Comparison of Three Selective Isolation Media for the Detection of L. monocytogenes in Foods

Abstract

391 different foods were examined in order to compare three selective isolation media for Listeria: blood agar with nalidixic acid, Palcam agar, Listeria selective Oxford medium (Oxford agar). The percentage of positive samples (presence of Listeria in 25 g) obtained on the three media after an enrichment procedure is identical: 15.8%. The species L. monocytogenes is found in respectively 8.43%, 8.43% and 8.18% of the foods examined on Palcam agar, Oxford agar and nalidixic acid agar; this represents more than 50% of the isolates of the genus Listeria. However, Palcam agar and Oxford agar offer the advantages of a great reduction of the development of the contaminating microflora and a clearly less fastidious reading. The technique used for the direct counting of L. monocytogenes on these two selective media does not allow the detection of a low contamination of foods (theoretical positivity threshold = 100 Listeria/g): 65.2% of the positive samples would not have been detected without prior enrichment. 41.1% of the isolated L. monocytogenes belong to serovar 1/2a, 5.8% to serovar 1/2b, 20.58% to serovar 1/2c, 5.8% to serovar 3b and 24.47% to serovar 4b.