Culture of Listeria monocytogenes on PALCAM agar (Polymyxin Acriflavin Lithium chloride Ceftazidime Aesculin Mannitol agar).
Cultivation 48 hours, aerobic atmosphere, 37°C.
PALCAM medium is highly selective due to the presence of lithium chloride, ceftazidime, polymyxin B and acriflavine hydrochloride.
It allows the easy differential diagnosis of L.monocytogenes by utilising the double indicator system:
- esculin and ferric ammonium citrate
- mannitol and phenol red
L. monocytogenes hydrolyzes esculin to form esculetin and dextrose. Esculetin reacts with ferric ammonium citrate and forms a brown-black complex seen as a black halo around colonies. Listeria monocytogenes does not ferment mannitol but contaminants such as Enterococci, Staphylococci and Micrococci ferment mannitol (colour change from red to yellow).