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BAM: Enumeration of Escherichia coli and the Coliform Bacteria
September 2002
Bacteriological Analytical Manual
Chapter 4
Enumeration of Escherichia coli and the Coliform Bacteria
Authors: Peter Feng, Stephen D. Weagant, Michael A. Grant, William Burkhardt

Chapter Contents
* Conventional Method for Determining Coliforms and E. coli * LST-MUG Method for Detecting E. coli in Chilledor Frozen Foods Exclusive of Bivalve Molluscan Shellfish * Bottled Water * Examination of Shellfish and Shellfish Meats * Analysis for E. coli in citrus juices * Other Methods for Enumerating Coliforms and E. coli * References |
Escherichia coli, originally known as Bacterium coli commune, was identified in 1885 by the German pediatrician, Theodor Escherich (14, 29). E. coli iswidely distributed in the intestine of humans and warm-blooded animals and is the predominant facultative anaerobe in the bowel and part of the essential intestinal flora that maintains the physiology of the healthy host (9, 29). E. coli is a member of the family Enterobacteriaceae (15), which includes many genera, including known pathogens such as Salmonella, Shigella, and Yersinia. Although moststrains of E. coli are not regarded as pathogens, they can be opportunistic pathogens that cause infections in immunocompromised hosts. There are also pathogenic strains of E. coli that when ingested, causes gastrointestinal illness in healthy humans (see Chap. 4A).
In 1892, Shardinger proposed the use of E. coli as an indicator of fecal contamination. This was based on the premise that E. coli isabundant in human and animal feces and not usually found in other niches. Furthermore, since E. coli could be easily detected by its ability to ferment glucose (later changed to lactose), it was easier to isolate than known gastrointestinal pathogens. Hence, the presence of E. coli in food or water became accepted as indicative of recent fecal contamination and the possible presence of frankpathogens. Although the concept of using E. coli as an indirect indicator of health risk was sound, it was complicated in practice, due to the presence of other enteric bacteria like Citrobacter, Klebsiella and Enterobacter that can also ferment lactose and are similar to E. coli in phenotypic characteristics, so that they are not easily distinguished. As a result, the term "coliform" was coined todescribe this group of enteric bacteria. Coliform is not a taxonomic classification but rather a working definition used to describe a group of Gram-negative, facultative anaerobic rod-shaped bacteria that ferments lactose to produce acid and gas within 48 h at 35°C. In 1914, the U.S. Public Health Service adopted the enumeration of coliforms as a more convenient standard of sanitary significance.Although coliforms were easy to detect, their association with fecal contamination was questionable because some coliforms are found naturally in environmental samples (6). This led to the introduction of the fecal coliforms as an indicator of contamination. Fecal coliform, first defined based on the works of Eijkman (12) is a subset of total coliforms that grows and ferments lactose at elevatedincubation temperature, hence also referred to as thermotolerant coliforms. Fecal coliform analyses are done at 45.5°C for food testing, except for water, shellfish and shellfish harvest water analyses, which use 44.5°C (1, 3, 30). The fecal coliform group consists mostly of E. coli but some other enterics such as Klebsiella can also ferment lactose at these temperatures and therefore, beconsidered as fecal coliforms. The inclusion of Klebsiella spp in the working definition of fecal coliforms diminished the correlation of this group with fecal contamination. As a result, E. coli has reemerged as an indicator, partly facilitated by the introduction of newer methods that can rapidly identify E. coli.
Currently, all 3 groups are used as indicators but in different applications. Detection...