USP <61> Microbiological Examination of Nonsterile Products: Microbial Enumeration Tests

USP General Chapter <61> describes harmonized procedures for the quantitative enumeration of mesophilic bacteria and fungi that may grow under aerobic conditions in nonsterile pharmaceutical products and raw materials. In practice, the chapter is used to generate the two core reportable values for routine microbial enumeration: TAMC (Total Aerobic Microbial Count) and TYMC (Total Combined Yeasts and Molds Count).

USP <61> addresses how many microorganisms are present, whereas USP <62> addresses specified microorganisms, and acceptance criteria are handled separately through monographs and USP <1111>.

Q4B Annex 4A(R1) states that USP <61>, Ph. Eur. 2.6.12, and JP 4.05 can be considered interchangeable in the ICH regions for the analytical procedure.

USP <61> covers
USP <61> is a quantitative bioburden chapter for nonsterile materials and products. It is intended to enumerate microorganisms that grow under the defined test conditions and report TAMC and TYMC. It should not be presented as the chapter that sets the product-specific microbiological limit itself.
Enumeration methods
The chapter includes membrane filtration, plate-count methods, and the Most Probable Number (MPN) approach. Membrane filtration is generally preferred for filterable products because it can help remove inhibitory effects through rinsing. Plate-count methods remain appropriate for many soluble or suspendable products. The MPN method should be described more cautiously: it is generally the least accurate method for microbial counts and is usually reserved for cases where filtration or plating is not feasible.
Method suitability
Before routine use, method suitability should demonstrate that the product matrix does not inhibit microbial recovery under the chosen conditions. The harmonized text also requires a negative control using the selected diluent in place of the test preparation to verify testing conditions. For inoculated suitability work, the familiar “factor of 2” concept applies: for solid media, recovery should not differ by more than a factor of 2 from the calculated value for the standardized inoculum.

Media and incubation conditions
For the standard plate conditions described in the harmonized text, TAMC uses Soybean-Casein Digest Agar incubated at 30 to 35 °C for 3 to 5 days, and TYMC uses Sabouraud Dextrose Agar incubated at 20 to 25 °C for 5 to 7 days.

The harmonized text gives explicit examples for prescribed limits: a limit of 10^1 cfu corresponds to a maximum acceptable count of 20, 10^2 corresponds to 200, and 10^3 corresponds to 2000. That is the practical interpretation point many readers miss. It is also worth stating that when replicate counts are performed, USP <1111> says acceptance criteria are based on the individual results or on the average of replicate counts, depending on the test design.