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
Chapter Contents
∙ Conventional Method for Determining Coliforms and E. coli ∙ LST-MUG Method for Detecting E. coli in Chilled or Frozen Foods Exclusive of Bivalve Molluscan Shellfish 月经变少 ∙ Bottled Water ∙ Examination of Shellfish and Shellfish Meats ∙ Analysis for E. coli in citrus juices ∙ Other Methods for Enumerating Coliforms and E. coli ∙ References |
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Escherichia coli, originally known as 老鹰打饱嗝歇后语Bacterium coli commune, was identified in 1885 by the German pediatrician, Theodor Escherich (14, 伦敦塔桥简介29). E. coli is widely distributed in the intestine of humans and warm-blooded animals and is the predominant facultative anaerobe in the bowel and part of the esntial intestinal flora that maintains the physiology of the healthy host (9, 29). E. coli姜花 is a member of the family Enterobacteriaceae (15), which includes many genera, including known pathogens such as Salmonella, 古人感谢师恩名句Shigella, and Yersinia. Although most strains of E. coli are not regarded as pathogens, they can be opportunistic pathogens that cau infections in immunocompromid hosts. There are also pathogenic strains of E. coli that when ingested, caus gastrointestinal illness in healthy humans (e Chap. 4A).
In 1892, Shardinger propod the u of E. coli as an indicator of fecal contamination. This was bad on the premi that 阅读方法和技巧E. coli is abundant 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 gluco (later changed to lacto), it was easier to isolate than known gastrointestinal pathogens. Hence, the prence of E. coli in food or water becam
e accepted as indicative of recent fecal contamination and the possible prence of frank pathogens. Although the concept of using E. coli as an indirect indicator of health risk was sound, it was complicated in practice, due to the prence of other enteric bacteria like Citrobacter, Klebsiella and Enterobacter that can also ferment lacto and are similar to E. coli in phenotypic characteristics, so that they are not easily distinguished. As a result, the term "coliform" was coined to describe this group of enteric bacteria. Coliform is not a taxonomic classification but rather a working definition ud to describe a group of Gram-negative, facultative anaerobic rod-shaped bacteria that ferments lacto 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 becau 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 bad on the works of Eijkman (12) is a subt of total coliforms tha
t grows and ferments lacto at elevated incubation temperature, hence also referred to as thermotolerant coliforms. Fecal coliform analys are done at 45.5°C for food testing, except for water, shellfish and shellfish harvest water analys, which u 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 lacto at the temperatures and therefore, be considered 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 ud as indicators but in different applications. Detection of coliforms is ud as an indicator of sanitary quality of water or as a general indicator of sanitary condition in the food-processing environment. Fecal coliforms remain the standard indicator of choice for shellfish and shellfish harvest waters; and E. coli is ud to indicate recent fecal contamination or unsanitary processing. Almost all the methods ud to detect E. coli, total coliforms or fecal coliforms are enumeration methods that are
bad on lacto fermentation (4). The Most Probable Number (MPN) method is a statistical, multi-step assay consisting of presumptive, confirmed and completed phas. In the assay, rial dilutions of a sample are inoculated into broth media. Analysts score the number of gas positive (fermentation of lacto) tubes, from which the other 2 phas of the assay are performed and then us the combinations of positive results to consult a statistical tables (Appendix 2), to estimate the number of organisms prent. Typically only the first 2 phas are performed in coliform and fecal coliform analysis, while all 3 phas are done for E. coli. The 3-tube MPN test is ud for testing most foods. The 5-tube MPN is ud for water, shellfish and shellfish harvest water testing and there is also a 10-tube MPN method that is ud to test bottled water or samples that are not expected to be highly contaminated (3).
There is also a solid medium plating method for coliforms that us Violet Red Bile Agar, which contains neutral red pH indicator, so that lacto fermentation results in formation of pink colonies. There are also membrane filtration tests for coliform and fecal coliform that measure aldehyde formation due to fermentation of lacto. This chapter also includ
es variations of above tests that u fluorogenic substrates to detect E. coli (18), special tests for shellfish analysis, a brief consideration of bottled water testing and a method for testing large volumes of citrus juices for prence of E. coli有形资产负债率 in conjunction with the Juice HACCP rule.
I. Conventional Method for coliforms, fecal coliforms and E. coli
A. Equipment and materials
1. 韩博士驱动助理Covered water bath, with circulating system to maintain temperature of 45.5 ± 0.2°C. Water level should be above the medium in immerd tubes.
