- It is a genus of Gram-negative bacteria, with Legionella pneumophila being the most common species that causes disease in humans.
- It thrives in warm water environments, such as cooling towers, hot water systems, and spas.
- It causes Legionnaires’ disease (a severe form of pneumonia) and Pontiac fever (a milder flu-like illness).
The Legionella bacterium got its name from the American Legion because of the 1976 Legionnaires’ disease outbreak in Philadelphia.
Origin of the Name
- In July 1976, a severe pneumonia outbreak occurred among attendees of an American Legion convention at the Bellevue-Stratford Hotel.
- The U.S. Centers for Disease Control and Prevention (CDC) investigated and, in January 1977, identified a previously unknown bacterium as the cause.
- Scientists named it Legionella pneumophila:
- “Legionella” → in reference to the American Legion.
- “pneumophila” → from Greek, meaning “lung-loving” (pneumo = lung, phila = loving).
Legionella is commonly found in natural and man-made water systems.
Where it is found in water
- Natural water sources – lakes, rivers, and streams (usually in low concentrations).
- Man-made water systems (where it becomes dangerous due to amplification):
- Hot water tanks and geysers
- Cooling towers (HVAC systems)
- Jacuzzis and hot tubs
- Showers and taps (especially when rarely used)
- Decorative fountains and humidifiers
How it is detected in water
- Water sampling & culture tests – samples are grown on special charcoal-yeast extract agar.
- PCR testing – detects Legionella DNA rapidly.
- Dip-slide or rapid antigen tests – for field screening, though less accurate.
Legionella can be effectively controlled and removed from water systems by a combination of temperature management, disinfection, and system maintenance.
1. Temperature Control (Most Effective)
Legionella grows best between 20°C and 45°C (68–113°F).
- Hot water systems → Keep stored water ≥60°C and circulating water ≥50°C.
- Cold water systems → Keep below 20°C.
- Thermal disinfection → Periodically flush the system with 70°C water for at least 30 minutes.
2. Chemical Disinfection
a) Chlorination
- Maintain 0.5–1 mg/L free residual chlorine in hot and cold systems.
- Shock chlorination → 20–50 mg/L chlorine for 1–2 hours, followed by thorough flushing.
b) Monochloramine
- More stable than chlorine and penetrates biofilms better.
c) Chlorine Dioxide
- Very effective against biofilms and commonly used in large buildings.
d) Copper-Silver Ionization
- Ions disrupt bacterial cell walls; good for hospitals and hotels.
e) Ozone or UV
- UV light inactivates Legionella at points of use (e.g., showerheads), but it has no residual effect in pipes.
3. Physical & System Maintenance
- Descale and clean tanks and pipes to remove biofilm and sediment.
- Flush rarely used outlets weekly to prevent stagnation.
- Replace old fittings where biofilm accumulates.
4. Point-of-Use Filters
- 0.2-micron filters on taps and showerheads (used in hospitals for immunocompromised patients).
Important Note
Even after removal, constant monitoring and preventive maintenance is required because Legionella can recolonize quickly if conditions return to warm and stagnant.