Legionnaires' disease ...

  Company profile
  Plant & equipment
  Legionnaires' disease 
  Contact us
How do you get it ?
Legionnaires' disease is caused by inhaling airborne droplets or particles containing viable legionella bacteria. legionellae bacteria
The legionella bacteria is to be found widely in natural waters eg lakes, rivers, streams etc. In nature numbers appear to be too low to cause any problem. The danger arises when we provide an environment particularly suited to this bacteria.

To appreciate the chain of events leading up to exposure and possible infection it helps to know something about the bacteria.

The organism
The legionella family is a large one. Over forty different species of the bacteria have been identified and legionellae-like amoebal pathogens are also proposed. Most members of this family are inoccuous.

However something like seven or eight are known to cause the disease and one in particular predominates in its virulence to humans.

This particular species is referred to as Legionella pneumophila, and accounts for almost 90% of legionnaires' disease infections. Of this latter species there are some sixteen sero groups, of which sero group 1 is the most virulent, accounting for some 50% of L. peumophila infections .

Legionella is a water borne 'gram negative' bacteria.

legionellae bacteria

This means that it is a relatively delicate organism with a thin cell wall structure.

diagramof the legionellae bacteria

Typically sausage shaped, and some 2-6 microns long, usually with a thin flagella when found in nature.

It has a high tolerance of different environmental conditions.

At below 15oC it becomes dormant. It can survive higher temperatures better than most bacteria, even up to 80oC for short periods of time.

legionellae growth & temperature range

Free legionella are destroyed easily by UV light and by a range of chemical biocides.

effects of UV & sunlight on legionellae growth

It has one characteristic that favours its ability to withstand adverse environmental conditions.

It has the ability to infect, grow and multiply within larger tougher cells such as amoeba and protozoa. sequence showing proliferation of legionellae inside amoeba

It also uses algae as a habitat in which to live. Eventually rupturing and releasing many legionella into the water.

This latter behaviour can make it very difficult to eradicate the presence of legionellae once colonisation of a water system has occurred.

Where nutrient is severely limited the interface between liquid and solid is more likely to be higher in nutrient then the bulk of the liquid.

This will encourage the formation of biofilms (or slimes) on otherwise clean surfaces.


In turn the biofilm will act as both nutrient source and protected environment for legionella.

Thus acting as a source of outbreak for infection at a later date when conditions are more favourable. This also has implications for the validity of sampling for legionella and for determining the effectiveness of cleaning and disinfection operations.