Some of the basic points that need to be addressed and the principles involved in selecting test methods are discussed under the section on chemical laboratories.

In selecting methods for microbiological testing, consider the requirements of the clients, the requirements of the regulatory organizations and the suitability of the method for the purpose, assuming there are no other limiting factors such as cost, competency, duration to perform the tests etc.

There are several sources from which to select test methods. In selecting methods, the analysts need to be mindful of the conditions under which the microorganisms are present in the matrix, specially the stresses arising from storage, processing and use of chemicals. These stresses results in debilitation of some enzymes in the microorganisms, resulting in reduced responses to the biochemical tests. This requires regeneration of the microorganisms using enrichment and selective enrichment media. Different test methods described in literature vary in their techniques of enrichment, and this could cause variations in test results. The test methods used generally in medical diagnostic tests for microorganisms may not work effectively with food microorganisms that have undergone stresses.

It is advisable to select well-validated test methods, written by organizations engaged in writing test methods. Among the different sources of test methods the following are usually accepted as more reliable.

• Official Methods, Association of Official Analytical Chemists (AOAC)
• Microbiology Manual of the Food and Agriculture Organization (FAO)
• Bacteriological Analytical Manual (FDA)
• American Public Health Association (APHA)
• International Standards Organization (ISO)

Even with the methods developed and published by the above sources, the biochemical reactions shown by the microorganisms could vary due to matrix effects. The “Compendium of methods for microbiological examination of foods” provides helpful information for the more meaningful interpretation of test data. Some documents that could provide useful information on test methods are listed below, but there are also other sources among various organizations that provide acceptable test methods.

(a) Compendium of methods for microbiological examination of foods (seek the most recent edition),

(b) Lenore Clasceri, Arnold E Greenberg, Andrew W Eaton. Standard Methods for Examination of water and wastewater. 20th edition or more recent edition.

(c) FAO manual on Microbiological Analysis, (1995), Nutrition paper 14/4.

(d) Official Methods of Analysis, Association of Official Analytical Chemists [Available in the form of Text Book or Compact Disc]

(e) H. H. Huss et al. (2003) Assessment and management of Seafood safety and Quality.

FAO Fisheries Technical paper 444 ISBN - 92-5- 104954-8

There are other sources that describe non-standard methods, in-house methods and methods associated with equipment. While these are important and are accepted by ISO 17025 International Standard, the laboratories using such methods are required to generate validation information that will satisfy the assessors when they are seeking accreditation. It is much safer for a microbiological testing laboratory to use test methods that are already validated and more commonly practiced.

Validation guarantees the ruggedness of the methods and provides evidence of their suitability. However, an analyst using a validated method still needs to provide evidence of his/her competence gained through regular and long practice, including performance checks in intra- and inter-laboratory comparison studies, as well as engaging in proficiency testing.