A Review of the Concept of Coliforms
Revised July 31, 2013
Remember the definition of coliforms:
- These are strains that share the characteristics of being gram-negative rods and also fermenting lactose with the production of acid and gas.
- You may have seen an extended definition which states that they ferment lactose (with visible gas production) in Lactose Lauryl Tryptose Broth (LLTB) and also Brilliant Green Lactose Bile (BGLB) Broth. These media are important in the Presumptive Tests where we are selectively enriching for them and also detecting their presence among the mixed cultures in these media; these media are selective against Gram-positive bacteria, especially BGLB.
- We could have used LLTB and BGLB in the Completed Tests when we tested the isolates we obtained off EMB Agar, but we simply tested for acid and gas production in Lactose Fermentation Broth. (And instead of doing the gram stain, we assumed that growth on EMB Agar was indicative of being gram-negative.)
- You may have seen in some of the literature that coliforms are "aerobic or facultatively anaerobic." This is an old-school description of what we call facultative anaerobes. They do grow as strict aerobes unless we give them an opportunity to grow under anaerobic conditions – such as fermentation. As they all ferment lactose, they will all ferment glucose as well. (If any organism ferments a sugar, it is assumed they can ferment at least glucose.)
Also remember that coliforms are not the problem but rather an easily-detectable type of organism that is associated with and therefore indicative of certain problems:
- If a coliform is identified as Escherichia coli, that means the environment was contaminated with fecal matter. It is a lot easier to look for E. coli than any of the other intestinal pathogens which may be bacterial, protozoan or viral!
- Remember we mentioned (and it is often forgotten) that well water is tested for coliforms. Many coliforms are found growing naturally in soil, and if they are detected in drinking water, that means that surface soil runoff is getting into the well, bringing problems that affect the water's safety.
- It is true that certain types of E. coli and Klebsiella happen to be pathogenic, but our emphasis was on the use of coliforms as indicator organisms of problems associated with the environment in which they are found.
The term "coliform" should not imply that there is a taxonomic group consisting of coliforms:
- There is no genus or species that is classified as a coliform.
- Any particular coliform isolate is usually identified as Escherichia coli or some other species of enterics, and any of these species can naturally consist of lactose-positive and negative strains. For example, many strains of E. coli which cause intestinal illness do not ferment lactose at all and tend to resemble Shigella to a great degree. Remembering the strict definition of coliforms, these lactose-negative organisms would not be called "coliforms."