The genus Listeria constitutes Listeria monocytogenes, Listeria ivanovii, Listeria seeligeri, Listeria welshimerii, Listeria innocua, Listeria grayi, Listeria murrayi and Listeria denitrificans. Among these, L. monocytogenes and L. ivanovii are associated with diseases in humans. The pathogenicity of L. ivanovii is uncertain. L. monocytogenes is found in a wide variety of habitats, including the normal microflora of healthy ruminants, gastrointestinal tract of asymptomatic humans and environmental sources including river water, sewage, soil, silage, fertilizers and decaying vegetation (4).
Listeria Identification Agar also known as Polymyxin-Acriflavin-Lithium chloride-Ceftazidime-Aesculin-Mannitol (PALCAM) Agar was formulated by Van Netten et al (1) and is recommended for the isolation of L. monocytogenes from foods. PALCAM medium is highly selective due to the presence of lithium chloride, ceftazidime, polymyxin B and acriflavin hydrochloride. PALCAM medium is a differential diagnostic medium utilizing two indicator systems, as esculin and ferric citrate and mannitol and phenol red.
Peptic digest of animal tissue serves as the main source of nutrients for the organisms. Dextrose, starch and mannitol are the carbohydrate and energy sources. Sodium chloride maintains the osmotic equilibrium of the medium. Phenol red is the pH indicator dye that exhibits changes in the pH of the medium. L. monocytogenes hydrolyzes esculin to form esculetin and dextrose. Esculetin reacts with ammonium ferric citrate and forms a brown-black complex seen as a black halo around colonies. L. monocytogenes does not ferment mannitol but contaminants such as Enterococci and Staphylococci ferment mannitol and is indicated by color change from red to yellow. Under microaerophilic conditions, strict aerobes such as Bacillus species and Pseudomonas species are inhibited. The addition of egg yolk (2.5% v/v) to PALCAM Agar has been reported to aid repair of damaged cells (2). Medium containing blood when overlaid on PALCAM Agar enables to differentiate and enumerate haemolytic Listeria species (3).
Depending upon the type of sample used, selective enrichment broth should be used prior to inoculation onto PALCAM Agar. Generally Listeria Selective Enrichment Medium is used for dairy products and Listeria Selective Enrichment Medium UVM (M890A), Fraser Secondary Enrichment Broth (M1083) are used for meats and poultry. On PALCAM Agar, colonies of Listeria appear as grey-green with a black precipitate, following inoculation and incubation at 35°C for 24-48 hours under aerobic or microaerophillic conditions.
Storage and Shelf-life:
Store below 30°C in tightly closed container and the prepared medium at 2-8°C. Use before expiry date on the label.
References:
1. Van Netten P., Peralse I, Van de Mosdik A., Curtis G.D.W., Mossel D. A.A., 1989, Int. J. Food Microbiol., 8(4):299.
2. int Veld P.H. and de Boer E., 1991, Int. J. Food Microbiol., 13:295.
3. Van Netten P., van Gaal B. and Mossel D. A. A., 1991, Lett. Appl.Microbiol, 12:20.
4. Watkin J., Sleath K. P., J. Appl. Bacteriol., 50: 1-9, 1981.