Detection of Listeria monocytogenes in pigs and pork

In this study, we surveyed hogs (n = 300) as well as pork products (ground pork and raw chitterlings) for Listeria monocytogenes. Pig specimens collected before (tonsil swabs) and after slaughter (tonsils, lymph nodes, carcass swabs, and rectal contents) were examined for L. monocytogenes by enrichment with conventional enrichment broths followed by subculturing to selective agar. A multiplex polymerase chain reaction assay targeting the highly conserved 16S rRNA gene of the Listeria species as well as the hlyA gene unique to L. monocytogenes was used to screen aliquots of the enrichment (method I) as well as to confirm presumptive Listeria colonies from Columbia agar with 0.05% glucose supplemented with polymyxin B-acriflavine-lithium chloride-ceftazidime-aesculin-mannitol (PALCAM; method II). Subculturing to PALCAM agar was the more sensitive of the two methods on the basis of the overall detection of Listeria. For hog tissues, method I detected L. monocytogenes (0.87% positive) and no other Listeria spp. in all samples (n = 1,849). In contrast, method II detected significantly more (P < 0.05) L. monocytogenes (2.38%) and Listeria spp. (0.38%) in these tissues. For small intestines (n = 300 raw chitterlings), L. monocytogenes was identified in 8.3% of enrichments with University of Vermont modified Listeria enrichment broth; plating to PALCAM slightly improved recovery (9%). Overall, ground pork samples (n = 340) harbored L. monocytogenes (45% positive) and other Listeria species (1.5% positive), as determined by method I. Subculturing to PALCAM significantly (P < 0.05) improved the detection of L. monocytogenes (50.2%) but not that of other Listeria species (1.7%). L. monocytogenes isolates (n = 243) were assigned to serotype 1 (53.5%), serotype 4 (25%), and serotypes other than 1 and 4 (21.4%).