The heightened awareness and concern surrounding the presence of Listeria monocytogenes in food has resulted in the development of many media for its isolation (3). Listeria Identification Broth also known as Polymyxin-Acriflavin-Lithium chloride-Ceftazidime-Aesculin-Mannitol (PALCAM) Broth is prepared as described by van Netten et al (1) for selective enrichment of Listeria species.
Peptic digest of animal tissue and yeast extract provide growth nutrients. High amount of lithium chloride and added selective supplement (FD061) (2) containing polymyxin B, acriflavin hydrochloride and ceftazidime inhibit accompanying microflora and allow the growth of Listeria species. Soya lecithin has similar properties as that of egg yolk; hence additional supplementation of egg yolk emulsion is not required.
After incubation at 30°C for 24-48 hours, approximately 0.1 ml of the broth is streaked on Listeria selective agars such as Listeria Identification Agar (PALCAM) (M1064) or Listeria Oxford Agar (M1145). The combination of mannitol and phenol red helps the detection of mannitol fermentation while esculin and ammonium ferric citrate together help in detection of esculin hydrolysis.
L. monocytogenes hydrolyses esculin resulting in the formation of black coloured medium.
L. monocytogenes does not ferment mannitol, therfore its differentiation from contaminants such as Enterococci and Staphylococci can be made as the later will ferment mannitol and produce a color change from red to yellow. Incubation under microaerophilic conditions serves to inhibit strict aerobes such as Bacillus and Pseudomonas species. Techniques for the isolation of L. monocytogenes will depend on the material under test. It is usual for the test sample to be first inoculated into an enrichment broth to allow multiplication before isolation and identification. Depending on the types of samples used, the appropriate method and selective enrichment broth should be used.
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. et al, 1989, Int. J. Food Microbiol., 8:299.
2. Lund A. M., 1991, J. Food Prot., 54:602.
3. Farber J. M. and Peterkin P., 1991, Microbiol. Rev. 55: 476-511