The Faucet Tested Negative. So Where Did the Legionella Come From?

When a case of healthcare-associated Legionnaires’ disease occurs, investigators naturally start looking at the building water system.

Showers.

Sinks.

Hot water.

Fixtures.

That is exactly what investigators did following a case of Legionnaires’ disease at Besançon University Hospital in France.

The results created an unusual problem.

The hospital water samples were negative.

The patient's water flosser was not.

A Legionnaires’ Disease Case With an Unexpected Source

The case was published in the CDC journal Emerging Infectious Diseases in February 2026.

A 48-year-old patient with multiple myeloma was admitted to the hospital for an autologous hematopoietic stem cell transplant. During his hospitalization, he developed fever and later respiratory symptoms. Testing identified Legionella pneumophila serogroup 1.

Because the infection occurred in a highly vulnerable hospitalized patient, investigators conducted an environmental investigation to determine the source.

They tested water from the patient's sink and shower, including samples collected with and without the installed filter. They also tested water from the toilet.

No Legionella was detected in those samples.

Investigators also reviewed routine environmental testing of the water network supplying the hospital ward. Ten routine tests performed during 2024 had also been negative for Legionella.

Then investigators noticed something else in the patient's bathroom.

A personal water flosser.

The Device Had Been Brought From Home

According to the published investigation, the water flosser belonged to the patient and had been brought into the hospital from his home without the healthcare team being informed.

It had therefore not been included in the patient's equipment inventory when he was admitted.

Investigators sampled the device by filling its reservoir with sterile water, operating the flosser and collecting water from the jet.

The sample contained Legionella pneumophila serogroup 1 along with L. pneumophila serogroups 2 through 14.

The measured concentration was 300 CFU/L.

But detecting Legionella in the device alone would not establish that it was related to the patient's infection.

So investigators went further.

The Genetic Evidence Was the Key

Researchers performed whole-genome sequencing on the L. pneumophila serogroup 1 isolate recovered from the patient and the isolate recovered from the water flosser.

The two isolates had identical genomes.

Both belonged to sequence type 42.

The researchers concluded that the clinical isolate and the isolate recovered from the water flosser had a common origin.

In their published conclusion, they reported the case as Legionnaires’ disease transmitted by a contaminated water flosser, supported by the genomic identity of the isolates recovered from the patient and the device.

That makes this much more than an interesting positive environmental sample.

Investigators had a Legionnaires’ disease case, negative samples from the usual hospital water sources, Legionella recovered from a personal water device, and a genetic match between the patient and device isolates.

Where Did the Water Flosser Become Contaminated?

This is where careful wording matters.

The investigators believed the Legionella found inside the device likely originated from the patient's home water system.

However, they were unable to investigate the patient's home, so that source was not confirmed.

The device reservoir also showed signs of fouling. The authors reported that this was consistent with use of nonsterile water and a lack of regular cleaning.

We therefore know that the device was contaminated.

We know the patient's clinical isolate matched the isolate from the device.

We do not know exactly when or where the device originally became contaminated.

That distinction is important.

Why a Water Flosser Matters

Water flossers contain a reservoir, internal tubing and a system designed to discharge water under pressure.

The researchers noted that nonsterile water can contribute to biofilm accumulation within device tubing and that the pressurized water jet directed into the mouth can generate aerosols capable of being inhaled.

That combination is particularly important when dealing with patients who are already at increased risk from Legionella exposure.

The authors specifically called attention to immunocompromised patients and recommended greater awareness of water-containing devices brought into healthcare facilities by patients or family members.

The Bigger Water Management Lesson

The most interesting part of this case may not actually be the water flosser.

It is what the water flosser represents.

Healthcare facilities invest enormous effort into managing building water systems. Hot water temperature, disinfectant residual, stagnation, flushing, fixture management and environmental testing can all be part of that process.

But patients and staff can introduce additional water-containing equipment into that controlled environment.

That equipment may have its own reservoir.

Its own tubing.

Its own maintenance history.

Its own biofilm.

And potentially its own microbial population.

In this case, the hospital's routine water-network testing had been negative, and investigators did not detect Legionella in the patient's sink, shower or toilet samples.

The unexpected source was a device that had entered the room with the patient.

Water Management Does Not Always Stop at the Faucet

There is a simple lesson here for healthcare water-management and infection-prevention teams.

When investigating a possible water-related infection, the question should not only be:

Where does water come out of the building plumbing?

It may also be worth asking:

What equipment in this environment contains, stores or sprays water?

A water flosser is an unusual example.

That is precisely why this case is valuable.

It demonstrates that an environmental investigation sometimes has to extend beyond the fixtures normally associated with a building water management program.

The hospital water system can be carefully monitored, and an exposure pathway can still exist somewhere investigators did not initially expect to find it.

Sometimes the most important water source in the room is not the faucet at all.