Legionella is the causative agent of Legionnaires’ disease, a severe form of pneumonia with a mortality rate of around 10%. Contaminated water systems, including cooling towers and hot water systems, serve as primary reservoirs for this opportunistic pathogen. Traditional plate culture methods remain the regulatory standard for monitoring Legionella, but these methods are slow—often requiring 7–10 days for results—and suffer from overgrowth by non-Legionella bacteria. Additionally, traditional methods fail to detect viable but non-culturable (VBNC) bacteria—cells that remain infectious but do not grow on standard culture media.
Molecular methods like PCR-based detection provide faster and more sensitive Legionella identification. However, a key limitation persists: PCR detects DNA from both live and dead bacteria, leading to false positives and unnecessary or even wasteful remediation efforts. To address this challenge, Promega has developed a viability qPCR method that retains the speed of molecular testing while distinguishing viable bacteria from non-viable remnants. In this third blog in our Legionella blog series, we cover how molecular detection methods can be refined to provide actionable results for Legionella monitoring.
Distinguishing Live Legionella from False Positives with Viability qPCR
Viability qPCR represents a major advancement in Legionella detection by addressing one of the biggest challenges in molecular testing: distinguishing live, potentially infectious bacteria from dead, non-threatening remnants. This method builds on standard qPCR but integrates a “Viability PCR Reagent”, a proprietary compound that selectively binds nucleic acids from compromised cells and/or viral particles. When bacteria die, their cell membranes break down, allowing the reagent to enter and chemically modify the DNA. This modification prevents the DNA from being amplified during qPCR, effectively removing non-viable bacteria from the results.
Live bacteria, on the other hand, have intact membranes that block the reagent from entering the cell. DNA remains unmodified and amplifiable, ensuring that only viable Legionella are detected. Unlike other viability PCR methods that require photoactivation to activate the reagent, this approach works without light exposure, simplifying the workflow.
This method ensures that only viable Legionella contribute to the qPCR signal, providing a more accurate picture in the contamination assessment. Additionally, by incorporating calibration controls that convert PCR data to viable genomic units (vGU), viability qPCR enables results that correlate more closely with culture-based enumeration. All of this results in a faster, more precise and quantitative assessment of Legionella in water systems.
To learn more about the how the viability qPCR at Promega works, check out this poster.
The Impact of Viability qPCR in Water Safety
Accurate detection of Legionella in water systems isn’t just about finding bacteria—it’s about understanding risk. The presence of DNA alone doesn’t necessarily indicate a live threat, which is why traditional PCR methods can sometimes lead to misleading results. By distinguishing between live and dead cells, viability qPCR provides a clearer picture of contamination, helping water safety professionals make more informed decisions.
One of the biggest advantages of this approach is the ability to reduce false positives. Standard PCR can pick up DNA from bacteria that are no longer viable, potentially triggering unnecessary remediation efforts. By focusing only on living cells, viability qPCR helps ensure that interventions are targeted where they’re actually needed.
Speed is another critical factor. Culture-based methods, which remain the regulatory gold standard for Legionella detection, rely on growing bacteria from water samples on specialized agar plates. After incubation, colonies that resemble Legionella are counted and further tested to confirm their identity.1 This approach provides a direct measure of viable bacteria, but it has significant drawbacks: 1) long incubation time delays decision-making, and 2) overgrowth by non-Legionella bacteria or the inability to detect viable but non-culturable (VBNC) cells can lead to inaccurate results.
In addition to accuracy and speed, viability qPCR reduces costs by preventing unnecessary disinfection procedures, while also identifying gaps in sanitation performance, processes, or chemical efficacy. By providing a more precise assessment, viability qPCR helps reduce the risk of over-treatment and the associated expenses, but also highlights areas where improved sanitation practices can be implemented. With these advantages, viability qPCR represents a shift in how Legionella are monitored—prioritizing meaningful, real-world data that informs both cost savings and enhanced sanitation protocols over raw detection numbers.
Industries Benefit from This Approach
Legionella monitoring is essential across industries where water systems impact public health and operational safety. Viability qPCR can be used to improve detection strategies in settings where accuracy and response time are essential.
In healthcare and long-term care facilities, for example, waterborne pathogens pose a serious risk to immunocompromised patients.2 Routine monitoring is necessary to prevent outbreaks, but traditional methods can make it difficult to distinguish between a real contamination event and harmless bacterial remnants. By providing a clearer assessment of viable Legionella, viability qPCR helps facilities respond appropriately while avoiding unnecessary disruptions.
For cooling towers and industrial water systems, Legionella testing is both a safety measure and a regulatory requirement.3 The ability to rapidly detect only live bacteria allows facility managers to make informed maintenance decisions, reducing the likelihood of unexpected downtime and costly decontamination efforts.
In municipal and environmental water testing where large-scale monitoring programs are in place, viability qPCR offers a way to streamline data collection while improving accuracy. By producing results that better correlate with culture-based methods, this approach supports regulatory compliance while ensuring public water systems remain safe.
In today’s environment of heightened water safety concerns, innovative approaches are essential to safeguarding public health. Viability qPCR delivers rapid, accurate results that simplify and streamline decision-making and proactively identify risks before they escalate. This method optimizes sanitation protocols and reduces unnecessary costs while reinforcing efforts to protect public health. As testing methods evolve to address real-world threats, Viability qPCR could be a robust advancement in water safety management.
Complete the Series with these Legionella blogs:
Understanding and Combatting Legionella in Water Systems
Strengthening Water Safety Measures with Advanced Detection
References
- Eble, D., Gehrig, V., Schubert-Ullrich, P., Köppel, R., & Füchslin, H. P. (2021). Comparison of the culture method with multiplex PCR for the confirmation of Legionella spp. and Legionella pneumophila. Journal of Applied Microbiology, 131(5), 2600–2609. https://doi.org/10.1111/jam.15103 ↩︎
- Barker, K. A., Whitney, E. A., Blake, S., & Berkelman, R. L. (2015). A review of guidelines for the primary prevention of legionellosis in long-term care facilities. Journal of the American Medical Directors Association, 16(10), 832–836. https://doi.org/10.1016/j.jamda.2015.05.015 ↩︎
- Paranjape, K., Bédard, É., Whyte, L. G., Ronholm, J., Prévost, M., & Faucher, S. P. (2020). Presence of Legionella spp. in cooling towers: The role of microbial diversity, Pseudomonas, and continuous chlorine application. Water Research, 169, 115252. https://doi.org/10.1016/j.watres.2019.115252 ↩︎
Anna Bennett
Latest posts by Anna Bennett (see all)
- No More Dead Ends: Improving Legionella Testing with Viability qPCR - March 18, 2025
- Strengthening Water Safety Measures with Advanced Detection - February 18, 2025
- Understanding and Combating Legionella in Water Systems with Viability PCR - January 21, 2025
