This monograph has been assessed by members of the IUMS–ICFMH Working Party on Culture Media and given ‘Approved’ status.

PALCAM, developed by van Netten et al. (1989), is a selective differential medium for the isolation and direct counting of Listeria spp. from faecal and biological specimens as well as food and heavily contaminated environmental samples. It is an improved modification of RAPAMY (van Netten et al., 1988a), ALPAMY (van Netten et al., 1988b) and Oxford (Curtis et al., 1989) media. Selectivity is achieved by the use of microaerobic incubation and the inhibitory components lithium chloride, acriflavine, polymyxin and ceftazidime. Its diagnostic properties are based on the use of two indicator systems: (i) aesculin and ferric iron and (ii) D-mannitol combined with phenol red. The selective system aims to suppress virtually all unwanted organisms, including most Enterococcus spp. The double diagnostic system allows easy distinction of Listeria spp. from enterococci and staphylococci. Listeria spp. are aesculin positive but mannitol negative, producing green colonies with black haloes against a pink-purple background. When direct counting is applied to foods processed for safety, solid medium repair has to precede the use of PALCAM.