Microbial Enumerations Tests – What Every Cosmetic Chemist Should Know about USP 61

This is a guest post by Rachelle Baker, Lab Manager, Cosmetic Test Labs

Microbial contamination of cosmetic and personal care products is a frequent concern within the industry.  Contamination with microorganisms can cause both product and monetary losses for manufacturers.  Although reports of contaminated cosmetics causing injury to humans are rare, the economical and brand-related impact of such events is considerable. In the worst case, microbial contamination of cosmetic and personal care products can result in infection to the consumer.

Product contamination

Microorganisms can enter a product at the time of manufacturing, or during normal consumer use. Microbial growth in personal care products can take place very rapidly. Microbial concentrations in products in the period after manufacturing depend on the cleanliness and sterility of the manufacturing process and the microbial growth potential inside the product.

Cosmetic preservation

Cosmetic and personal care products typically contain chemical preservatives to prevent microbial growth during their shelf life. Preservative challenge testing is used to verify the effectiveness of preservative systems.  Although preservative challenge testing is an essential part of ensuring quality and stability of a product, it cannot indicate for certain that contamination with a problematic microorganism will not occur at some point in within its shelf life.

Often, contamination is caused by microorganisms that are not included in the various official preservative challenge test methods. In addition, sometimes errors are made during manufacturing, such as a slipped decimal place, when calculating the amount of preservative to add. For these and many other reasons, it is extremely important to run total microbial count tests, like the USP <61>, on personal care products to verify that they are not contaminated with microbes that have the potential to grow, even in seemingly well preserved products.

Microbial count tests

The USP <61> test, along with other similar methods, is a full quantitative analysis of products to determine the Total Aerobic Microbial Count (TAMC) and Total Yeast and Mold Count (TYMC) present in them. These tests are recommended for every batch of finished product. Should a contamination event ever arise, this method will be useful to test raw materials as well, to assist in determining the source of contamination.

One special aspect of the full USP <61> procedure is that it includes neutralization and recovery validations to ensure that the methods used to determine the presence of the microorganisms in each product are sufficient to detect them. This is important, because preservatives in cosmetic products have the potential to inhibit growth of microorganisms on laboratory agar plates, causing false-negative results.

Acceptable standards

Cosmetics and personal care products are not expected to be completely free from microorganisms. However, an acceptable standard for their microbial limits has not been established. Some manufacturers and agencies within the cosmetic industry recommend a maximum of 100 colony forming units (CFU) of bacteria or fungi per gram or milliliter of product. Everybody agrees that, pathogenic (disease-causing) microorganisms such as P. aeruginosa and S. aureus, among others, should not be detectable.

By having USP <61> tests conducted on every batch of finished product, manufacturers can have confidence that their products are contaminant free and safe for consumers. Independent testing labs, such as Cosmetic Test Labs are a great resource for cosmetic formulators and manufacturers for microbiological testing of their products.


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