USP 51 Antimicrobial Effectiveness Test

Welcome to Antimicrobial Test Laboratories' USP <51> Resource Page.

Below, you will find a summary of the USP 51 method, along with some of its strengths and weaknesses.

In addition to the summary of the USP 51 test you will find below, we have put together a page we're calling "Getting the Most from Preservative Efficacy Testing."

Summary of the USP <51> Antimicrobial Effectiveness Test:

Each test microorganism is grown in liquid or on solid medium, depending on the microorganism. Microorganisms tested in the USP 51 test follow:

C. albicans
A. niger
E. coli
P. aeruginosa
S. aureus

The test microorganisms are either harvested by centrifugation or by washing surface growth from solid medium into a sterile vessel.
The concentrations of test microorganisms are standardized by resuspending harvested microorganisms in sterile saline to yield ~1 X 10^8 CFU/ml.
A sufficient volume of test product (typically 10ml) is distributed across 5 separate containers, and each container is inoculated with a separate test microorganism (mentioned above).
The final microorganism target concentration varies for various product types, but ranges between 1 x 10^3 and 1 x 10^5.
The initial concentration of viable microorganisms in the test product is determined by standard dilution and plate count methods.
Each inoculated test product is incubated at 22.5 ± 2.5°C and sampled to determine microorganism concentration "antimicrobial effectiveness" at 7, 14 or 28 day intervals depending on the product category.
The microorganism concentration at each interval is compared to the initial concentration, and the antimicrobial effectiveness is determined based on the guidelines set forth for each product category.

Strengths of the USP <51> Preservative Challenge Method:

The USP 51 Antimicrobial Effectiveness Test specifies the initial target inoculum concentration, which allows for a fairly reproducible comparison of products that fall in the same product category.
The USP 51 challenges preserved products with a variety of microorganisms representing a broad spectrum of manufacturing, nosocomial and house hold contaminants, including gram-negative and gram-positive bacteria, common fungi and mold.
The initial inoculum concentration is relatively high, thus providing a critical antimicrobial challenge prior to the product meeting the shelf.

Weaknesses of the USP <51> Preservative Challenge Method:

The USP has divided products into four categories for the purpose of testing, and each category has separate antimicrobial effectiveness "passing" criteria.
Antimicrobial effectiveness criteria, with respect to C. albicans and A. niger, is poorly defined across the 4 categories, for a sample taken from 14 and 28 day intervals can have the same microorganism concentration as the initial inoculum, and still meet the "pass" criteria for these two microorganisms.

Antimicrobial Test Laboratories has a great deal of expertise in preservative efficacy testing, and in particular the USP <51> challenge test. For more information about the <USP 51> Preservative Challenge Method, Contact the Lab Today!