Listeria monocytogenes isolated on selective chromogenic plating media necessitates further characterization to confirm its presence in dairy foods

Listeria monocytogenes colonies were isolated using Listeria enrichment broth and Listeria selective agar from the
dairy (paneer, cheese and ice cream) samples. The isolated colonies were differentiated on chromogenic medium,
Agar Listeria according to Ottaviani and Agosti (ALOA). Isolated colonies have appeared bluish-green with a halo-
zone on ALOA medium. This enabled confirmation of L. monocytogenes by β-D-glucosidase and phosphatidyli­
nositol phospholipase-C (PI-PLC) enzymes activity. However, variations were observed in the biochemical tests
among the bluish-green isolates. Expression of virulent internalin genes (InlA, InlB, and InlC) was not observed in
the bluish-green colonies (except one colony) isolated from ALOA medium. Surprisingly 16S rRNA data revealed,
only one isolate was L. monocytogenes among four bluish-green isolates. These results signify that, bluish-green
colonies on ALOA chromogenic medium can be misidentified as L. monocytogenes. Moreover, the identified
L. monocytogenes has shown atypical catalase negative activity. Therefore, the L. monocytogenes isolated on a
chromogenic medium is warranted for further characterization