Antimicrobial effectiveness testing is commonly performed on pharmaceutical, cosmetic, and personal care products. The antimicrobial preservative challenge test consists of inoculation of the product with a high concentration of known microorganisms. USP <51> specifies the use of three bacteria and two fungi:
- Bacteria: Escherichia coli, Psuedomonas aeruginosa, Staphylococcus aureus
- Fungi: Candida albicans, Aspergillus brasiliensis
The initial concentration of each challenge microorganism listed above is determined using standard dilution and plating techniques. For Category 1 (injectable products), surviving microorganisms are counted at days 7, 14, and 28 of the challenge study. For Categories 2, 3, and 4 (topical, oral, and antacid products), surviving microorganisms are counted at days 14 and 28 of the challenge study. A log reduction is calculated for each challenge microorganism, at each time interval. The antimicrobial effectiveness of the product is determined using USP <51> test acceptance criteria.