How to Test for Legionella in a Cooling Tower

A hand with a blue rubber glove holding on to a petri dish with bacteria in it. It shows that the cooling tower tested positive for Legionella.

Legionella testing is the only way to determine if your cooling tower system is clean and it gives building owners, operators, managers and landlords an opportunity to find out if their system contains high levels of Legionella and to prevent the spread of Legionnaires’ disease. Legionnaires’ disease is most often seen in warmer and wetter summer months when the Legionella bacteria may rapidly increase in cooling towers and spread through water droplets leaving the tower. Legionella is common in the environment and grows best in the warm water.

Therefore, when testing cooling towers, there is a likely chance of finding it, although it may include organisms that are no longer active.  If it is found, it is important to know how much legionella is there. If the amount of the bacteria is high, people are more likely to be exposed and could become ill.

If elevated levels of bacteria are measured, building owners and operators should take the corrective measures to clean and disinfect the unit(s). Then they will need to retest it to confirm that that the problem has been addressed. There are three popular methods to test for Legionella:

  • Cultured Samples: Water samples from the cooling tower are cultured on a buffered charcoal yeast extract and typically take about three to five days to confirm. However, the culture results may make take longer because some strains take longer to form colonies.
  • Direct Fluorescence Antibody (DFA): Water samples from the cooling tower are determined by conjugate tests that stain the organism with fluorescence dye. This method of testing gets results faster but is unable to tell the difference between live and dead bacteria and may result in false results.
  • DNA Amplification: A new method for fast detection of Legionella bacteria that is growing popular. The test uses a polymerase chain reaction process to detect portions of DNA that is common to Legionella. Results come in just one day.

Testing samples should be consistent and taken in the same place, in the same way, every time. It is recommended for cooling towers that water samples, which are free from sediment be collected from the tower basin at the furthest most point away from the make-up-water. Keep in mind, the best prevention for legionellosis is maintenance of the water systems in which Legionella grows.

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