The bacterium Legionella is a naturally occurring microbe commonly found in fresh water sources like lakes and streams, but it becomes a public health concern when it contaminates and amplifies within human-made water systems. Exposure occurs when people inhale tiny, contaminated water droplets, known as aerosols, which can lead to a serious form of pneumonia called Legionnaires’ disease, or a milder flu-like illness called Pontiac fever. Because the risk is tied directly to the presence and concentration of the bacteria, testing is a fundamental component of any water management program. Testing helps facility managers and homeowners evaluate the effectiveness of control measures and mitigate the risk of infection.
Identifying High-Risk Water Systems
Legionella bacteria thrive in water systems that maintain temperatures between 68°F (20°C) and 113°F (45°C), with the optimal growth temperature around 95°F (35°C). Complex water infrastructure, especially systems where water can become stagnant or where biofilm accumulates, creates an ideal environment for the bacteria to multiply. Testing should focus on these high-risk environments, particularly those that generate water aerosols that people can inhale.
Large facilities like hospitals, hotels, and manufacturing plants present an elevated risk due to extensive, centralized plumbing systems. Specific equipment known to harbor the bacteria includes:
- Cooling towers and evaporative condensers, which are part of large air conditioning systems and create contaminated water mist.
- Hot tubs and spa pools, which are maintained within the bacteria’s favored temperature range and produce significant aerosols.
- Domestic hot water tanks, particularly those storing water below 140°F (60°C).
- Infrequently used outlets like emergency eyewash stations, decorative fountains, and showerheads.
Best Practices for Sample Collection
Accurate Legionella testing requires proper sample collection under sterile conditions to prevent external contamination. Personnel often wear personal protective equipment, such as gloves, especially when sampling aerosol-generating sources like cooling towers. The water sample must be collected in a sterile container, typically a one-liter bottle, pre-dosed with a chemical neutralizer like sodium thiosulfate.
Sodium thiosulfate is necessary to immediately neutralize residual disinfectants, such as chlorine, in the water sample. Without this neutralization, the disinfectant would continue to kill the bacteria in transit, leading to a falsely low or negative test result. When sampling a faucet or shower, two types of samples are often recommended: a “pre-flush” sample to capture stagnant water, and a “post-flush” sample collected after the water has run for a prescribed time.
Sampling must also account for biofilm, the slimy layer where Legionella often resides within plumbing surfaces. Therefore, collecting a bulk water sample is frequently supplemented with a swab sample taken from the inside of faucet aerators or showerheads. After collection, samples must be kept cold and transported to the laboratory quickly. Analysis should begin within 24 hours of collection, and no later than 48 hours, to ensure the viability and integrity of the bacteria are maintained.
Methods of Laboratory Analysis
Once a water sample reaches the laboratory, it undergoes analysis using one of two primary methods to determine the presence and concentration of Legionella. The traditional method, known as the culture method, is considered the gold standard for environmental testing. This technique involves filtering the sample and placing it onto a specialized growth medium, allowing viable bacteria to grow into visible colonies over 7 to 14 days.
The culture method provides a quantifiable result reported in Colony Forming Units per liter (CFU/L), reflecting the concentration of living, potentially infectious bacteria. Since it yields a living isolate, this method also permits the identification of the specific Legionella species and serogroup, such as L. pneumophila serogroup 1. The other common technique is the Rapid Molecular Method, typically Quantitative Polymerase Chain Reaction (qPCR).
qPCR detects the genetic material (DNA) of the Legionella bacteria and is significantly faster, often providing results within 24 to 48 hours. While speed is an advantage, qPCR cannot differentiate between live and dead bacteria, meaning it may detect DNA from non-viable organisms. Results are reported in Genomic Units, which do not directly correlate with the Colony Forming Units used in the culture method, but offer useful information for a water management plan.
Understanding Test Results and Response
Interpreting Legionella test results involves comparing the detected concentration of bacteria against established action levels. These action levels are expressed in CFU per unit of volume and are used as triggers for corrective action. Specific thresholds vary based on the building type and local health jurisdiction; for instance, a hospital with vulnerable patients may have a much lower threshold than a general commercial facility.
In many general systems, a result showing less than 100 CFU/L is considered acceptable, suggesting that current control measures are effective. A low-level detection (100 to 1,000 CFU/L) signals that the system’s management program should be urgently reviewed and resampling is necessary to confirm the issue. Concentrations exceeding 1,000 CFU/L represent a high contamination level and indicate a significant health risk requiring immediate, system-wide remediation.
When a high concentration is detected, the immediate response involves an urgent review of the water management plan and implementation of corrective measures. These actions often include system cleaning, disinfection using methods like hyperchlorination, or thermal eradication by raising the water temperature to lethal levels. After disinfection is complete, follow-up testing must be performed at frequent intervals to confirm the effectiveness of the remediation and ensure the bacteria’s concentration remains under control.