Legionella vs Pseudomonas

Legionella vs Pseudomonas: What’s the Difference & Why Test for Both

Waterborne bacteria are a serious concern in built environments. Two of the most commonly tested pathogens are Legionella and Pseudomonas. While they share some risk settings (dampness, water systems, biofilms), they behave differently — and testing for one does not guarantee safety from the other. In this guide, we’ll compare them, explain why testing both is advisable, and show how that fits into a water safety programme.

What is Legionella?

Legionella bacteria cause Legionnaires’ disease, a potentially fatal pneumonia linked to water systems like showers, taps, and cooling towers. They thrive in warm, stagnant water (20°C–45°C), and are controlled through temperature management, disinfection, and system design. Legionella risk is highest in systems that produce aerosols which can be inhaled.

What is Pseudomonas (Pseudomonas aeruginosa)?

Pseudomonas aeruginosa is an opportunistic pathogen found in water systems, particularly in healthcare environments. It poses a contact infection risk, especially for immunocompromised individuals, and thrives in biofilms, fittings, and poorly disinfected systems. Its control requires good design, regular flushing, and suitable disinfection regimes.

Key Differences

Legionella typically infects via inhalation of aerosolised water droplets, while Pseudomonas spreads through contact or ingestion. Legionella grows best in warm water, whereas Pseudomonas can persist even at lower temperatures. Legionella is a focus for nearly all water systems; Pseudomonas is a particular concern in healthcare and high-risk environments.

Why Test for Both?

Testing both gives full confidence in water safety management. Each bacterium presents unique risks — Legionella through inhalation, Pseudomonas through contact. Both can co-exist in biofilms, and both require different corrective actions if detected. Testing for both demonstrates robust compliance, particularly in healthcare, dental, and care settings.

How to Integrate Dual Testing

Use a 2-in-1 Legionella and Pseudomonas kit, such as the one provided by HSRA, which includes UKAS-accredited lab analysis and clear reporting. Combine this with your routine flushing and monitoring programme for year-round assurance.

Summary

Testing water only for Legionella may give you some reassurance — but without testing for Pseudomonas, you may miss substantial risks, especially in healthcare or vulnerable environments. HSRA’s 2-in-1 kit provides both tests in one package, giving you more complete peace of mind.