Comparison of BCYEα+AB agar and MWY agar for detection and enumeration of Legionella spp. in hospital water samples

Abstract

Background: This study illustrates for the first time the performance (sensitivity and selectivity) of the selective medium BCYEα +AB suggested by the new edition of ISO 11731 for legionella isolation and enumeration. We compared the efficacy of the selective BCYEα +AB medium with that of the highly selective MWY medium.

Results: Legionella spp. was detected in 48.2 and 47.1% of the samples by BCYEα +AB and MWY agar, respectively. For optimal detection of Legionella spp., most protocols recommend using selective media to reduce the number of non-Legionella bacteria. Agreement between the two media was 86.7%.

Conclusions: According to the results, both media have a very similar performance and they both have advantages and disadvantages over each other. In AB medium there is the risk of being less selective so more interfering microbiota may grow but in MWY medium there is the risk of being too selective. The low selectivity of the AB medium could be resolved if other treatments are applied after filtration, e.g. acid and/or heat treatment, but it must be taken into account that these treatments still reduce the number of viable Legionella. In conclusion, we recommend using MWY as a selective medium for the detection of Legionella spp. as it is easier discern suspected colonies and facilitate the final Legionella spp.

Keywords: Culture media; Environmental monitoring; Legionella.

Fig. 1

Comparison of selective ability of MWY agar and BCYEα+AB. The presence of Legionella spp. and relationship with other bacteria. Background flora was measured through semi-quantitative counting, where zero was no background flora and 3+ was massive contamination (see Additional file 1)

Fig. 2

Comparison of Legionella counts (CFU/l) on different media. Scatter plot in logarithmic scale comparing Legionella spp. detection by MWY and BCYEα +AB counts. Kendall’s tau correlation coefficient was computed taking into account only samples positive for Legionella spp. on both media