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MPL Laboratories
12 Wilson Drive
Sparta, NJ 07871

Phone: 973-300-9715
A Full Service Microbiology Laboratory


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USP 51 Antimicrobial Effectiveness Testing


As described in USP <51>, this test demonstrates the effectiveness of added antimicrobial preservatives. Products packaged in multidose or multiuse containers may contain preservatives to prevent the growth of microorganisms that could be introduced during repeated uses of the product. Antimicrobial effectiveness testing is commonly performed on pharmaceutical, cosmetic, and personal care products such as injectable or topical medications, sunscreens, shampoos and conditioners, toothpaste and oral rinses, or eye care and nasal solutions.  Antimicrobial effectiveness testing (AET) is also known as preservative efficacy testing (PET) or challenge testing.

Routine Testing

The antimicrobial preservative challenge test consists of inoculation of the product with a high concentration of known challenge microorganisms. At a minimum, USP <51> specifies the use of these three bacteria and two fungi:

       •Bacteria: Escherichia coli, Pseudomonas 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. The effectiveness of the product’s preservative system is evaluated by comparing the initial concentration of microorganisms to the test product at various time intervals over a period of 28 days. 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.

Suitability Testing

The USP <51> suitability test is used to establish the routine testing method, which in the presence of the product, allows the detection of the challenge microorganism (s). If the product demonstrates antimicrobial properties, the ability of the media and dilutions used to promote microbial growth must be established. Neutralizers may need to be added to the product to compensate for observed antimicrobial properties.

Compendial Product Categories

Based upon the USP <51> criteria for antimicrobial effectiveness, compendial products are grouped into four categories:

Category 1: Injections; other parenterals including emulsions, otic products, sterile nasal products, and ophthalmic products made with aqueous bases or vehicles

Category 2: Topically used products made with aqueous bases or vehicles; nonsterile nasal products and emulsions, including those applied to mucous membranes

Category 3: Oral products other than antacids, made with aqueous bases or vehicles

Category 4: Antacids made with an aqueous base


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