What do legionella CFU levels mean?
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Key points
- Results are reported in colony-forming units per litre (cfu/L): the number of live legionella cells in the sample that grew into countable colonies.
- The HSG274 Part 2 ladder: below 100, system under control; 100 to 1,000, review the scheme and resample; above 1,000, immediate remedial action.
- “Not detected” and “below 100” are different results: one found nothing, the other found a little.
- A count is one outlet, one day. It never certifies the whole system, which is why resampling and trends matter more than any single number.
- Cooling towers and healthcare premises work to different, lower thresholds under HSG274 Part 1 and HTM 04-01.
When a legionella laboratory report arrives, the number that matters is the count in colony-forming units per litre, written cfu/L. HSG274 Part 2 turns that number into action with a three-rung ladder: below 100 cfu/L the system is considered under control, between 100 and 1,000 cfu/L you review and resample, and above 1,000 cfu/L you act immediately. This page explains what the unit means, what each rung actually requires you to do, and why no single result is an all-clear.
What a colony-forming unit is
The culture method, BS EN ISO 11731, works by growing the bacteria in the sample on selective plates for around ten days. Each colony that appears is counted as having started from one live cell or clump of cells: one colony-forming unit. The laboratory scales that up to a count per litre of water. So a result of 350 cfu/L means the sample contained enough live legionella to produce the equivalent of 350 colonies from each litre. It is a count of what was alive and able to grow, which is exactly the property that matters for control.
The action ladder
| Result (cfu/L) | What it indicates | What you do |
|---|---|---|
| Not detected | No legionella found at the method's detection limit | No action beyond routine monitoring |
| Below 100 | System under control | Continue routine monitoring; record the result |
| 100 to 1,000 | Control may be slipping | Review the control scheme, resample, investigate any localised cause; if a repeat sample confirms a similar level, disinfect and review again |
| Above 1,000 | Control has failed | Resample to confirm, take immediate remedial action including disinfection, review the risk assessment and control measures, and consider whether occupants need to be informed |
This is the ladder for hot and cold water systems in HSG274 Part 2. Cooling towers work to different thresholds under HSG274 Part 1, and healthcare premises to lower ones under HTM 04-01, so read a result against the regime your system belongs to.
Rung by rung: what to actually do
Below 100 cfu/L. File the result in the log book and carry on. The useful work here is trend-watching: a result that was not detected last year and reads 80 this year is still under control, but the direction of travel deserves attention at the next risk assessment review.
100 to 1,000 cfu/L. Treat it as a warning light. Review the written scheme and the monitoring records for the weeks before the sample: were temperatures in range, flushes done, tanks inspected? Investigate localised causes near the outlet, such as a dead leg, scale build-up or a faulty blending valve. Resample the same outlet to confirm whether the result repeats, and if a further sample comes back in the same band, disinfect the affected part of the system and review the controls again.
Above 1,000 cfu/L. Act now, not at the next review. Resample promptly to confirm the result, review the risk assessment and control measures to find why control failed, and carry out remedial work, which in practice usually means disinfection of the system or the affected section. Consider whether the people using the building need to be told, particularly where showers or other spray outlets are in use. Then resample after the work: the fix is only demonstrated by a count back under control. The full response sequence is in what to do if legionella is detected.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Why a single low result is not an all-clear
Legionella does not spread itself evenly through a water system. It lives in biofilm, and biofilm sits in patches: the base of a calorifier, a little-used branch, a scaled tap insert. A sample is a snapshot of the water that happened to leave one outlet on one day, so a clean result tells you about that outlet at that moment, not about the system as a whole. This is why the guidance leans on routine control measures rather than one-off testing: keeping hot water at 50°C or more at outlets and cold below 20°C suppresses growth everywhere, all the time, which no spot sample can verify on its own. The legionella temperature checker tests those readings against the HSG274 thresholds.
Species and serogroups on the report
Beside the count, the report identifies what was found: legionella species, and for Legionella pneumophila the serogroup. This detail matters because L. pneumophila serogroup 1 is associated with the great majority of Legionnaires' disease cases, so its presence weighs more heavily than the same count of a less pathogenic species when you decide how urgently to act. Where legionella sits in the wider system, and how the count relates to the conditions that produced it, is covered in legionella in water and legionella sampling and action levels.
Below 100, carry on and watch the trend. Between 100 and 1,000, find the cause and resample. Above 1,000, act immediately, disinfect, and prove the fix with another sample. The ladder only works if you climb it.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
What is a safe level of legionella in water?
There is no level that makes a system “safe” in the sense of certified, but HSG274 Part 2 treats a count below 100 cfu/L as indicating the system is under control, with no action needed beyond routine monitoring. Any detection at or above 100 cfu/L triggers review and resampling, and above 1,000 cfu/L triggers immediate remedial action. Bear in mind the count is a snapshot of one outlet on one day, so even a low result is a point on a trend, not an all-clear for the whole system.
What happens if legionella is above 1,000 cfu/L?
Above 1,000 cfu/L, HSG274 Part 2 requires immediate action: resample to confirm, review the risk assessment and the control measures to find the cause, and carry out remedial work, which commonly means disinfecting the affected part of the system. Consider whether occupants need to be informed, particularly where outlets producing spray are in use. Record everything, then resample after the work to confirm the count has come back under control.
What does 'not detected' mean on a legionella result?
Not detected means the laboratory found no legionella colonies in the sample at the limit the method can detect. It is a stronger result than a count below 100 cfu/L, where bacteria were found but at a low level. It still describes only that sample, from that outlet, on that day: legionella lives in biofilm and its distribution in a system is patchy, so a not-detected result at one outlet does not prove the whole system is free of the bacteria.
Is 100 cfu/L of legionella dangerous?
100 cfu/L is the threshold at which HSG274 stops treating the system as under control and asks you to act: review the control scheme, resample, and investigate whether there is a localised cause such as a dead leg or a failing temperature regime. It is an action trigger, not a prediction of illness. Risk of infection depends on whether the contaminated water is turned into breathable spray and who is exposed, which is why the guidance responds to the count by fixing the system rather than by counting differently.
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