Sunday, February 17, 2013

Coliforms and operational definitions

Recently, I was talking about coliforms with my colleagues from the lab. ‘Coliforms’ is the name given to a group of bacteria that usually serve as indicators of fecal contamination in water and food samples, because most coliforms come from the intestinal flora of animals. For this reason, water and food that contain too high levels of coliforms are deemed unfit for human consumption (the US EPA recommends a limit of 10 or less per liter in drinking water). The most famous coliform, Escherichia coli, is a commensal in our intestine, although some pathogenic strains exist as well (for instance O157:H7, involved in deadly outbreaks in the US and Europe). Coliforms behave similarly as fecal pathogenic bacteria, notably regarding their survival in water. We can thus assume that a sample devoid of coliforms will also be free of fecal pathogens. Commercially-available tests for coliforms, such as the culture medium CHROMagar ECC, also permits us to differentiate between total coliforms and fecal coliforms (mostly E. coli), simply based on the coloration of the bacterial colonies on agar plates.

At some point during the discussion, I found myself wondering whether the term ‘coliforms’ could be used as a synonym to Enterobacteria (bacteria from the family Enterobacteriacaea). I thought that the terms were somewhat equivalent, but I checked in my BBOM and here’s what I read (pp.906-907):

“Coliforms are defined as facultative aerobic, gram-negative, nonspore-forming, rod-shaped Bacteria that ferment lactose with gas formation within 48 hours at 35°C. However, this is an operational rather than a taxonomic definition, and the coliform group actually includes a variety of organisms. Most coliforms are members of the enteric bacterial group. For example, the coliform group includes the organism Escherichia coli, a common intestinal organism, and the organism Klebsiella pneumoniae, a less common pathogenic intestinal inhabitant. However, Enterobacter aerogenes, an organism not found in the enteric group or in the intestine, is also classified as a coliform because of its fermentative properties.”

So all coliforms belong to the family Enterobacteriacaea, but not all coliforms inhabit the gut of animals, and not all Enterobacteriacaea are coliforms. The key point in this quote is that ‘coliform’ is an operational definition. In other words, we define coliforms as those bacteria that can do so and so (ferment lactose, etc.). In our example, coliforms happen to coincide with the taxonomic group of Enterobacteria, but in principle belonging to this group is not a prerequisite to be classified as a coliform.

Operational definitions are quite widespread in science. For instance, during my PhD thesis I worked on the effect of dissolved organic carbon (DOC) on the degradation of hydrocarbon pollutants by bacteria; DOC is simply defined as all organic matter in solution that can pass a filter of a certain pore size! Another example from microbiology is the Gram coloration: bacteria treated with a series of chemical will be stained either red or purple, which is interpreted as Gram-negative and Gram-positive, respectively.

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