Mode of Action
PALCAM Agar provides a quantitative cultivation of Listeria monocytogenes, while, at the same time, inhibiting the Gramnegative and most of the Gram-positive accompanying bacteria. The selectivity of the medium results form its content of polymyxin, acriflavin, ceftazidime and lithium chloride. L.monocytogenes breaks down the esculin in the medium to glucose and esculetin. Esculetin forms an olive-green to black complex with iron(III) ions which stains the colonies of L.monocytogenes. Mannitol-positive accompanying bacteria
such as staphylococci grow as yellow colonies, if they are not inhibited.
According to Hammer et al. (1990) PALCAM Agar is superior, with respect to selectivity, compared to other listeria media.
Typical Composition (g/litre)
Peptone 23.0; yeast extract 3.0; starch 1.0; sodium chloride 5.0; agar-agar 13.0 (= Columbia Agar); D(-)mannitol 10.0; ammonium iron(III) citrate 0.5; esculin 0.8; glucose 0.5; lithium chloride 15.0; phenol red 0.08.
Suspend 35.9 g in 500 ml of demin. water, autoclave 15 min at 121 °C). Dissolve the contents of 1 vial of PALCAM Listeria Selective Supplement acc. to VAN NETTEN et al. in 1 ml sterile distilled water and add to the sterile medium cooled to 50 °C. If necessary rinse the vial with 1 ml of sterile distilled water. Mix well and pour plates.
pH: 7.2 ± 0.2 at 25 °C.
The prepared plates (incl. supplement) are clear and dark-red.
Experimental Procedure and Evaluation
Inoculate by spreading the sample on the surface of the medium and incubate at 35 °C for up to 48 hours preferably under microaerophilic conditions (using Anaerocult® C or Anaerocult® C mini). L. monocytogenes grows as grey-green coloured colony with a black zone. If the colonies are very close together the whole medium is coloured black-brown. PALCAM Listeria Selective Agar is highly selective. If, however, mannitol-positive enterococci or staphylococci do grow, they appear yellow with a yellow zone. Further biochemical tests should be carried out. Suspicious colonies should be confirmed with biochemical or serological