**1. Introduction**

The most common protocol for environmental surveillance of *Legionella* spp. involves the use of buffered charcoal yeas<sup>t</sup> extract agar enriched with 1 g/<sup>L</sup> alpha-ketoglutarate agar (BCYEα). Although this method has a proven record of effectively isolating and enumerating *Legionella* species from environmental or clinical specimens, its sensitivity and selectivity are often hampered by the presence of contaminating flora in the water samples, which may influence the final count of *Legionella* spp. due to overgrowth or inhibition. Therefore, culture on selective agar media—i.e., BCYEα supplemented with glycine, vancomycin, polymyxin B, cycloheximide (GVPC) or with glycine, polymyxin B, vancomycin, anisomycin, bromothymol blue, and bromocresol purple (MWY)—that is capable of inhibiting most non-*Legionellaceae* bacteria is the preferred solution to isolate *Legionella* spp. from environmental specimens [1–3].

Given that the recovery of *Legionella* spp. strictly depends on the type of agar being used, quality-assured culture media for water testing are key to consumer safety. However, the overall performance of commercially available nonselective (BCYEα) and selective (MWY and GVPC) media

for *Legionella* spp. isolation has only been partially addressed. In this regard, the last studies on the quality assurance of these media date back to 2004 [4] and 2010 [5].

According to the literature, these types of media are very di fficult to prepare, store, and test as minor di fferences in pH, cation content, and agar composition can heavily influence growth rates, plating e fficiency, and colony formation [6–8]. Thus, the quality control of culture media used for *Legionella* spp. detection is now, more than ever, essential to achieve a satisfactory degree of comparability among water-testing results from di fferent laboratories.

For over two decades, our group has been conducting *Legionella* spp. testing in numerous hospitals and health facilities in the Piedmont region of Italy. Furthermore, since 2007, our laboratory has been a permanent member of the External Quality Assessment (EQA) scheme for *Legionella* spp. isolation from water, which relies on the use of the BCYE α and MWY media manufactured by Oxoid Ltd. (Basingstoke, UK), with satisfactory z-scores of performance throughout. In particular, by conducting parallel seeding experiments, we have previously found that BCYE α allows a higher yield and recovery rate of *Legionella* spp. positive samples (93%) compared to that obtained with MWY (78%). Based on these findings, we were the first in 2011 [9] to recommend the use of BCYE α as a nonselective medium, in addition to MWY or GVPC, for optimal detection of *Legionella* spp. in environmental water samples. The combined use of one selective and one nonselective media for improved *Legionella* spp. detection was later on incorporated in the second edition of ISO 11731 [10].

Besides the recovery rate, we have also been actively involved in evaluating the sensitivity and selectivity of nonselective vs. selective media for the isolation and enumeration of *Legionella* spp., an investigation that has been more recently extended to BCYE α-AB media (Ditommaso et al., unpublished data).

In this study, we report on a comparative assessment between the sensitivity and selectivity of the aforementioned media from Oxoid Ltd. (Basingstoke, UK) and the corresponding media manufactured by Xebios Diagnostics GmbH (Düsseldorf, Germany) for the detection of *Legionella* spp. from environmental water samples.
