The five indicators
Five Tests. A Clearer Picture of Water Hygiene.
Each microbiology test provides a different insight into your water system. Explore what each test detects, what results may indicate, and where the test is most relevant.
Total Viable Count at 22°C (TVC 22°C)
The Total Viable Count at 22°C measures the number of bacteria in your water sample that are capable of growing when incubated at 22 degrees Celsius for 72 hours. This temperature mimics typical environmental or ambient conditions, so this test picks up a broad range of organisms — including many that naturally occur in soil, sediment, and surface water.
On its own, a high TVC at 22°C doesn’t automatically mean your water is unsafe to drink — some of these bacteria are entirely harmless. However, an unexpected increase from a previously established baseline, or a consistently elevated count, is a reliable signal that something has changed within the water system.
What it detects
Environmental and mesophilic bacteria — organisms that grow at oroom r ambient temperature
Elevated levels suggest
Biofilm growth in pipework, stagnation in little-used outlets, deterioration in water treatment, or ingress of environmental contamination
Most relevant to
Routine water quality monitoring, all property types, baseline and trend analysis over time
UK drinking water standard
No absolute limit, but significant increases from baseline require investigation under Drinking Water Inspectorate guidance
The 22°C Total Viable Count is one of the most useful monitoring tools in routine water safety management. Rather than signalling a specific pathogen, it provides a general picture of the bacterial population in your water. Think of it as a health check: when the count is stable and within expected ranges, your water system is likely performing as it should. When the count rises unexpectedly, it’s a prompt to investigate further — before a minor issue becomes a more serious one. Many organisations use this test as part of a scheduled monitoring programme to track trends and catch early signs of system deterioration.
Total Viable Count at 37°C (TVC 37°C)
The Total Viable Count at 37°C measures bacteria that thrive specifically at human body temperature. Samples are incubated at 37°C for 24 hours — and any organism that grows under those conditions is the kind of organism your body might have to deal with if that water were ingested or inhaled. For this reason, the 37°C count is considered a more direct indicator of potential health risk than the 22°C count.
A significantly elevated TVC at 37°C is a stronger indicator of concern than the same result at 22°C. It can suggest inadequate disinfection, contamination from human or animal sources, or that warm water is being distributed at temperatures that encourage harmful bacterial growth.
What it detects
Thermotolerant bacteria — organisms capable of surviving and multiplying at human body temperature (37°C)
Elevated levels suggest
Inadequate disinfection, potential contamination, warm water distribution problems, or tofailure maintain correct hot water temperatures (55°C+ in distribution pipework)
Most relevant to
Healthcare, food preparation, care homes, dental surgeries — any setting where infection risk to vulnerable individuals is elevated
UK drinking water standard
No absolute limit, but any count should be stable. Significant increases trigger mandatory investigation under Drinking Water Inspectorate regulations
While the 22°C count tells you about the general bacterial environment in your water, the 37°C count focuses your attention on organisms that are best adapted to the human body — and therefore most capable of causing infection. In settings such as care homes, GP practices, dental surgeries, and hospital premises, elevated 37°C counts are taken seriously and investigated promptly. They can also point to specific problems in how hot water is being managed: if water in your system is consistently distributed below 55°C in hot-water pipework, or stored below 60°C in your cylinder, you have the conditions for thermotolerant bacteria — and potentially Legionella — to multiply.
Faecal Indicator Organism]
E. coli (Escherichia coli)
Escherichia coli — commonly known as E. coli — is a bacterium found in the lower intestines of humans and warm-blooded animals. Its presence in a water sample is direct, unambiguous evidence of faecal contamination. This is never acceptable in drinking water, regardless of the quantity detected. Even a single colony-forming unit (CFU) per 100ml of water constitutes a failed result under UK drinking water regulations.
E. coli is used as the primary indicator of faecal contamination in water precisely because it is specific, reliable, and measurable. If it is present, it means sewage or faecal matter has found its way into the water supply — and where E. coli is present, more dangerous pathogens may also be.
What it detects
Faecal coliform bacteria — direct evidence that sewage or animal waste has contaminated the water supply
Health risks
Gastroenteritis, severe stomach cramps and diarrhoea, urinary tract infections, and in vulnerable individuals — haemolytic uraemic syndrome (HUS), a serious complication affecting the kidneys
UK standard
ZERO TOLERANCE — 0 CFU per 100ml is the absolute legal standard under the Water Supply (Water Quality) Regulations 2016. No level is acceptable.
Action required
Immediate investigation of the contamination source and appropriate remedial action. Water should not be consumed until the system is cleared and a re-test confirms the all-clear.
The 22°C Total Viable Count is one of the most useful monitoring tools in routine water safety management. Rather than signalling a specific pathogen, it provides a general picture of the bacterial population in your water. Think of it as a health check: when the count is stable and within expected ranges, your water system is likely performing as it should. When the count rises unexpectedly, it’s a prompt to investigate further — before a minor issue becomes a more serious one. Many organisations use this test as part of a scheduled monitoring programme to track trends and catch early signs of system deterioration.
Coliform Bacteria (Total Coliforms)
Coliform bacteria are a broad group of microorganisms that include E. coli but also encompass many other species found in the environment, in soil, and in the digestive systems of animals. Unlike E. coli, not all coliforms originate from faecal sources — some are entirely environmental in origin. However, their presence in treated drinking water is still a sign that the protective barriers within the water system have been compromised.
Think of coliforms as an early warning system. They are easier to detect than specific pathogens and often appear before more dangerous organisms do. A positive coliform result tells you that the treatment process has been breached somewhere — even if the source and severity are not yet clear.
What it detects
A broad group of bacteria including both environmental organisms and potential faecal indicators — serving as a general hygiene marker for treated water systems
Elevated levels suggest
Treatment failure, cross-contamination, temperature fluctuations, inadequate disinfection, or biofilm within pipework allowing organisms to colonise and be released
Most relevant to
0 CFU per 100ml for treated drinking water at point of use. Any detection requires investigation, even without confirmed E. coli
UK drinking water standard
All property types — particularly useful as a routine monitoring tool for identifying water quality degradation before it escalates
The value of testing for total coliforms alongside E. coli is that it gives you an earlier, broader picture of potential problems. A water sample could test negative for E. coli but still contain environmental coliforms — indicating that the system’s defences have slipped, even if direct faecal contamination hasn’t occurred. In practice, coliform contamination is often traced back to issues such as a faulty or uncovered storage tank, pipework that hasn’t been adequately disinfected after maintenance, or stagnant water in sections of the distribution system that are rarely used. Finding coliforms gives you the chance to investigate and correct the issue before it potentially progresses to a more serious contamination event.
Pseudomonas aeruginosa is an opportunistic pathogen — meaning it causes little harm to healthy individuals with normal immune function, but can cause severe and even life-threatening infections in people whose immune systems are weakened. It is naturally resistant to many standard disinfectants and a wide range of antibiotics, making it one of the more challenging organisms to eliminate once it has established itself in a water system.
Pseudomonas thrives in biofilm — the thin, protective layer of bacteria and organic matter that coats the inside of water pipes, fittings, and outlets when hygiene is poor or water flow is insufficient. It is particularly associated with healthcare-acquired infections and is a specific concern in any setting that serves or treats people with compromised immunity.
What it detects
Pseudomonas aeruginosa — an antibiotic- and disinfectant-resistant bacterium that colonises water systems via biofilm, often in low-flow or stagnant areas
Health risks
Respiratory infections, wound infections, ear infections, urinary tract infections, and in immunocompromised patients — bloodstream infections that can be life-
Who is at highest risk
Immunocompromised patients, the elderly, newborns, people on chemotherapy, those with cystic fibrosis or burns, and anyone in a clinical or care setting
Most relevant to
Dental surgeries, GP practices, care homes, hospitals, neonatal units — specifically required under NHS guidance HTM 04-01
Pseudomonas aeruginosa has been responsible for some of the most serious healthcare-associated infection outbreaks in recent history — including an outbreak in a Northern Ireland neonatal unit that led to significant regulatory change and the development of the current NHS water safety guidance (HTM 04-01). For dental practices, GP surgeries, care homes, and any other premises where vulnerable people receive care or treatment, testing for Pseudomonas is not optional — it is an expectation. The organism’s ability to survive in biofilm and resist standard chlorine-based disinfection means it can persist in water systems for extended periods if not actively managed. Testing for Pseudomonas aeruginosa gives you the evidence base to demonstrate that your water safety management programme is working — and to act promptly when it isn’t.
