The significance of various coliform organisms in water has long been a subject of considerable study. Collectively, coliform bacteria (also called total coliforms) are considered indicators of possible inadequate sanitation, fecal contamination, and pathogenic and opportunistic pathogenic bacteria. However, completely and accurately differentiating coliform bacteria remains a challenge.
Until recently, the definition of coliform bacteria did not include the genera and species of Enterobacteriaceae. The traditional definition of coliform bacteria, based on lactose fermentation with the production of gas, excluded some strains of Escherichia coli and included some strains of enteric pathogens (e.g., Salmonella and Shigella).1 However, as a result of DNA hybridization tests and other molecular methods, researchers now classify many species of Enterobacteriaceae as coliform bacteria.1–4 There are 18 Enterobacteriaceae genera with species or distinct biogroups that ferment lactose and produce gas during fermentation. Sixty-eight of the 134 named Enterobacteriaceae (59%) are coliforms.2 These must be considered along with traditional coliforms (e.g., Citrobacter, Enterobacter, Escherichia, and Klebsiella) during coliform differentiation. Information on nomenclature, classifications, original literature citations, and detailed information for genera and species in the family Enterobacteriaceae and newly identified species, with their publication date, is available.5
Users must decide whether a coliform culture requires a precise taxonomical identification to protect public health or because of regulatory requirements. Identifying the species is not a normal practice in the standard water-testing laboratory.
A history of coliform bacteria identification and differentiation can be found in the literature.3,4 Other recent reviews1,2,4 detail coliform differentiation and identification, and discuss the many variables involved.
Completely and accurately identifying coliform bacteria is a daunting task; an Enterobacteriaceae reference laboratory may be needed. A practical solution for a water laboratory is to consider several coliform-differentiation methods, to word reports precisely, and to include comments qualifying the reported results (see 9225 E for examples).
In the following sections, both cited references and a bibliography are available for further information.