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

•We present the first bovine conjunctiva explant model for infection studies.•Listeria monocytogenes was isolated from a healthy bovine eye.•2 L. monocytogenes isolates were markedly less able to invade conjunctiva explants.•Those isolates were also significantly more sensitive to lysozyme.

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.