Coliform bacteria, a group of closely related bacteria found in soil, water and the intestinal tract of animals, have been used as indicators of unsanitary conditions in food and beverage production for more than a century, including tests for water and dairy products originating in the early 1900s. Today, the coliform count is a frequent hygienic indicator in several food and beverage industries.
The term coliform has no taxonomic status, but coliforms are characterized as Gram-negative, non-spore-forming rods defined by their ability to ferment lactose to produce acid and/or carbon dioxide gas. Examples of genera considered to be coliforms include Citrobacter, Enterobacter, Escherichia and Klebsiella.
The most well-known coliform, Escherichia coli (E. coli), is a common resident of the intestines of warm-blooded animals but can also be found in the natural environment and transmitted into food manufacturing facilities as well as drinking water sources. Most E. coli are harmless, but some strains can cause serious food poisoning and disease.
Rather than looking for a specific species, testing for coliforms gives a broader view of organisms in ingredients, finished products, and the overall production environment. In this way, coliforms serve as “indicator organisms,” and testing for their presence at large rather than an individual genus or species can signify that unhygienic conditions exist that may harbor pathogenic organisms. If a sample area is under control, the number of these indicator organisms would be contained in relatively small amounts.
Notably, some coliforms have historically been referred to as fecal coliforms, but research has since shown that the more appropriate and current terminology is thermotolerant coliforms because of their tendency to grow and ferment lactose at an elevated temperature. While testing for thermotolerant coliforms was at one time used to estimate the number of E. coli in a sample, some pathogenic E. coli (notably E. coli O157: H7) do not grow well at these elevated temperatures so the practice has all but stopped.
Coliform standards are included in certain regulatory documents, from the EPA’s “Total Coliform Rule” involving drinking water to the FDA’s Grade “A” Pasteurised Milk Ordinance.
Enumeration of coliforms (using selective media such as 3M™ Petrifilm™ Coliform Count Plates) can help monitor the performance of food and beverage processing, equipment cleaning and sanitation and quality of ingredients. Increased levels of coliform indicate areas that may have experienced poor sanitation and should be addressed.
Coliforms naturally found in foods are killed by most heat processing conditions (e.g., pasteurization of milk). Therefore, their presence in foods that have already been processed indicates an inadequate heat process or post-processing contamination that also must be investigated and dealt with.