Legionella is invisible in water. The bacteria are far too small to see with the naked eye, and contaminated water looks, smells, and tastes completely normal. Even heavily contaminated water systems show no obvious change in water clarity or color. The only way to confirm Legionella is present is through laboratory testing, though certain visible warning signs in your water system can signal conditions where the bacteria thrive.
Why You Can’t See Legionella in Water
Individual Legionella bacteria measure between 2 and 20 micrometers long. For context, a human hair is about 70 micrometers wide, so even the largest Legionella cells are roughly a quarter the width of a single strand of hair. Under a microscope, they appear as thin, rod-shaped cells. They stain poorly with the standard laboratory technique used to classify bacteria (Gram stain), which is one reason they went unidentified for so long before the famous 1976 outbreak that gave them their name.
Even at dangerous concentrations, Legionella doesn’t cloud the water or produce a visible color change. The threshold where experts consider urgent action necessary is around 50,000 colony-forming units per liter. That sounds like a lot of bacteria, but it’s still an astronomically small amount of biological material suspended in water. You could fill a glass from a tap with Legionella levels ten times over that danger threshold and it would look perfectly clear.
Visible Signs That Suggest Risk
While you can’t see Legionella itself, you can sometimes see the environments where it’s most likely to be growing. The CDC specifically recommends checking water features, hot tubs, and cooling towers for visible slime or biofilm before use. Biofilm is a slimy, sometimes greenish or brownish coating that forms on surfaces in contact with water. It’s made of many different microorganisms bound together in a sticky matrix, and Legionella can embed itself within this layer, where it’s protected from disinfectants and feeds on other microbes.
Other visual red flags include sediment or scale buildup inside water heaters, rusty or discolored water coming from taps that haven’t been used in a while, and visible debris in cooling tower basins. None of these guarantee Legionella is present, but they all indicate the kind of stagnant, nutrient-rich conditions that let the bacteria multiply. If you’re reopening a building after a prolonged shutdown, these are the things to look for before restoring normal water use.
Where Legionella Grows
Legionella occurs naturally in lakes, rivers, and streams at low levels that rarely cause problems. The danger comes from engineered water systems that inadvertently create ideal growing conditions: warm temperatures, stagnant water, and surfaces for biofilm to form. Cooling towers, building plumbing, hot water heaters, decorative fountains, and hot tubs are the most common sources of outbreaks.
Temperature is the single biggest factor. Legionella grows best between 77°F and 113°F (25°C to 45°C), though it can survive and slowly multiply at temperatures as low as 68°F (20°C). Above 140°F (60°C), the bacteria are killed. This is why hot water storage tanks are recommended to be kept above that temperature, and why lukewarm water sitting in pipes is the highest-risk scenario. If your building has areas where hot water cools down before reaching the tap, or cold water warms up because pipes run through heated spaces, those are the zones where Legionella is most likely to proliferate.
How Testing Actually Works
Since you can’t see or smell Legionella, testing requires collecting a water sample and sending it to a lab. There are two main approaches.
The traditional method uses a special growth medium called BCYE agar. A water sample is spread on this medium and incubated for 7 to 10 days. Legionella grows slowly compared to other bacteria, and the culture plates can be overgrown by other organisms in the sample, which sometimes makes results unreliable. Positive cultures then need additional testing to confirm the specific species and strain.
A newer method called Legiolert simplifies the process. It can detect and quantify Legionella from water samples in 7 days without needing the extra confirmation steps. Testing of this method showed a specificity of 96.5%, meaning false positives are rare. PCR-based molecular testing can also detect Legionella DNA in water more quickly, though it identifies both living and dead bacteria, which can complicate interpretation.
What the Test Numbers Mean
Test results are reported in colony-forming units per liter (CFU/L). For building water systems other than cooling towers, here’s how the levels break down:
- Below 1,000 CFU/L: At or near the detection limit. No action typically needed.
- 1,000 to 10,000 CFU/L: Legionella may be actively multiplying. Investigation and increased monitoring are warranted.
- Above 10,000 CFU/L: Active growth has occurred and remediation is needed.
- Above 50,000 CFU/L: Considered an imminent outbreak risk. Exposure should be stopped immediately and a remediation plan put in place.
Cooling towers tolerate higher baseline levels because of their size and design, but even there, concentrations above 1,000,000 CFU/L require the system to be taken offline for treatment. Some jurisdictions, like New York State, use a different trigger: if 30 percent or more of collected samples test positive for any Legionella species, control measures and resampling are mandatory regardless of the specific concentration.
Practical Steps for Checking Your Water System
If you manage a building or are concerned about a home water system, start with what you can observe. Check for biofilm or slime on any surface that stays wet, including showerheads, faucet aerators, and the inside of water heater drain valves. Run taps that haven’t been used in more than a week to flush stagnant water. Make sure your water heater is set to at least 140°F (60°C) at the tank, keeping in mind that mixing valves should bring the temperature down at the tap to prevent scalding.
For larger buildings with complex plumbing, cooling towers, or hot tubs, routine Legionella testing is the only reliable way to know what’s in your water. Visual inspection catches the conditions that favor growth, but the bacteria themselves will never announce their presence. Clear water is not safe water, and murky water isn’t necessarily dangerous. Only a lab result tells you what’s actually there.