Use of a phospholipase-C assay, in vivo pathogenicity assays and PCR in assessing the virulence of Listeria spp.

Listeria spp. was isolated from 19.8% of animals with a history of reproductive disorders. A total of 333 faecal, genital swab and blood samples from 111 animals (cattle, buffaloes, sheep and goats) were subjected to PCR to detect virulence-associated genes (prfA, plcA, hlyA, actA and iap) and pathogenicity testing by the phosphatidylinositol phospholipase-C (PI-PLC) assay, and by mouse and chick embryo inoculation. One isolate of Listeria ivanovii recovered from a genital swab from a sheep was found to be pathogenic. Virulence assessment was then carried out on two L. ivanovii and 29 Listeria monocytogenes isolates from various sources using these assays. Haemolytic L. monocytogenes isolates lacking the plcA gene and PI-PLC activity were deemed non-pathogenic when assessed by mouse and chick embryo inoculation tests, in spite of having the hlyA gene. The results suggested that the PI-PLC and PCR assays are reliable in vitro alternatives to in vivo pathogenicity tests for L. monocytogenes.