Abstract
BackgroundThis 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.ResultsLegionella 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%.ConclusionsAccording 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. count.
Highlights
This study illustrates for the first time the performance of the selective medium Buffered Charcoal Yeast Extract agar with α-ketoglutarate (BCYEα) +AB suggested by the new edition of ISO 11731 for legionella isolation and enumeration
A total of 263 hot water samples, all taken from hospital potable water faucets, were cultured on two different media (i.e., BCYEα +AB and modified Wadowsky–Yee (MWY)) to isolate Legionella spp
The evaluation of the presence of Legionella in our samples was possible for 98.1% (258/263) of the samples seeded on MWY agar and 95.4% (251/263) of those seeded on BCYEα +AB agar, whereas all samples contaminated by overgrowth of Gram-negative bacteria were deemed as Legionella negative
Summary
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. Legionella is a fastidious gram-negative bacterium: in the late 1970s, Feeley and Gorman prepared a new agar medium, cysteine F-G agar-iron, with L-cysteine hydrochloride and soluble ferric pyrophosphate [11], while Feeley et al modified the medium by replacing casein acid hydrolysate with yeast extract and by adding activated charcoal as a scavenger of radicals and peroxides [10] and the resulting charcoal yeast. Pasculle et al supplemented the CYE medium with N-(2-Acetamido)-2-aminoethanesulfonic acid (ACES) buffer, thereby obtaining buffered charcoal yeast extract agar (BCYE) which, under aerobic conditions, enabled a better recovery of Legionella [27], and Edelstein further increased this medium’s sensitivity by adding α-ketoglutarate (i.e., BCYEα agar) [8]
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