Characterisation of Listeria monocytogenes isolates from cattle using a bovine caruncular epithelial cell model.

Listeria monocytogenes is an important foodborne pathogen in human and veterinary health, causing significant morbidity and mortality including abortion. It has a particular tropism for the gravid uterus, however, the route of infection in reproductive tissues of ruminants (i.e. placentome), is much less clear. In this study, we aimed to investigate a bovine caruncular epithelial cell (BCEC) line as a model for L. monocytogenes infection of the bovine reproductive tract. The BCEC infection model was used to assess the ability of 14 different L. monocytogenes isolates to infect these cells. Lysozyme sensitivity and bacterial survival in 580 µg lysozyme/ml correlated with attenuated ability to proliferate in BCEC (p = 0.004 and p = 0.02, respectively). Four isolates were significantly attenuated compared to the control strain 10403S. One of these strains (AR008) showed evidence of compromised cell wall leading to increased sensitivity to ß-lactam antibiotics, and another (7644) had compromised cell membrane integrity leading to increased sensitivity to cationic peptides. Whole genome sequencing followed by Multi Locus Sequence Type analysis identified that five invasive isolates had the same sequence type, ST59, despite originating from three different clinical conditions. Virulence gene analysis showed that the attenuated isolate LM4 was lacking two virulence genes (uhpT, virR) known to be involved in intracellular growth and virulence. In conclusion, the BCEC model was able to differentiate between the infective potential of different isolates. Moreover, resistance to lysozyme correlated with the ability to invade and replicate within BCEC, suggesting co-selection for surviving challenging environments as the abomasum.

A new bovine conjunctiva model shows that Listeria monocytogenes invasion is associated with lysozyme resistance.

Listerial keratoconjunctivitis ('silage eye') is a wide spread problem in ruminants causing economic losses to farmers and impacts negatively on animal welfare. It results from direct entry of Listeria monocytogenes into the eye, often following consumption of contaminated silage. An isolation protocol for bovine conjunctival swabbing was developed and used to sample both infected and healthy eyes bovine eyes (n=46). L. monocytogenes was only isolated from one healthy eye sample, and suggests that this organism can be present without causing disease. To initiate a study of this disease, an infection model was developed using isolated conjunctiva explants obtained from cattle eyes post slaughter. Conjunctiva were cultured and infected for 20 h with a range of L. monocytogenes isolates (n=11), including the healthy bovine eye isolate and also strains isolated from other bovine sources, such as milk or clinical infections. Two L. monocytogenes isolates (one from a healthy eye and one from a cattle abortion) were markedly less able to invade conjunctiva explants, but one of those was able to efficiently infect Caco2 cells indicating that it was fully virulent. These two isolates were also significantly more sensitive to lysozyme compared to most other isolates tested, suggesting that lysozyme resistance is an important factor when infecting bovine conjunctiva. In conclusion, we present the first bovine conjunctiva explant model for infection studies and demonstrate that clinical L. monocytogenes isolates from cases of bovine keratoconjunctivitis are able to infect these tissues.