Atypical colonial morphology and low recoveries of Listeria monocytogenes strains on Oxford, PALCAM, Rapid'L.mono and ALOA solid media.

The performance of four commercial media, polymyxin-acriflavine-LiCl-ceftazidime-aesculin-mannitol (PALCAM), Oxford, Rapid'L.mono (Bio-Rad, Marne la Coquette, France) and Agar Listeria according to Ottaviani and Agosti (ALOA: AES Laboratoire, Combourg, France; Biolife, Milan, Italy), used to detect and enumerate 176 Belgian strains of Listeria monocytogenes of human and food origin, was evaluated. Four strains showed a low recovery and/or atypical colonies on one or more media. These results showed that a combination of these media, especially alternative media (Rapid'L.mono and/or ALOA) with esculin-containing media (PALCAM and/or Oxford), should therefore be recommended to detect or enumerate atypical strains of L. monocytogenes. In outbreak case investigation for example, incubation of plates should be extended to at least 96 h if no colonies are typical or growth does not appear after 48 h. This is a cost/benefit calculation that should be done in the context of recent listeriosis risk assessments.