Comparison of PALCAM and modified Oxford plating media for isolation of Listeria species in poultry meat following UVM II or Fraser secondary enrichment broths

All combinations of two secondary selective enrichment broths, UVM II (University of Vermont II) and Fraser, and two selective plating media, PALCAM (polymixin B, acriflavin, lithium chloride, ceftazidime, aesculin, D -mannitol) and MOX (modified Oxford) were compared for isolating Listeria spp. from retail raw chicken meat. Percentages of samples with Listeria spp. were significantly (P<0·001) higher with PALCAM than with MOX medium (95% and 87%, respectively) and in both cases were independent of the enrichment broth used. Higher percentages of contaminated samples with Listeria monocytogenes were also obtained with PALCAM medium (31%) than with MOX (27%). With both solid media or both broths, or the four media simultaneously, the percentage of carcasses with detectedL. monocytogenes increased to 32%. The percentage of isolated L. innocua, L. monocytogenes andL. welshimeri strains were independent of the culture media used. Nevertheless, L. ivanovii was mainly isolated from PALCAM and L. grayi from MOX. The PALCAM medium proved to be more selective (P<0·001) than MOX, and fewer different Listeria spp. micro-organisms grew when the secondary enrichment broth was Fraser, both in PALCAM and MOX media. Significant differences (P<0·001) were found in the differential efficacy of PALCAM and MOX agars. The double differential system of the PALCAM medium (esculin and mannitol) resulted in the isolation of only 1·04% (2/192) false positives (all Bacillus spp.) as opposed to 11·46% (22/192) from the MOX medium (mainly Enterococcus spp.). The enrichment broth used did not have any influence on the number of false positives found.