Means, Ranges and Replicates: Improving Microbial Plate Counting

A microbial colony is a visible cluster of microorganisms growing on the surface of or within a solid medium. This may be from a single cell or an amalgam of the same organism of more than one cell or a mix of different organisms. Bioburden levels are commonly measured when conventional methods are deployed in terms of colony-forming units (CFUs). These units provide an estimation of the number of viable bacteria or fungal cells found on a sample, expressed against a unit of measurement, such as per millilitre or per milligram.

When counting CFUs on solid microbiological culture media (agar) there are some aspects of ‘counting’ that need to be considered. These are:

a) Rounding and averaging

b) Significant figures

c) Countable range

d) Statistical error from low counts

This article looks at each of these aspects of the microbial plate count.

Sandle, T. (2025) Means, Ranges and Replicates: Improving Microbial Plate Counting, European Journal of Parenteral and Pharmaceutical Sciences, 30 (1): https://www.ejpps.online/post/means-ranges-and-replicates-improving-microbial-plate-counting 
 

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)