Coliform bacteria
Indicator bacteria for fecal contamination in water.
Coliform bacteria are Gram-negative, rod-shaped microbes that do not form spores and can be either motile or non-motile. They produce acids and gases using the enzyme β-galactosidase, thriving best at temperatures between 35 and 37 °C. These bacteria can grow with or without oxygen, and their presence is a common sign that food, milk, or water may be unsanitary. They occur naturally in water, soil, and plants, and are abundant in the intestines of warm-blooded animals, where they live without normally causing illness. Because they are easy to grow in a lab, testing for coliforms serves as a warning that disease-causing agents—such as harmful bacteria, viruses, protozoa, or parasites—might also be present, making a sample unsafe to drink. All drinking water sources must be checked for total coliform bacteria.
Typical genera include *Citrobacter*, *Enterobacter*, *Klebsiella*, and *Escherichia*. *Citrobacter* are rod-shaped, facultative anaerobes with flagella all around, ranging from 0.6 to 6 μm long. They live harmlessly in the gut but can cause urinary tract infections, blood infections, brain abscesses, pneumonia, abdominal infections, meningitis, and joint infections if they get the chance. Infections from *Citrobacter* species have a death rate of 33 to 48 percent, with infants and people with weak immune systems at higher risk. *Enterobacter* are motile, flagellated rods that can cause blood infections, respiratory infections, urinary tract infections, surgical site infections, and in severe cases, meningitis, sinusitis, and bone infections. To detect them, samples are first grown on MacConkey agar to confirm lactose fermentation; an indole test then separates them from *Escherichia*, since *Enterobacter* are indole-negative while *Escherichia* are positive. *Enterobacter* differ from *Klebsiella* because they are motile. *Klebsiella* are non-motile, Gram-negative rods 1 to 2 μm long. They are facultative anaerobes with a capsule made of complex acid polysaccharides, which lets them survive drying for months. *Klebsiella pneumoniae* is the most common species in humans, found in animal guts, sewage, and soil. On carbohydrate-rich media, their colonies look greyish-white with a slimy surface. A selective agar containing ornithine, raffinose, and Koser citrate makes *Klebsiella* form yellow, wet colonies.
- Optimal growth temperature
- 35–37 °C
- Mortality rate for citrobacter infection
- 33–48%
- E coli incubation period
- 12–72 hours
- E coli optimal growth temperature
- 37 °C
- E coli o157 us cases 2021
- 10
- E coli o157 us states affected 2021
- 7
- E coli o157 canada cases 2021 2022
- 14
Lore & Background
Coliform bacteria are Gram-negative non-spore forming bacilli that produce acids and gases via β-galactosidase at 35–37 °C. They are aerobes or facultative aerobes and inhabit the gastrointestinal system of warm-blooded animals, as well as aquatic environments, soil, and vegetation. Typical genera include Citrobacter, Enterobacter, Klebsiella, and Escherichia. Citrobacter are peritrichous facultative anaerobic bacilli that can cause infections with a mortality rate of 33–48% in susceptible individuals. Enterobacter are motile, flagellated bacilli that can cause various infections and are distinguished from Escherichia by being indole negative. Klebsiella are non-motile, encapsulated bacilli that can withstand drying for months; Klebsiella pneumoniae is the most common species found in humans and animals. Escherichia species, particularly E. coli, are the most common cause of disease in humans among coliforms, with multiple pathotypes causing gastrointestinal syndromes. E. coli is almost exclusively of fecal origin and its presence confirms fecal contamination.
Reader's Guide
Coliform bacteria serve as a key indicator of low sanitary quality in foods, milk, and water. Their presence in a sample suggests that pathogenic organisms of fecal origin may also be present, including disease-causing bacteria, viruses, protozoa, and multicellular parasites. Because coliforms are easy to culture and not normally the cause of serious illness themselves, they are used as a proxy for potential contamination. Every drinking water source must be tested for total coliform bacteria. Detection methods include PCR amplification of the beta-galactosidase gene for general coliforms, and amplification of beta-D glucuronidase or verotoxin genes for E. coli. Chemiluminescent in-situ hybridization uses probes targeting 16S rRNA to label E. coli colonies. Violet red bile agar and the membrane filter method are culture-based approaches that rely on lactose fermentation and gas production to confirm coliform presence. E. coli can be specifically identified by its metallic green sheen on eosin methylene blue agar or by growth on Tryptone Bile X-Glucuronide agar. The distinction between general coliforms and E. coli is important because E. coli is almost exclusively of fecal origin, making it a more definitive indicator of fecal contamination.
Did You Know?
- Coliform bacteria are defined by their possession of β-galactosidase, which allows them to produce acids and gases at 35–37 °C.
- Escherichia coli can be distinguished from most other coliforms by its ability to ferment lactose at 44 °C in the fecal coliform test.
- Klebsiella species have a capsule composed of complex acid polysaccharides that allows them to withstand drying for several months.
- In 2021–2022, E. coli O157 outbreaks were linked to spinach in the United States and to a specific brand of Original Kimchi in Canada.
Frequently Asked Questions
Who is Coliform bacteria?
Coliform bacteria are a loose family of Gram-negative, rod-shaped, non-spore-forming microbes that can respire with or without oxygen. They naturally inhabit the intestines of warm-blooded animals as well as water, soil, and plant surfaces. In the aquarium community they're the 'background characters' nobody wants to notice in high numbers.
What are Coliform bacteria's powers/role in the tank?
Their signature ability is fermenting lactose through the enzyme β-galactosidase, producing both acid and gas—exactly the reaction lab technicians exploit to detect them. They reproduce fastest in the 35–37 °C range and may or may not be motile depending on the species. In a home aquarium, a sudden coliform spike is the telltale sign that organic waste or fecal material has entered the water column.
Why is Coliform bacteria important to aquarium keepers?
They are the standard indicator organisms for fecal contamination, so their detection is the fastest, most affordable 'red flag' that water quality has slipped. Because they're easy to culture on simple lab media, both public-health labs and hobbyist test kits use them as a first-line screening step before hunting for more dangerous pathogens.
What are Coliform bacteria's weaknesses?
Standard disinfectants such as chlorine and UV sterilization knock them down quickly, which is why municipal treatment plants and aquarium UV units keep their numbers in check. They also grow sluggishly outside their 35–37 °C comfort window, so typical tropical-tank temperatures around 24–28 °C naturally slow their reproduction.
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