Legionella doubling time: unknown and underestimated

When a building is temporarily closed, for example for closing days, holidays, or due to unforeseen circumstances, water stagnates in the pipes. However, many operators underestimate how quickly Legionella bacteria can multiply during this period, especially when the temperature in the pipes reaches a favorable level.

We recently visited a site that was closed for two consecutive days each week. During this closure, the water in the cold water pipes and hot water taps reached 33°C . This is precisely the temperature range conducive to the proliferation of Legionella.

Why is the 25 to 45 °C range so critical?

Legionella thrives between 25°C and 45°C , with an ideal temperature between 35°C and 38°C. Under these conditions, bacteria can multiply relatively rapidly. In the laboratory, the doubling time is sometimes only a few hours. In pipes, where conditions are less favorable, or should be, a doubling can reasonably be estimated every 8 to 12 hours at optimal temperatures and when the system is out of service. This calculation does not take into account existing biofilm or other nutrients.

Source: own layout based on the following studies, Kusnetsov, J. M., Ottoila, E., & Martikainen, P. J. (1996). Growth, respiration and survival of Legionella pneumophila at high temperatures. Journal of Applied Bacteriology, 81(4), 341–347.Personnic, N., Striednig, B., & Hilbi, H. (2023). The Legionella Lqs-LvbR regulatory network controls temperature-dependent growth onset and bacterial cell density. mBio, 14(2)Borella, P., et al. (2017). Temperature-driven growth of Legionella in lab-scale systems and interaction with protozoa. Water Science and Technology, 75(3), 655–663.

At 33°C and without flow, it is therefore perfectly possible that an undetectable quantity could quickly transform into an increased concentration.

The example below shows how quickly a population can grow when we assume:

• Initial concentration: slightly less than 100 cfu of Legionella bacteria per liter (low value, often undetectable)

• Doubling time: 12 hours (conservative and realistic estimate at 30-35 °C and at rest)

Note: This is a theoretical calculation example for illustrative purposes, based on a limited research area. In real-world installations, biofilm, nutrients, materials, and water quality also play a role.

Calm	Number of bacteria (theoretical)
12 hours	200
24 hours	400
36 hours	800
48 hours	1,600
72 hours	6,400
96 hours	25,600
7 days (168 hours)	± 25 million*

Although the value after 7 days is an arithmetic figure, and we did not expect to encounter such a value due to limitations such as nutrient availability, the table shows how quickly stagnant water leads to increased risk.

After just 48 hours, a low initial concentration can reach a significantly high, alarming, and potentially pathogenic level. After a few days of inactivity, contamination levels can be very high , especially in the presence of biofilm and deposits. This calculation also doesn't account for dead zones, or traditionally underused taps, where inactivity naturally lasts much longer.

Closed buildings: an increased risk, that's for sure.

In the event of temporary or partial closures, such as for renovations, a combination of risk factors is often observed:

• long-term impasse

• potentially dangerous water temperatures

• Insufficient or non-existent flushing of the main line and branches

This creates ideal conditions for Legionnaires' disease, in both hot and cold water pipes.

If a building is subsequently put back into service without a proper flushing or disinfection procedure, users may be exposed to increased risks, for example through showers, faulty taps and other aerosol-generating taps.

Some essential preventative measures:

• Ensure that the water meets the temperature requirements:

  • cold water at a temperature below 25°C (ideally even below 20°C)

  • Hot water supply at least 60°C

  • hot water circulation at a temperature of at least 55°C

• Avoid prolonged service interruptions: regular rinsing cycles are essential.

• Before reopening after closing: perform a very thorough rinse (with cold water and hot water).

• If in doubt, have a risk analysis and sampling carried out by a specialist partner.

  
  

Conclusion

The doubling time of Legionella illustrates how quickly a seemingly harmless, even invisible, situation can transform into a serious health risk. In our example, two days of inactivity at 33°C are enough for low concentrations to reach problematic levels.

Prevention, monitoring and proper management of water facilities remain the best protection.