Monday 26 April 2021

Why it’s time to strengthen and widen the microbial test panel

The testing of culture media, and conducting microbiological method suitability studies, disinfectant efficacy studies, Antimicrobial Effectiveness Test (or Preservative Efficacy Test), and associated activities, requires the use of a test panel of microorganisms. These organisms need to be representative of the intended application and be of a suitable range in order to demonstrate that a low level of viable cells can be recovered without any indication of the inhibition of growth or excessive growth-promotion (typically defined as the recovery of between 50 to 200% of the challenge count). This activity provides the basis for the release of culture media or for verifying that a method is suitable for recovering any microorganisms that might be present in a product sample.

This approach has been established for several decades and in many cases, the types of organisms selected for the panel are drawn from guidance presented in the major pharmacopeia. The test panels of organisms recommended by compendia along with specific standards are in place to allow for reproducibility between laboratories. In the case of most standards (such as the disinfectant efficacy norms), these are designed to be multi-industry. Hence the presented organisms may or may not be suitable for the intended application.


In addition, with the compendia, all too often the organisms recommended for, say, the Microbial Limit Test method verification, are applied to the release of all culture media used for all activities, be that the test for sterility or for the recovery of organisms from water systems.

Not only is this inappropriate – for the microbiologist should be reviewing the panel and deciding upon the appropriate organisms – the recommended panels have not kept pace with improvements with our understanding of the types of microorganisms likely to cause contamination. Take, for example, the inclusion of Salmonella in the panel for the recovery of so-called ‘objectionable microorganisms’ for use with the Microbial Limits Test (1). To my knowledge and based on discussions with microbiologists over a couple of decades, no Salmonella has ever been recovered from a raw material.

Equally the selection, especially in relation to the testing of culture media used for cleanroom environmental monitoring, has not moved forwards with the findings from the human microbiome project and the depth of species richness found on the human skin microbiome.

The argument that the test panel quoted in the compendia is representative and thus if culture media release testing can recover such organisms then it can reasonably be assumed that any other similar organisms can be recovered does not really hold up. Unless that is, the only concern is with the recovery of non-fastidious mesophilic organisms.

Instead, I maintain that in terms of the appropriate panel, this should be based on:

  • What is currently being recovered, and 
  • What should theoretically be recovered.

To read the full article, the reference is:

Sandle, T. (2021) Why it’s time to strengthen and widen the microbial test panel, European Journal of Parenteral and Pharmaceutical Sciences, 25 (4):

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (

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