatives present) ranged from 0 to 50% higher on letheen agar than on SMA except for 3 strains of Staphylococcus, which were 8-29% lower. Fungal counts were about the same on all media. Cosmetics (10 g) representing 4 preservative systems (hexachlorophene, benzoin, formaldehyde, and parabens) were inoculated with diluted cultures. Counts at 10−1 and 10−2 dilutions were typically 10-200% higher on letheen agar; however, in one case (benzoin, S. aureus, 10−1) the count was 400 on SMA vs 20 000 on letheen agar. Although differences in fungal counts were not as great, letheen agar partially neutralized the preservatives’ action. Results show that product dilution does not sufficiently reduce the effects of preservative carryover and neutralizers should be incorporated into plating media for this purpose.