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Comparison of two culture methods for the enumeration of Legionella pneumophila from potable water samples
Citation:
Boczek, L., M. Tang, C. Formal, D. Lytle, AND H. Ryu. Comparison of two culture methods for the enumeration of Legionella pneumophila from potable water samples. JOURNAL OF WATER AND HEALTH. IWA Publishing, London, Uk, 19(3):468-477, (2021). https://doi.org/10.2166/wh.2021.051
Impact/Purpose:
Legionella spp. can be difficult to culture out of water systems and to recover from environmental sources. Buffered Charcoal Yeast Extract (BCYE) Agar has been used historically as the culture media for growth of Legionella. However, Legionella is a slow growing bacterium. It takes 7-10 days for incubation, and then it needs further testing for verification of genus, species, as well as serogroups using latex agglutination tests specific for L. pneumophila. BCYE media can also be overgrown by other microorganisms that are present in the water sample. This is a common occurrence due to the slow growing nature of Legionella spp. compared to other organisms that can grow faster. If overgrowth by non-target organism is a concern, other treatment techniques such as acid treatment, heat treatment, and the use of selective additives (e.g., antibiotics) need to be added to the sample or media. These treatments will suppress the growth of the non-target organisms significantly, but they have also been known to decrease the detection of Legionella spp. in the samples. Due to these difficulties, there is a need for a robust, easy method that gives reliable results on the presence of these organism in water systems. IDEXX Laboratories Inc. (Westbrook, ME) has created the Legiolert product which can grow and detect the presence of L. pneumophila from potable and non-potable water samples in 7 days. Legiolert uses the Quanti-Tray system (IDEXX Laboratories Inc.) which allows for enumeration of L. pneumophila, and the trays can be investigated to pull out an individual isolate for further testing. The manufacturer claims that there is no need for further verification for L. pneumophila due to the high selectivity against non-pneumophila species. While this method shows promising practical applications (i.e., simplistic and less laborious) over the traditional culture method, further testing against various types of environmental water samples is necessary to ensure its reliability. The objective of this research effort was to compare qualitive and quantitative results of L. pneumophila from the traditional culture isolation method using BCYE agar with the Legiolert product in drinking water collected from a building model home plumbing system (HPS). In addition, the specificity of Legiolert against L. pneumophila in testing environmental water was investigated using a molecular confirmatory assay.
Description:
Legionella infections have steadily increased in the United States over the last 20 years, and most of these infections have been attributed to contaminated water. The gold standard for confirmation of Legionella presence in water is through culturing with Buffered Charcoal Yeast Extract (BCYE) agar since it was first used in the mid 1970’s. Following many modifications, this method is still time consuming, expensive, and can take longer than 10 days for full confirmation. Legiolert (IDEXX Laboratories Inc., Westbrook, ME) is a newer and simpler culture product that was claimed to be able to quantify Legionella pneumophila from water systems in 7 days with high sensitivity and specificity and doesn’t need further confirmation for the presence of L. pneumophila. We compared the culturablility of L. pneumophila occuring in a simulated home plumbing system using both Legiolert and BCYE agar methods. Out of 185 water samples, Legiolert and BCYE method detected the presence of L. pneumophila in 83% and 85% of the samples, respectively. The two methods were determined to be statistically equivalent for culturability of L. pneumophila, though the detected levels by Legiolert were slightly higher than the BCYE method. Molecular confirmation of positive and negative wells with Legiolert also showed a high specificity of 96.5% (i.e., 3.5% false positives and 0% false negatives).
URLs/Downloads:
DOI: Comparison of two culture methods for the enumeration of Legionella pneumophila from potable water samples
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