AXIO Proficiency Testing | Coliform Test Results in LGC Proficiency Testing Schemes
Conclusion The definition of coliforms is affected by the method used, and the interpretation of results, by individual laboratories. Coliform definitions also vary according to sample type and by national standards. As there is no taxonomic classification of coliforms, organisms classified as coliforms according to one method may not be classified or enumerated as coliforms by a different method. When participating in a global proficiency testing scheme many labs will be using different definitions of coliform and the results may therefore differ depending upon strains present in the sample. This is not a limitation of the method or laboratory but rather a result of the fact that coliforms are not defined by taxonomy. If you find that you have a result that is different to others in a round of proficiency testing in which an organism was included that does not fit your country regulations or the methods definition of coliform, you should reassess the results based on the sample contents and organisms included to explain any unsatisfactory results. In most cases, LGC AXIO Proficiency Testing addresses this by removing colour- coded assessments of such results, so as to not disadvantage laboratories applying a different definition or interpretation of coliforms compared to the majority. In microbiology there are also almost always exceptions to the rules. Microbiological methods have been developed over time to easily test for the most common, typical microorganisms of interest in foods and water, however there are many strains which may not behave exactly as expected under the conditions of the test; these are known as atypical strains.
This means methods may not be 100% accurate all of the time, especially with less common and atypical strains, and some degree of compromise and careful interpretation of the results may be needed. For example, if an organism that could have originated from an enteric source is found in a sample, then this could indicate poor sanitary quality, regardless of whether the organism produces gas or not under the conditions of the test. Coliforms, which may not behave as a textbook definition of a coliform under laboratory conditions, do exist in nature and could quite feasibly be present in food and water. For this reason, PT samples should contain a range of strains with different characteristics, including atypical strains, in order to reflect real life challenges. Such strains should be included infrequently to reflect their prevalence in nature, but to give participants the opportunity to challenge their methods, and to see how they perform compared to other laboratories carrying out the same tests. This may occasionally make AXIO proficiency testing samples more challenging than some other PT samples, but we strongly believe that it represents a better value to participants than providing PT samples that always contain the same strains and produce predictable results with no educational element.
As a result of these investigations , from January 2022, for sample MC16, AXIO will offer participants the option to report their coliform results according to method of confirmation. The options will be coliforms (presumptive), coliforms (confirmed by gas) and coliforms (confirmed other than gas). This additional information should enable a better interpretation of participant results.
We welcome any comments or feedback on this issue, please contact microbiology.bury@lgcgroup.com
References 1. ISO 4832:2006: Microbiology of food and animal feeding stuffs — Horizontal method for the enumeration of coliforms — Colony-count technique 2. ISO 9308-1:2014: Water quality — Enumeration of Escherichia coli and coliform bacteria — Part 1: Membrane filtration method for waters with low bacterial background flora
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www.lgcstandards.com/AXIO
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