Survival of Listeria monocytogenes in Ground Beef or Liver During Storage at 4 and 25°C

Survival and growth of three Listeria monocytogenes strains (Scott A, Brie-1, and ATCC 35152) were studied in ground beef or liver during storage from freshness to spoilage at 4 and 25°C. Cells were enumerated on Plate Count Agar, Trypticase Soy Agar, and selective media, including McBride Listeria Agar (MLA), Cyclohexanedione Nalidixic Acid Phenylethanol Agar (CNPA), LiCl Phenylethanol Moxalactam Agar (LPM), and LPM with potassium tellurite (LPMT). Aerobic natural microflora in the fresh uninoculated samples ranged from 10² to 10⁴ CFU/g, and L. monocytogenes inocula were ca. 10³ or 10⁵ CFU/g. Total aerobes after 2 weeks at 4°C were >10⁸ or >10⁷/g in meat or liver respectively, while recovered numbers of L. monocytogenes remained unchanged during a storage of over 30 d in either ground meat or liver. Samples stored at 25°C confirmed recovery but absence of multiplication of the organism. LPM or LPMT provided the best selective environment for direct plating of meat. Despite differences in composition and spoilage pattern of meat and liver, no difference was observed in the fate of L. monocytogenes in these foods.