The Rapid Isolation and Identification of Listeria species from Food Samples

The combination of the chromogenic culture medium ALOA (AES Laboratoire, available in the UK from Microgen Bioproducts) and the biochemical identification system Microgen® Listeria ID has been demonstrated to facilitate the isolation and identification of all Listeria species from food samples within 3 days.

Food samples were pulsified (Pulsifier®, Microgen Bioproducts Ltd) and then incubated for 24 hours in half Fraser broth after which a sample was inoculated onto ALOA agar plate medium. After 24 hours incubation colonies were examined to provide a presumptive identification based on their colour. Listeria species produce blue/ green colonies with L. monocytogenes and L. ivanovii producing a halo (clear zone) around the colonies. Any other organisms growing on the medium produce white colonies which are easily differentiated from Listeria species.

Colonies suspected of being L. monocytogenes or L. ivanovii were inoculated directly into the Microgen® Listeria ID to provide final biochemical identification.

The ALOA agar medium has been fully validated under the AFNOR regulatory framework in France and is also cited in the BAM method for Listeria species. In the AFNOR studies, ALOA agar plates yielded more colonies of Listeria species after 24 hours incubation than PALCAM or OXFORD agar plates after 48 hours incubation. In both the ALOA method and the OXFORD agar method, there was a 100% correlation for the isolation of L. monocytogenes using a range of food samples.

The combination of ALOA and the Microgen® Listeria ID has recently gained AFNOR approval in France.

This work was presented at ASM, 2004 by S.A. Clark (Microgen Bioproducts, UK), J-P Aurel (AES Laboratoire, France) and G. Dunn (Microbiology International, USA).