High-risk clonal groups of Avian Pathogenic Escherichia coli (APEC) demonstrate heterogeneous phenotypic characteristics in vitro and in vivo

Avian Pathogenic Escherichia coli (APEC), a major bacterial pathogen of poultry, is comprised of a diverse range of high-risk clonal groups. However, phenotypic interactions with the avian host cell and how they may differ between lineages remains poorly understood. Therefore, the ability of predominant and outbreak-associated APEC clonal groups to invade and survive within avian host cells, as well as virulence within the Galleria mellonella infection model was investigated. The molecular characterisation of APEC isolated from an outbreak of colibacillosis in turkey poults in the UK, identified APEC sequence type (ST)-101 as the dominant clonal group, carrying a high number of virulence factors. As such, ST-101 was compared as an outbreak-associated lineage to a range of predominant APEC high-risk clonal groups (ST-23, ST-140, ST-95, ST-117). Utilising in vitro cell culture models, APEC isolates displayed comparable adhesion to 8E11 chicken epithelial gut and HD11 chicken macrophage cell lines. However, a trend of increased invasion of the 8E11 cells, and intracellular survival within HD11 macrophages was observed for ST-95, ST-101, and ST-140 APEC, relative to ST-23 and ST-117, suggestive of pronounced phenotypic differences between clonal groups. However, in HD11 cell assays, no difference in magnitude of elicited immune response was observed between lineages, indicating lineages had differing capacities to resist phagocyte killing. In vivo virulence in the Galleria mellonella infection model was also observed to differ between APEC genotypes, with ST-117 inducing the highest mortality, despite the comparatively lower epithelial invasion and intramacrophage survival to other lineages. Collectively, this suggests a distinct phenotypic profile associated with high-risk clonal groups within APEC, potentially allowing the future development of broad-spectrum disease management strategies.