Enumeration of Coliforms, Faecal Coliforms and of E. Coli in Foods using the MPN Method

3. PRINCIPLE

The terms "coliform" and "faecal coliform" have no taxonomic validity and, therefore, are only meaningful when expressed in terms of the analytical test parameters of medium, time and temperature of incubation. For further discussion of definitions, please see Chapter 8 in reference
8.2. Coliforms, faecal coliforms, and E. coli are considered "indicator organisms."

The presence of "indicator organisms" in foods processed for safety may indicate one of the following possibilities: 1. inadequate processing and/or post-processing contamination; and/or 2. microbial growth (8.2). The presence of faecal coliforms and E. coli may indicate faecal contamination; however, it must be understood that these microorganisms can survive and multiply in a variety of non-intestinal environments, including the processing plant (8.2). When assessing the presence of "indicator organisms" in a sample, one must assess the results against the tolerance limits specified by government standards or guidelines, health agencies, or a laboratory's in-house specifications, keeping in mind that established standards and guidelines are specifically linked to the method used to develop these standards.

As indicated in section 1, the presence of coliforms, faecal coliforms and aerogenic E. coli in food and water may be determined by means of the MPN procedure. Briefly, this method involves serially diluting out the target organisms in the sample, in 5-replicate aliquots, to extinction. (8.4) The probable level of the target organisms is then statistically estimated from an MPN table.

Gas production is used as an indication of ability to ferment lactose from LST broth (presumptive coliform test); gas production from BGLB broth is considered confirmation of coliform presence; gas production at 44.5 or 45^o C from EC broth is used as confirmation of faecal coliform presence; and appearance of typical nucleated, dark-centred colonies with or without metallic sheen when positive EC broths are streaked onto L-EMB agar are indicative of E. coli. The typical colonies on L-EMB agar must be confirmed by further biochemical tests to prove the presence of E. coli.

4. DEFINITION OF TERMS

See Appendix A of Volume 2.

5. COLLECTION OF SAMPLES

See Appendix B of Volume 2.

For shellfish and fish products you may also need to consult the following:

Canadian Shellfish Sanitation Program Manual

Fish Products Inspection Manual

Fish Products Standards and Methods Manual