Analysis of ISO 11731:2017 method to assess Legionella pneumophila in water with high background : And how it differentiates from its earlier variant ISO 11731:1998

Abstract: Legionella pneumophila is a human pathogen commonly found in natural and artificial aquatic environments and can cause a condition called legionellosis. Monitoring for legionellae is therefore important for protecting public health and identifying its environmental sources is a way to prevent illness. This has resulted in development of several control strategies to identify these sources. One of these strategies is to construct a valid method to detect Legionella pneumophila and monitoring these methods is a way to ensure the method remain effective at tracing infection.  The current version of standardized method is called ISO 11731:2017 and supersedes its former version called ISO 11731:1998. The former version uses a combination of heat and acid solution treatment to reduce interfering microorganisms in water with high background, whereas the current version separates the treatment by subdividing the sample in three parts. One part is subjected to heat treatment, one with acid solution treatment and one remains untreated. Therefore, the aim of this study is to analyse how this difference in method strategy will affect detection of Legionella pneumophila between the current and its former version of ISO 11731. To do this, this study divided the experiment into two parts: experiment A was aimed at evaluating the validity of the method and experiment B was designed to study repeatability in terms of dispersion and performance data range. For experiment A: 14 samples were tested using both ISO 11731:2017 and 11731:1998 to see how the results differentiated. Six are natural samples and was appointed based on their previous results that showed positive for Legionella. Four samples were spiked with different serotypes of Legionella and the remaining four were spiked with both Legionella and Legionella-inhibited bacteria. For experiment B, three certified reference material with different concentration of Legionella pneumophila serotype 1 was tested in repeatability conditions with each sample producing ten replicates.  In conclusion, based on results assessed in this study ISO 11731:1998 was more suitable to analyse water with higher concentration of interfering microorganisms. By a combination of heat and acid solution treatment: it maximizes the reduction of interfering microorganisms which facilitates Legionella to cultivate on agar. ISO 11731:2017 was more efficient in recovering different serotypes of Legionella. Although, there were a significant increase in dispersion and performance data range results in ISO 11731:2017. This indicates that since there is an additional dilution step added in acid solution treatment: it increases the risk of human error and therefore a greater vulnerability to the method.