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Summary of dissertation

Enterococcus cecorum is a bacteria that normally lives in the gut of chickens without causing problems. In recent decades, however, it has increasingly been linked to serious illnesses in broiler chickens. These infections can lead to painful conditions like inflammation of the bones in the legs and/or vertebral column and heart sac infection. As a result, affected birds often become lame or even paralyzed, which harms their health and welfare, and subsequently causes major financial losses in poultry farming.

This dissertation set out to better understand how E. cecorum causes disease and what makes some strains more harmful than others. In particular, it looked at how the bacteria enter the body, whether other infections make the problem worse, and how disease-causing strains differ from harmless ones found in healthy birds.

Experiments in young broilers showed that the bacteria can cause typical lesions whether they are applied via aerosol or given orally. Adding other common chicken viruses and bacteria did not increase disease rates, though certain combinations—especially when chicken anaemia virus or Mycoplasma synoviae were present along with respiratory viruses—did make it easier to recover the bacteria from the birds.

Another part of the research focused on testing how harmful different strains are using chicken embryos. The study found that injecting the bacteria into the egg white (albumen) gave the clearest distinction between harmful and harmless strains. Interestingly, the harmful strains were more resistant to the egg’s natural defenses, particularly lysozyme, an enzyme that kills bacteria. This suggests that testing for lysozyme resistance could be a good way to identify the more harmful strains.

Before the bacteria can cause lesions in the bones or heart sac, it first has to move from the gut into the bloodstream. In the final study, the ability of different strains to cross gut cells was tested. Using miniature “gut models” grown in the lab (organoids), it was shown that harmful strains invaded more effectively than harmless ones, especially at higher doses. This model now provides a useful tool for studying how the bacteria interact with the chicken’s immune system and for testing future prevention strategies.

In summary, this research shows that E. cecorum infections can start either via the respiratory tract or oral uptake of the bacterium, and that harmful strains have special traits that help them survive, spread, and cause disease. The new testing methods developed—embryo lethality assays, lysozyme resistance assay, and organoid studies—provide valuable ways to study this pathogen further. Overall, these findings improve our understanding of E. cecorum as an emerging poultry disease and open the door for better prevention and diagnosis in the future.

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