Coliform Bacteria: E. coli, Enterobacter & more

Coliform bacteria are a key topic in drinking water hygiene.

For operators of drinking water systems, they are highly relevant – especially Escherichia coli, which serves as a reliable indicator of faecal contamination. Other coliform bacteria such as Enterobacter may indicate general hygiene deficiencies – and therefore potentially pose serious health risks to consumers.

What are coliform bacteria?

Coliform bacteria are a group of microorganisms that occur naturally in the environment – for example, in soil, on plants and in the intestines of animals and humans.

Coliform bacteria are capable of fermenting lactose at 35–37 °C, producing acid and gas – a key characteristic that allows for their identification in standardised microbiological testing procedures.

Their presence in drinking water indicates hygiene deficiencies in the pipework and may suggest a potential contamination with pathogenic bacteria – especially in the case of faecal indicators like Escherichia coli.

Under normal conditions, coliform bacteria do not usually multiply in drinking water – unless biofilms or other growth-promoting conditions exist. Their detection is therefore primarily seen as a sign of recent or persistent contamination.

Types of coliform bacteria

Total coliforms:

This group includes bacteria such as Enterobacter, Citrobacter or Klebsiella, which are commonly found in the environment.

They are generally not pathogenic, but indicate general hygiene deficiencies or external contamination.

Faecal coliform bacteria:

This group includes microorganisms originating from the intestines of humans or animals.

Escherichia coli (E. coli) is the most important representative – its presence in drinking water clearly indicates faecal contamination.

Thermotolerant coliform bacteria:

These are coliform bacteria that can grow at elevated temperatures (44 ± 0.5 °C) and ferment lactose with gas production.

This characteristic is used as a technical criterion to indicate faecal origin in water analysis.

E. coli belongs to the group of thermotolerant coliform bacteria but is usually detected separately and specifically, as it is a clear faecal indicator.

Are coliform bacteria a health risk?

Coliform bacteria such as Enterobacter, Citrobacter or Klebsiella occur naturally in the environment as well as in the intestines of humans and animals.

For healthy individuals, they generally do not pose a health risk. However, certain strains can act as opportunistic pathogens under specific conditions – for example, in people with weakened immune systems.

Good to know: Opportunistic pathogens are microorganisms that only cause illness when the immune system is weakened or natural barriers are disrupted. These bacteria take advantage of such “opportunities” (Latin: opportunitas) to cause disease.

Escherichia coli (E. coli) is a different case: certain strains (e.g. EHEC) can cause severe gastrointestinal illness and always indicate faecal contamination when found in drinking water.

How do coliform bacteria enter drinking water?

Coliform bacteria typically enter the drinking water system from external sources.

Common causes include leaking pipes, faulty backflow preventers or incorrectly connected installations that allow contaminated water to enter the pipe network.

But risks are not limited to internal plumbing: microbiological contamination can also occur in the catchment areas of water abstraction plants – especially due to intensive agricultural activity.

When organic substances such as slurry or silage effluent enter groundwater or surface water, the risk of faecal contamination increases significantly.

This mainly affects systems using untreated or only lightly treated raw water.

If coliform bacteria are detected during drinking water testing, quick and systematic action is required. Although these bacteria usually do not pose an immediate health risk, their presence indicates hygiene deficiencies – for example due to leaks, backflow or stagnant water.

What to do if coliform bacteria are detected in drinking water

If coliform bacteria are detected during routine drinking water testing, immediate and systematic action is required.

Although these bacteria are generally not harmful, their presence indicates potential hygiene deficiencies – such as leaks, backflow, or stagnant water.

Further sampling should be conducted to confirm the findings and to help identify the source of contamination.

If Escherichia coli or Enterococci are detected – both indicators of faecal contamination – prompt notification of the local authority is necessary, and appropriate immediate measures should be implemented.

These may include informing users, issuing boil water advisories, or disinfecting the system.

The detection of other coliform bacteria requires assessment by the local authority, especially if they occur repeatedly or in high concentrations.

Only after the drinking water has been deemed safe through follow-up testing and approved by the relevant authority should it be returned to unrestricted use.

Overview of coliform bacteria

Below, we briefly present the most important genera.

Note: The abbreviation “spp.” stands for the Latin term species pluralis, meaning “multiple species”. For example, Enterobacter spp. refers to several (unspecified) species within the genus Enterobacter.

Escherichia coli (E. coli) as a key indicator

Escherichia coli (E. coli) is a significant member of the coliform bacteria group and serves as a reliable indicator of faecal contamination.

It naturally inhabits the intestines of humans and animals.

The presence of E. coli in drinking water indicates a potential health risk and is not acceptable under UK regulations, which mandate zero detection per 100 ml. Immediate investigation and remedial actions are required to identify and eliminate the source of contamination.

Origin of the name “E. coli”

Escherichia: Named after the discoverer of the bacterium, Theodor Escherich.

Coli: “Coli” means “from the colon” (Latin: colon), referring to the bacterium’s natural habitat.

“E. coli” is the abbreviated form of Escherichia coli.

Enterobacter spp.

Bacteria of the genus Enterobacter are common environmental organisms found in soil, on plants or in moist environments.

They are classified as environmental bacteria, but under certain conditions – such as in immunocompromised individuals – they can act as opportunistic pathogens.

Their presence in drinking water may point to hygiene deficiencies in the pipework or to secondary contamination either within the plumbing system or at the point of use.

Klebsiella spp.

Klebsiella pneumoniae and related species can be found both in the environment and in the human gut.

In water analysis, their detection cannot be clearly linked to faecal contamination, as they may persist in damp environments (e.g. plant material or biofilms).

Nevertheless, their presence is considered a hygiene concern.

Citrobacter spp.

Citrobacter species are found in soil, wastewater and occasionally in the intestinal tract.

These environmental bacteria do not necessarily indicate faecal contamination, but their detection in drinking water may serve as a technical warning signal – particularly if they occur repeatedly.

Hafnia alvei

A less common member of the coliform group is Hafnia alvei.

This bacterium is occasionally found in food and surface water.

Although it has low clinical relevance, its presence in drinking water should still be interpreted as an indicator of hygiene irregularities.