Coliform bacteria

Escherichia coli

Coliform bacteria are defined as Rod shaped Gram-negative non-spore forming and motile or non-motile bacteria which can ferment lactose with the production of acid and gas when incubated at 35–37 °C. Due to the limited ability of certain coliform bacteria to ferment lactose, the definition has changed to bacteria containing the enzyme β-galactosidase. They are a commonly used indicator of sanitary quality of foods and water. Coliforms can be found in the aquatic environment, in soil and on vegetation; they are universally present in large numbers in the feces of warm-blooded animals. While coliforms themselves are not normally causes of serious illness, they are easy to culture, and their presence is used to indicate that other pathogenic organisms of fecal origin may be present. Such pathogens include disease-causing bacteria, viruses, or protozoa and many multicellular parasites. Coliform procedures are performed in aerobic or anaerobic conditions.

Typical genera include:[1]

Escherichia coli (E. coli) can be distinguished from most other coliforms by its ability to ferment lactose at 44 °C in the fecal coliform test, and by its growth and color reaction on certain types of culture media. When cultured on an eosin methylene blue (EMB) plate, a positive result for E. coli is metallic green colonies on a dark purple medium. Also can be cultured on Tryptone Bile X-Glucuronide (TBX) to appear as blue or green colonies after incubation period of 24 hours. Escherichia coli have an incubation period of 12–72 hours with the optimal growth temperature being 37 °C. Unlike the general coliform group, E. coli are almost exclusively of fecal origin and their presence is thus an effective confirmation of fecal contamination. Most strains of E. coli are harmless, but some can cause serious illness in humans. Infection symptoms and signs include bloody diarrhea, stomach cramps, vomiting and occasionally, fever. The bacteria can also cause pneumonia, other respiratory illnesses and urinary tract infections.[2][3]

An easy way to differentiate between different types of coliform bacteria is by using an eosin methylene blue agar plate.[4] This plate is partially inhibitory to Gram (+) bacteria, and will produce a color change in the Gram (-) bacterial colonies based on lactose fermentation abilities.[4] Strong lactose fermenters will appear as dark blue/purple/black, and E.coli (which also ferments lactose) colonies will be dark colored, but will also appear to have a metallic green sheen. Other coliform bacteria will appear as thick, slimy colonies, with non-fermenters being colorless, and weak fermenters being pink.

E. coli on EMB agar

See also

References

  1. ↑ The Microbiology of Drinking Water (2002) – Part 1 -(h2o) Water Quality and Public Health; Department of the Environment
  2. ↑ Todar, K. "Pathogenic E. coli". Online Textbook of Bacteriology. University of Wisconsin–Madison Department of Bacteriology. Retrieved 2007-11-30.
  3. ↑ "Escherichia coli". CDC National Center for Emerging and Zoonotic Infectious Diseases. Retrieved 2012-10-02.
  4. 1 2 Leininger, Dagny Jayne; Roberson, Jerry Russel; Elvinger, François (May 2001). "Use of Eosin Methylene Blue Agar to Differentiate Escherichia Coli from Other Gram-Negative Mastitis Pathogens". Journal of Veterinary Diagnostic Investigation. 13 (3): 273–275. doi:10.1177/104063870101300319. ISSN 1040-6387.
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