FOOD MICROBIOLOGY: INDICATOR ORGANISMS
COLIFORM BACTERIA
A group: Gram-negative, non-spore-forming rods that ferment lactose to acid and gas. Genera include Escherichia, Enterobacter, Klebsiella and Citrobacter.
E. COLI
One species inside the coliform group. Lives in the gut of warm-blooded animals. Most strains are harmless; a few, such as O157:H7, cause illness.
Figure 1: E. coli is one species inside the wider coliform group, and it is the stricter faecal indicator.
Introduction
E. coli and coliform bacteria are often named together, but they are not the same thing. Coliforms are a whole group of bacteria, while E. coli is one species inside that group, and it is the stricter signal of faecal contamination.
Quick comparison
Quick comparison: E. coli and Coliform Bacteria
| Aspect | E. coli | Coliform Bacteria |
|---|---|---|
| What it is | One species: Escherichia coli | A group of related genera |
| Genera covered | Escherichia only | Escherichia, Enterobacter, Klebsiella, Citrobacter |
| Shape and stain | Gram-negative rod, non-spore-forming | Gram-negative rods, non-spore-forming |
| Lactose | Ferments lactose to acid and gas | Ferment lactose to acid and gas (the group test) |
| IMViC pattern | Indole positive, methyl red positive, Voges-Proskauer negative, citrate negative | Mixed patterns across the genera |
| Natural home | Gut of warm-blooded animals, including people and cattle | Gut, soil, plants and water |
| Faecal signal | Strong: its presence means faecal matter reached the food or water | Weaker: some coliforms come from soil and plants, not only from faeces |
| Illness risk | Most strains harmless gut residents; enterohaemorrhagic strains such as O157:H7 make toxin and have a low infectious dose | Most coliforms do not cause illness; the group is an indicator, not a pathogen test |
| Test stages | Confirmed by IMViC and by growth at 44 to 45 °C in the faecal coliform test | Presumptive, confirmation and completed tests; MPN and membrane filter for water |
Scientific explanation
Definition
Coliform bacteria are Gram-negative, non-spore-forming rods that ferment lactose to acid and gas, usually found in the gut of people and animals. The group includes Escherichia, Enterobacter, Klebsiella and Citrobacter. Escherichia coli is one species in that group: a Gram-negative rod that lives in the gut of warm-blooded animals. Most E. coli strains are harmless gut residents, but some cause illness.
Working principle
Coliforms are used as indicator organisms: their presence signals a hygiene failure rather than proving disease. Because some coliforms live in soil and on plants, a coliform count alone cannot prove faecal contamination. E. coli is the stricter signal: finding E. coli in drinking water is a warning even when nobody is sick yet, since its presence means faecal matter reached the water, and disease-causing microbes from faeces may travel the same route. Enterohaemorrhagic E. coli, such as O157:H7, makes a toxin that damages the gut and, in some people, the kidneys; it lives in cattle and reaches food through undercooked minced beef, raw milk and raw produce watered with contaminated water, and its infectious dose is low.
Apparatus
Coliform testing uses lactose broth tubes for the presumptive test, selective media such as violet red bile agar and MacConkey agar, and IMViC reagents for confirmation. A Gram stain of one pink colony on MacConkey agar shows pink rods, which points to the coliform group.
Measurement procedure
The coliform test has three stages: presumptive, confirmation and completed tests. In water testing, the MPN method adds measured volumes, often 10 ml, 1 ml and 0.1 ml, five tubes each, to lactose broth; tubes that turn cloudy and produce gas are counted as positive, and the pattern, such as 5-3-1, is read against an MPN table to give the most probable number per 100 ml. The membrane filter method filters 100 ml through a sterile membrane laid on a medium and counts colonies per 100 ml. E. coli is confirmed by IMViC: indole positive, methyl red positive, Voges-Proskauer negative, citrate negative.
Factors affecting readings
MPN is a statistical estimate, not an exact count, and its precision grows with more tubes. Injured cells may not grow, and a mixed culture can mislead: if a Gram stain and IMViC results cannot belong to the same organism, the culture must be re-streaked and a single well-isolated colony re-tested.
Formulas and calculations
This topic has no standard formula. Coliform and E. coli numbers use the plate count formula, CFU/g = colonies counted divided by (volume plated in ml x dilution factor), or the MPN table for water, where the pattern of positive tubes gives the most probable number per 100 ml.
Industrial applications
- E. coli: faecal indicator testing of drinking water, raw milk and raw produce; pathogen testing for O157:H7 in minced beef.
- Coliforms: general hygiene testing of milk, ice cream, water and ready-to-eat foods; plant sanitation checks.
- Both: release and reject decisions in food and water labs, where E. coli calls for action before illness appears.
Advantages and limitations
E. coli: advantages are a strong, specific faecal signal and a clear confirmation test (IMViC); limitations are that it says nothing about other pathogens, and most strains are harmless, so its presence is a warning, not a diagnosis.
Coliform bacteria: advantages are a simple, fast group test that catches hygiene failure early; limitations are that soil and plant coliforms can give a positive result with no faecal contamination, so the group alone is a weaker signal.
FAQ
Is all E. coli dangerous?
No. Most E. coli strains are harmless gut residents. A few, such as enterohaemorrhagic O157:H7, cause illness, and its infectious dose is low.
Why is E. coli a better faecal indicator than total coliforms?
E. coli lives in the gut of warm-blooded animals, while other coliforms also live in soil and on plants. Finding E. coli points to faeces; finding coliforms alone does not.
What does IMViC tell us about E. coli?
E. coli gives indole positive, methyl red positive, Voges-Proskauer negative and citrate negative, which separates it from Enterobacter and Klebsiella.
Why act on E. coli in water when nobody is sick yet?
Its presence means faecal matter reached the water, and disease-causing microbes from faeces may travel the same route, so action is needed before illness appears.
References and further reading
FTN subjects used: Food Microbiology (Units 1, 2, 3, 4, 5 and 8) and General Microbiology (Units 1, 2, 3, 4, 5, 6 and 10), from the notes at ~/workspace/food-tech-notes/notes/.
Books used: Adams, M. R. and Moss, M. O., Food Microbiology, Royal Society of Chemistry, 2007 (book on disk at food-tech-notes/books/Food-Microbiology.pdf; Google Books page: https://books.google.com/books?id=BZNKI088iooC). Ray, B. and Bhunia, A., Fundamental Food Microbiology, 5th edition, as cited in the FTN notes. Essentials of Industrial Microbiology (book on disk at food-tech-notes/books/Essentials-of-Industrial-Microbiology.pdf).
Google Books pages used: Food Microbiology by Adams and Moss, https://books.google.com/books?id=BZNKI088iooC (contents and chapter list read on 2026-11-11).
Related study notes: Food Microbiology covers microbial growth, foodborne pathogens, spoilage, testing methods and laboratory practice.
