Total coliforms, E. coli and enterococci in water: what each indicator reveals

Three indicators, three different questions

When a water lab report lists total coliforms, E. coli and enterococci, it is not repeating the same measurement three times. Each indicator answers a different question: whether treatment and the network are working, whether there has been recent fecal contamination, and whether that contamination could carry pathogens more resistant than E. coli. In the article on water microbiology we saw why indicators are used instead of searching for every possible pathogen; here the focus is on reading each result separately and knowing what to do when one comes back positive.

Total coliforms: the broad group

According to the WHO Guidelines for drinking-water quality, total coliforms are Gram-negative, aerobic or facultatively anaerobic, non-spore-forming rods that grow in the presence of bile salts and ferment lactose with acid or aldehyde production within 24 hours at 35-37 °C. When they ferment lactose they produce the enzyme β-galactosidase, which is the basis of many chromogenic and enzymatic methods.

Citrobacter, Klebsiella and Enterobacter

The group was traditionally considered to include the genera Escherichia, Citrobacter, Klebsiella and Enterobacter, although the WHO points out that it is more heterogeneous and includes others such as Serratia and Hafnia. Many of these bacteria are of environmental origin: they live in soil and water and can survive and multiply within distribution systems, especially where there are biofilms. That is why the WHO does not consider them a useful indicator of fecal pathogens.

Their value lies elsewhere: they must be absent immediately after disinfection, so their presence indicates inadequate treatment. If they appear in the network or in a storage tank, they point to regrowth, biofilm formation or the ingress of foreign material such as soil or plant debris. The logic is similar to that of Enterobacteriaceae as a hygiene indicator on surfaces: they flag a system failure, not a specific pathogen.

Thermotolerant ("fecal") coliforms and E. coli

Within total coliforms, those that ferment lactose at 44-45 °C are called thermotolerant; in practice many laboratories still call them "fecal coliforms". In most waters the predominant genus is Escherichia, but some types of Citrobacter, Klebsiella and Enterobacter are also thermotolerant, so the group can include environmental organisms.

E. coli is distinguished from other thermotolerant coliforms by producing indole from tryptophan or by the enzyme β-glucuronidase. It is present in very high numbers in human and animal feces, rarely occurs without fecal contamination, and conditions in a distribution network hardly allow it to grow. That is why the WHO considers it the most suitable indicator of fecal contamination and accepts thermotolerant coliforms only as a less reliable alternative. To learn more about the species and its pathogenic strains, see the article on E. coli and STEC.

Intestinal enterococci: the more resistant indicator

Intestinal enterococci are Gram-positive bacteria, tolerant of salt and alkaline pH, which the WHO groups into four species: Enterococcus faecalis, E. faecium, E. durans and E. hirae. In human feces they are usually present at roughly one order of magnitude below E. coli, and most species do not multiply in water.

Their main advantage is persistence: they survive longer in the aquatic environment than E. coli, withstand drying better and are more resistant to chlorination. That is why they are used in raw water as an indicator of more persistent fecal pathogens, in drinking water to complement E. coli testing, and to verify water quality after network repairs or the installation of new pipes.

What a positive means and what action to take

• Total coliforms positive, E. coli negative: this does not prove fecal contamination, but it does point to a treatment or integrity failure. Resample, review disinfection and look for points of ingress or biofilms in the network.
• E. coli or thermotolerant coliforms positive: evidence of recent fecal contamination. The WHO calls for an immediate investigation: resampling and a search for the cause, such as inadequate treatment or a break in the network.
• Intestinal enterococci positive: also indicate recent fecal contamination and call for the same investigation. If they appear without E. coli, consider older contamination or chlorinated water in which E. coli has already been inactivated.

What the standards require

The WHO states that E. coli (or, failing that, thermotolerant coliforms) must not be detectable in any 100 ml sample of water intended directly for drinking. Directive (EU) 2020/2184 sets 0 per 100 ml for E. coli and intestinal enterococci as mandatory microbiological parameters (0 per 250 ml for bottled water), and keeps coliform bacteria as an indicator parameter, also at 0 per 100 ml, intended to verify that control measures are working. As reference methods it cites EN ISO 9308-1 or 9308-2 for E. coli and coliforms, and EN ISO 7899-2 for enterococci.

In the United States, the EPA's Revised Total Coliform Rule, mandatory since April 1, 2016, sets a maximum contaminant level (MCL) for E. coli and uses total coliforms as the trigger for a "find and fix" approach. After a total coliform positive, three repeat samples are taken within 24 hours; accumulating positives triggers a Level 1 assessment, and an E. coli MCL violation triggers a Level 2 assessment and immediate public notice.

Water in the food plant: ingredient and process tool

These limits are not foreign to the food industry. Directive 2020/2184 expressly includes in its definition water used in any food business for the manufacture, processing, preservation or marketing of products intended for human consumption. In turn, Regulation (EC) No 852/2004 requires an adequate supply of potable water, to be used whenever necessary to prevent food contamination; ice in contact with food must be made from potable water (or clean water in the case of whole fish), and recycled water must meet the same requirements as potable water unless the authority accepts otherwise.

In practice, it helps to separate two uses:

• Water as an ingredient: beverages, brines, doughs or ice. Any positive fecal indicator directly affects the product and batch traceability.
• Process water: washing raw materials, cooling, and cleaning equipment and surfaces. Here total coliforms are an early warning of biofilms in tanks, hoses or recirculation circuits.

Defining representative sampling points (plant inlet, tank outlets, points of use) and tracking trends over time is as important as the limit itself. Also remember that these results are expressed as presence/absence or counts per volume of water, not as the CFU per gram usual in food.

How they are detected with molecular methods

The reference methods for these indicators are still culture-based, with incubations of 24 to 48 hours plus confirmation. Real-time PCR makes it possible to identify the genera involved from their genetic material and get an early reading to guide the investigation. For water, TAAG offers the kits Ampliora 3.11 WaterScan (Escherichia coli, Citrobacter spp. and Klebsiella spp.), Ampliora 3.12 WaterScan (Enterococcus spp., Enterobacter spp. and Escherichia spp.) and Ampliora 6.1 WaterScan Plus, which brings all six targets together in a single kit. Because PCR detects genera and does not replicate the culture-based definition of "coliform" or "intestinal enterococcus", its results complement, rather than replace, the methods required by the applicable regulations.