Legionella Detection and Result Interpretation
G. Pinna B App Sc (Med Tech) B Sc MASM MAIRAH
Manager of Biotech Laboratories Pty Ltd, Brisbane, Queensland.


Statistical Data
Biotech Laboratories is a nationally accredited biological testing laboratory that has been analysing samples of water-cooled air-conditioning systems since 1990. The laboratory analyses approximately 8,000 samples per year and of these, some 5,500 are for Legionella bacteria.

The detection limit for Legionella by the Australian Standard method is 10 colony forming units per millilitre (CFU/mL) which is basically the same as bacteria per millilitre. The industry guidelines for acceptable results, which is also quoted in the Australian Standard AS 3666, Air-handling and water systems of buildings - Microbial control also state an acceptable limit of less than 10 CFU/mL.

In this laboratory over the 10-year period from 1993 to 2003, the average isolation of Legionella bacteria occurred at a rate of once in every 9 samples, i.e. 10.7%. The monthly isolation rate varies with the seasons and peaks in late summer each year. During the 10-year period the lowest monthly isolation rate was 1.9% and the highest was 23.8% (nearly once in every 4 samples). This is despite the fact that it would be expected that these samples came from air-conditioning systems that were being maintained by reputable and efficient water treatment specialists and being maintained to the requirements of AS 3666. It is important also to remember that in that 10-year period although 10% water samples were positive for Legionella, none resulted in a Legionnaires' Disease outbreak. This is despite the fact that on average 5% of all positive samples has a Legionella count exceeding 1,000 CFU/mL.

The data presented would suggest that reliance on maintaining cooling towers to Australian Standard AS3666 does not prevent contamination of the system with legionellae. It would be expected that at some time or another all water-cooled air-conditioning systems would show some level of contamination with Legionella. This does not necessarily reflect on the quality of the system maintenance.

The concentration of Legionella present is important however, and a level exceeding 1,000 CFU/mL requires immediate decontamination. A level of 10 to 100 CFU/mL is commonly found and although some remedial action is indicated it can often be regarded as insignificant so long as proper maintenance practices are being maintained. By maintaining the correct maintenance procedures and testing at a monthly interval, the risk of Legionella concentrations reaching the high risk level (>1,000 CFU/mL) before detection is extremely low. This tolerance level does not apply to spa pools, warm water, reticulated domestic supplies, water used for cooling drill bits or other machinery where aerosols are produced. In these cases even low Legionella levels are highly significant and immediate disinfection is required. Irrespective of the water source after a positive detection follow up testing for Legionella is required 3 to 5 days after remedial action is taken to ensure that the process was effective.

It must be remembered that the species of Legionella detected, be that Legionella pneumophila serogroup 1, Legionella pneumophila serogroup 2-14, or Legionella species has no effect on the actions that are required as a result of a positive detection. It is the total number of Legionella present per millilitre water that is important. If a water sample had a Legionella pneumophila count of 200 CFU/mL and a Legionella species (not pneumophila) count of 900 CFU/mL, then the total Legionella count for the water would be 1,100 CFU/mL which requires immediate shutdown and disinfection. This fact is supported by AS/NZS 3666.3 which classifies actions required based on the level of Legionella and makes no mention for a diminished risk status based on the speciation of the microorganism.
It is true that all cooling tower outbreaks to date of Legionnaires' Disease have been due to Legionella pneumophilia, but at the same time you must remember that 98% of all Legionella found in cooling towers are Legionella pneumophilia. It is therefore not surprising that all outbreaks have been due to this species and not one of the other Legionella species. It is important to remember that the group Legionella species includes L. micdadei, L. longbeachae, L. bozemanii, L. feeleii, and L. dumoffii. All these have been associated with serious human infections and in many cases have led to death.