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.
Leclercq
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Unité de Microbiologie MBLA, Université Catholique de Louvain, Place Croix du Sud 2, Box 12, B-1348 Louvain-la-Neuve, Belgium. alexlec@pasteur.fr
D002611:Cheese / Q000382:microbiology; D015169:Colony Count, Microbial; D003470:Culture Media / Q000737:chemistry; D005506:Food Contamination; D005516:Food Microbiology; D006801:Humans; D008089:Listeria monocytogenes / Q000166:cytology / Q000254:growth & development* / Q000302:isolation & purification*; D008088:Listeriosis / Q000382:microbiology*; D012680:Sensitivity and Specificity; D012703:Serotyping; D013997:Time Factors
D003470:Culture Media