Proliferative enteropathy (PE) is an infectious disease caused by an obligate
intracellular bacterium, Lawsonia intracellularis, and characterized by thickening of the intestinal epithelium due to enterocyte proliferation. The overall goals of this thesis were to improve the understanding of the pathogenesis of PE by evaluating phenotypic traits, genome variations and transcriptome patterns of L. intracellularis infection and to evaluate the adaptation of the bacterium to porcine and equine hosts. In the first section, the susceptibility of pigs to a homologous porcine L. intracellularis isolate continuously grown in vitro was assessed. A loss of virulence after 40 passages of the bacteria in culture was established. The comparative whole genome analysis of the pathogenic (low passage) and non-pathogenic (high passage) isolates identified the loss of a prophage-associated genomic island in the non-pathogenic variant. This chromosomal island proved not to be essential for the virulent phenotype, since it was not identified in horses clinically affected with PE. However, this genetic element
was associated with host-adapted L. intracellularis variants. While pathogenic porcine isolates harbor this genetic element, it was absent in equine isolates and PE-affected horses. Gene expression profiling of a porcine pathogenic isolate showed a wider transcriptional landscape compared with the non-pathogenic variant. In addition, genes highly activated in vitro by the pathogenic variant also were significantly expressed in vivo. However, genes identified in the genomic island were not expressed by intracellular bacteria either in vitro or in vivo. The proliferative changes exhibited by the infected enterocytes in vivo were associated with deregulation of the G1 phase of the host cell cycle and repression of membrane transporters related to nutrient acquisition,
characterizing a malabsorptive syndrome as the major mechanism involved in the poor
performance and growth of affected animals.
Finally, an alternative method for cultivation of L. intracellularis was developed in order to perform a cross-species experimental study evaluating the susceptibility of pigs and horses to a porcine and an equine L. intracellularis isolate. Evident clinical signs, longer periods of bacterial shedding and stronger serologic immune responses were
observed in animals infected with species-specific isolates indicating that host
susceptibility is driven by the origin of the L. intracellularis strain.