The US Pharmacopeia (USP) and the European Pharmacopoeia (Pharm Eur) "Microbial Limits Tests" are in the final stages of harmonization. They were signed off to Stage 6A at the November 2005
meeting of the Pharmacopeial Discussion Group held in Chicago, Illinois (1), and the harmonized versions have been published
(see USP 2006, supplement 2). The harmonized chapters (Stage 6B, see sidebar, "Stages of the Pharmacopeial Discussion Group process")
do not differ significantly from the drafts published in 2003 (2–4).
The format of the USP chapters changes dramatically with this harmonization. Whereas the Microbial Limits Tests were two chapters in the USP 29 (5, 6), they are now modified in the harmonized version to mirror the European format (see Table I).
Stages of the Pharmacopeial Discussion Group process
The implementation of the tests is on different schedules in the United States and in Europe. In the United States, the tests
were originally scheduled to become effective Aug. 1, 2007, but the implementation date has been postponed to May 1, 2009
on the basis of comments received by USP. In Europe, implementation has three different schedules depending upon the situation:
1. Substances covered by a monograph specification: use the methods of the European Pharmacopoeia (A) until the monograph is revised and implemented (projected date: January 2009).
2. Substances not covered by a monograph specification: use either the methods of the European Pharmacopoeia (A) or the harmonised methods (B) until January 2010. From January 2010: use harmonised methods (B).
3. Preparations: use either method of the European Pharmacopoeia (A) or the harmonised method (B) of chapter 5.1.4 until January 2010. For new preparations, use of harmonised method (B)
is advisable. From January 2010: Use harmonised method (B) of chapter 5.1.4 (7).
Table I: Harmonized chapter numbers.
USP ‹61› "Microbial Enumeration"
The microbial enumeration test is a basic, simple design to count the number of colony-forming units (CFUs) in a nonsterile
product or raw material. The preferred method is to put the material into a solution and then plate the aliquots to determine
the CFUs/g (or mL) of initial material. If the product cannot be put into a solution, the most probable number (MPN) method
has several provisions to use. A full description of the MPN method is outside the scope of this article, but interested readers
can refer to the discussion in the US Food and Drug Administration's Bacterial Analytical Manual (8).
The method of plating can be either pour plate, spread plate, or material filtration and then placing the membrane filter
on an agar plate surface. The membrane filtration method should only be used when few CFUs are expected to be found in the
material to be tested. Though membrane filtration is a good method to test a large volume of liquid, it can only count as
many as 100 CFUs/membrane.
The harmonized method provides much more detail than any of the current pharmacopeial methods in terms of demonstrating method
suitability (method validation) and media-growth promotion.