The Concept of Coliforms and their Enrichment, Detection, Isolation and Identification

Coliforms are considered indicator organisms in water analysis – not because they are "bad" in themselves, but because they are often associated with potential health problems. Two examples: (1) Coliforms are abundant in soil, and if they are found in well water, that would be an indication of probable surface soil contamination of the water. Usually such coliforms are identified as Klebsiella and Enterobacter. (2) Identification of an isolate from any source as Escherichia coli would then implicate the source as being contaminated by intestinal waste, which is the habitat of E. coli. It is much easier to find whether or not E. coli is present through the enrichment and isolation procedure than it is to test for each specific intestinal pathogen (bacterial, protozoan and viral).

The usual definition one encounters about coliforms is that they are gram-negative rods that ferment lactose rapidly with the production of gas (insoluble gas that is detectable in a Durham tube) in Lactose Lauryl Tryptose Broth and Brilliant Green Bile Broth at 35°C. Those which additionally do so in EC Broth at 44.5°C belong to the subset of coliforms called "fecal coliforms."

As explained in the manual and in lab, coliforms happen to be easy to detect with the appropriate selective-differential media. These media tend to inhibit gram-positive bacteria, and the presence of coliforms (among other organisms which would also be present) is suggested by gas from lactose fermentation in the Durham tube of the enrichment media. Subsequently coliforms are detected by their acidic colonies on EMB Agar (with lactose) which is the first step in isolation and identification.

One must realize that coliforms do not constitute a discrete taxonomic group. Usually they are ultimately identified as various species of enteric bacteria of the genera Escherichia, Enterobacter, Klebsiella and Citrobacter. A fecal coliform isolate usually is identified as E. coli. That does not therefore mean that these genera are to be categorized as coliforms! Out in the real world, one may encounter many "exceptional" strains of these organisms that do not conform to the definition of coliform. Many pathogenic E. coli strains do not ferment lactose, and it is sometimes very difficult to differentiate them from Shigella. Also, most strains of Citrobacter (such as the one we use in Experiments 14A and 17) very weakly attack lactose and may be initially confused with Salmonella when isolations are made from clinical or natural samples. So, considering the absolute definition of this non-taxonomic term, there can be no such thing as a non-lactose-fermenting coliform.

The enrichment media in the following table increase in selectivity from left to right as they increasingly select for the desired organisms. Still, we never expect a pure culture in any enrichment medium (selective or otherwise), so the subsequent isolation (on EMB Agar) is important as in any enrichment-isolation procedure as we see in Experiment 11. For a tube which shows growth and gas, suggesting the presence of coliforms, all it might have taken for a population of coliforms to develop in the tube could have been just one coliform cell in the inoculum. Cells of non-coliforms may or may not be able to grow; one way or another in the overall enrichment and isolation process they will be "sorted out."