The bacterium Legionella pneumophila is a naturally occurring organism found in fresh water sources worldwide. When this bacteria aerosolizes, becoming suspended in tiny water droplets, inhaling it can lead to a severe form of pneumonia known as Legionnaires’ Disease. Controlling the proliferation of this pathogen within man-made water systems relies heavily on temperature management. Understanding the thermal conditions that encourage growth versus those that ensure eradication is paramount for water system safety.
Temperature Range for Growth and Multiplication
Legionella pneumophila thrives within a specific intermediate temperature range, making standard warm water systems ideal breeding grounds. The bacteria exhibit optimal growth and multiplication when water temperatures are between 32°C and 42°C (89.6°F and 107.6°F). The organism flourishes best at approximately 35°C (95°F).
Temperatures outside this narrow window significantly inhibit the rate of reproduction. Below 20°C (68°F), the bacteria enter a dormant state, where they can survive for extended periods. Similarly, once water temperature rises above 45°C (113°F), the growth rate slows considerably. This survival range between 20°C and 45°C (68°F and 113°F) is the primary zone that water management protocols aim to avoid.
Specific Temperatures for Immediate Eradication
Eliminating Legionella requires subjecting the bacteria to temperatures significantly higher than its growth range. The required exposure time decreases dramatically as the heat increases. At a temperature of 50°C (122°F), 90% of the bacterial population will die, but this process can take between 80 and 124 minutes, depending on the specific strain.
Raising the temperature to 55°C (131°F) dramatically accelerates the process, killing 90% of the bacteria in approximately 20 minutes. Complete eradication at this level requires a sustained exposure of five to six hours.
The speed of destruction increases sharply as the water reaches 60°C (140°F). At this heat, 90% of the Legionella population is destroyed within just two minutes. To achieve a 100% kill rate, the water must maintain 60°C for a minimum of 30 minutes. This is a commonly targeted temperature for stored hot water in commercial buildings.
For near-instantaneous results, temperatures must be elevated beyond 60°C. A sustained temperature of 65°C (149°F) kills 90% of the bacteria in around 10 seconds, with total eradication achieved in two minutes. The most effective temperature for thermal disinfection is 70°C (158°F), which results in the immediate death of Legionella.
Practical Application of Thermal Disinfection
The data on killing temperatures is applied through a procedure known as thermal shock or heat disinfection, primarily used in large water systems like those in hospitals and hotels. This process involves intentionally increasing the temperature of the water heater or storage tank to 60°C (140°F) or higher. The goal is to send a wave of superheated water through the entire plumbing network to eliminate any bacterial colonization.
For the thermal shock to be successful, the high-temperature water must reach every part of the system, including the farthest and least-used fixtures. This means flushing all taps, showers, and other outlets until the water exiting them registers a temperature above the 50°C (122°F) threshold. The high temperature must be sustained to ensure the heat penetrates any accumulated scale or sediment within the pipes.
Implementing such high temperatures necessitates careful management to address the risk of scalding. Water at 60°C (140°F) can cause serious third-degree burns within seconds. Consequently, thermostatic mixing valves (TMVs) are often installed at the point of use, blending the superheated water with cold water to ensure a safe delivery temperature at the faucet.
How Biofilms Protect Legionella
The simple temperature-time data for eradication is often complicated by the presence of biofilms within water systems. A biofilm is a complex, protective layer of microorganisms encased in a self-produced slime that adheres to pipe surfaces. This slime layer acts as a physical barrier, insulating the Legionella from both chemical disinfectants and heat.
Within this protective layer, Legionella often survives as an intracellular parasite inside water-dwelling protozoa, such as amoebae. The amoebae serve as hosts, allowing the bacteria to multiply unchecked and providing an additional layer of thermal protection. Because the biofilm shields the bacteria, the effective temperature required to kill Legionella in a real-world plumbing system may need to be higher or maintained for longer periods than laboratory tests suggest.