Rapid alternatives to compendial microbiological methods

Over the past 80 years, few changes have been made to compendial microbiological methods, which include United States Pharmacopeia (USP) Chapter <61> ‘Microbiological Examination of Non-sterile Products: Microbial Enumeration Tests’,¹ USP Chapter <62> ‘Microbiological Examination of Non-sterile Products: Tests for Specified Microorganisms’,² and USP Chapter <71> ‘Sterility Tests’.³

Although the compendial methods are the standard, rapid technologies have recently emerged and, in some cases, have received regulatory approval as alternatives to traditional microbiological methods.

In 2000, the Parenteral Drug Association (PDA) published the first guidance document on how to validate and implement alternative rapid microbiological methods.⁴ The USP and European Pharmacopeia (EP) have also published similar guidances.^5,6

Rapid microbiological methods (RMMs) are grouped into three categories (EP 5.1.6): growth-based methods, direct measurement and cell component analysis. Growth-based methods detect a signal after a short incubation period in liquid or on solid media; examples include detection of CO₂ production by colorimetric methods. Direct measurement methods can detect cell viability without requiring growth of the micro-organism. One example of a direct measurement method combines fluorescent labelling and laser scanning cytometry to enumerate organisms. The third type of RMM is cell component analysis or indirect measurement; expression of certain cell components correlates to microbial presence. One example is amplification of DNA or RNA by polymerase chain reaction (PCR).