The new Legionella Testing Standard AS 5132: What does it mean for you?

This year a new Australian Standard, AS5132:2017 has been released. The title is ‘Waters-examination for Legionella spp. including Legionella pneumophila – using concentration’. Basically, the new standard will detect Legionella to a limit of 100 cfu /Litre. This is about 100 times more sensitive than the current AS 3896. The current standard has a limit of detection of 10 cfu /mL (not litre). So, what does this mean for your facility?

What is the Scope?

The target for the standard is reticulated water systems. It is not intended for cooling towers or industrial water. It does not replace the current AS/NZS 3896 that is a current requirement for testing in all Australian jurisdictions. In essence, this standard is more focused on building water systems like health and aged care facilities.

What is difference between the two standards?

Most of the methodology is the same for both standards. The major difference is that new standard uses a filtration method to concentrate the bacteria in the sample. The filter is then washed in a smaller volume of water (10 mL) and cultured in the same way as the old method. Of course, this requires the use of filters capable of catching bacteria and will add some time to the process. The sample result is multiplied by the concentration factor and then reported as cfu/L.

This new method is not recommended for cooling water systems etc. This is for a good and simple reason. It is very hard to filter cooling tower water through bacterial filters as they clog very quickly. It is almost impossible to filter more than 500 mL, and 250 mL can be a challenge! This makes the sample size that could be processed in their case very variable.

Some things to consider.

The standard does not specify a volume that must be collected, only that the result must be reported as cfu/L. This sounds OK on the face of it but it does create some problems.

The average 2 metre shower hose contains around 150 mL of water.

Showers hoses are a prime point for bacterial growth in a water system. So, if a sample volume of 100 mL is taken from the shower then it could be expected that there would be a quite large microbial content drawn from the stagnant water in the hose. Once this sample is concentrated and then reported as cfu/L the results look like the example below.

Concentrating the sample for Legionella also concentrates other microorganisms. The history of other bacteria interfering with Legionella culture results is long and well documented. Will concentrating a sample add to the problems of other bacteria in the water, that will also be concentrated by filtration?

Example Results

Let’s assume the result from both methods is a positive result of 2 organisms is found on the culture plate:

Using AS 3896

2 organisms from a 0.1 mL sample put directly on the culture plate = 2 X 10 = 20 cfu/mL (20,000 cfu/L).

The same result would be returned if a 250 mL sample was taken.

Using AS5132

2 organisms from a 0.1 mL sample put on the culture plate = 2 X 10 = 20 cfu from the concentrated sample. But the result needs to be report per Litre. The sample has been concentrated 10 X by the filtration method = 200 cfu

Allowing for a 100 mL sample originally drawn from the shower multiply by 10 = 2000 cfu/L. This is 2 cfu/mL and would not be detected by AS 3896.

So let’s assume that there are 2000 cfu/L in the first 100 mL of the sample. In most instances, further back from the shower hose there are likely to be much less bacteria in the water.

If a 1 litre sample is taken and the last 900 mL do not contain detectable Legionella then the 2000 cfu/L detected in the first 100 ml of example sample is diluted by 10 times to 200 cfu/L.

If a litre sample is taken and only 100mL filtered then the result would become 20 cfu/L. This is below the detection limit of the methods and would be reported as not detected.

So the point is?

It all seems a bit confusing but does need to be understood. Both the volume of water collected and the volume of water filtered can influence the final result. In deciding to use AS 5132 it is very important to consider the volume of water being sampled and filtered as smaller volumes will tend to skew the results towards positive results, and larger volumes in the opposite direction! The practicality of drawing a litre sample without contamination from some outlets must also be considered.

What else do I need to know?

Adding the concentration step to the Legionella testing method will add more time to processing samples. Inevitably this will add more cost to having samples tested.

There is no legal requirement for the new standard to be used in any jurisdiction in Australia. The new Queensland regulations only suggest that Legionella is detected using a recognised standard method and do not require the more sensitive method to be used. This is the current position for all other jurisdictions.

So depending on your ‘risk appetite’ you may choose to use the more sensitive method or stick with the old method. We recommend you make this decision based on your current risk management strategy.

A similar version of AS 5132 has been in operation in Europe for a number of years. However, the rates of disease in Europe are very similar to those reported in Australia each year ( 0.5 - 1.0 cases per 100,000 population. This suggests that successfully tackling the problem is not about the sensitivity of the testing – but the management of the system. We advise that you keep your focus on risk management through physical maintenance and rather than verification testing. Remember that whatever method you choose it is inherently inaccurate. It is wise to make decisions based on data you can rely on.

References

AS 5132:20017 Waters-examination for Legionella spp. including Legionella pneumophila – using concentration.

AS 3896: 2008 Waters-examination for Legionella spp. including Legionella pneumophila.