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USP Antimicrobial Effectiveness Testing

Many cosmetics, health and beauty aids, parenterals, topically used products, and oral products have the potential to support microbial growth.  Contamination may occur during manufacturing or, more often, by the end-user after the container is opened.  Repeated opening and closing of containers and contact with hands, skin, and mucous membranes or repeated withdrawal of individual doses can result in contamination.  Without some antimicrobial properties, products can allow the growth of many different microorganisms. 

A product may have intrinsic factors that inhibit microbial growth, or formulators can add antimicrobial preservatives to protect them from microbial growth.  When preservatives are added, the goal is to add only as much as is needed since all antimicrobial agents are considered toxic substances. 

To determine the antimicrobial properties of products, the USP <51> Antimicrobial Effectiveness Test is a valuable tool for evaluating either intrinsic antimicrobial properties, or different types and levels of added antimicrobial agents.

The USP <51> Antimicrobial Effectiveness Test involves adding specified microorganisms directly to the test product at relatively high concentrations to simulate contamination.  The product is held for one month, during which time the added microorganisms are enumerated weekly to determine if they are growing, dying off, or remaining near the initial inoculation level.  Inoculated microorganisms include Candida albicans, Aspergillus niger, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus.  The key to an accurate test is having fresh, active cultures standardized to a concentration that when added to the product, deliver between 100,000 to 1,000,000 cells per ml of the test product. 

The acceptance criteria for antimicrobial effectiveness is described in detail in USP <51>, but in general, a 1 to 3 log reduction in bacteria from the initial level should occur in one to two weeks, with no further increase in bacteria after two weeks. 

For yeast and mold, no increase from the initial inoculum level is permitted.

Microbac has experienced microbiologists who have been performing the USP <51> Antimicrobial Effectiveness test for over 20 years.  We have assisted both small and large processors in the development of effective antimicrobials in a wide range of products.  Contact us for further information on Antimicrobial Effectiveness testing and our other USP test capabilities.

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For more information, contact:
Dr. Kim Baughman, Director of Development
Microbac Laboratories, Inc.
kbaughman@microbac.com

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