Listeria monocytogenes is a food-borne pathogen of major veterinary importance. There has been a 3% increase in the number of bovine abortions caused by this bacterium over the last 17 years in the UK. With each abortion costing the dairy farmer around £630, Listeria infections have major welfare and economic consequences. L. monocytogenes has a particular tropism for the gravid uterus and, while the route of infection of the ruminant placentome is relatively unknown, invasion is thought to be mediated by the interaction of bacterial InlA and InlB with host E-cadherin and c-Met tyrosine kinase receptors, respectively.
We compared the ability of L. monocytogenes isolated from bovine abortions to infect cells of the feto-placental barrier compared to other clinical (conjunctivitis , meningitis ) and environmental isolates. Bovine caruncular epithelial cells (BCECs) were used to model the bovine reproductive tract. The intracellular viability of 14 L. monocytogenes isolates was assessed at 2 and 24 hours post-infection of BCECs. In addition, bacterial growth in Heart Infusion (HI) broth, InlA mRNA expression and multi locus sequence types (MLST)were determined.
Ten different sequence types were identified in the 14 isolates, nine with single isolates and five isolates with ST59, the latter all from clinical bovine cases (abortions, meningitis and keratoconjunctivitis). Four isolates showed significant attenuation at 24 hours post-infection, of which three also showed significantly slower growth rates in HI-broth and one had a reduced level of InlA expression, an essential virulence factor for cell invasion. However, none of the ST59 isolates were attenuated in their ability to infect and grow in BCEC cells. This suggests BCECs are a good bovine cell model to investigate invasive L monocytogenes isolates, especially ST59.