High-risk clonal groups of avian pathogenic <i>Escherichia coli</i> (APEC) demonstrate heterogeneous phenotypic characteristics <i>in vitro</i> and <i>in vivo</i>

Avian Pathogenic <i>Escherichia coli</i> (APEC), a major bacterial pathogen of poultry, comprises a diverse range of high-risk clonal groups. However how these lineages interact with avian host cells remains poorly characterized. This study examined the ability of key APEC clonal groups to adhere to, invade, and survive within avian host cells, alongside assessing their virulence in the <i>Galleria mellonella</i> infection model. Genomic analysis of APEC from a UK turkey colibacillosis outbreak identified ST-101 as the dominant clonal group, carrying numerous virulence factors. ST-101 was compared to other high-risk APEC clonal groups (ST-23, ST-140, ST-95, ST-117). Utilizing <i>in vitro</i> cell culture models, APEC isolates displayed comparable adhesion to 8E11 chicken epithelial gut and HD11 chicken macrophage cell lines. APEC ST-95, ST-101, and ST-140 demonstrated increased invasion of 8E11 cells, and intracellular survival within HD11 macrophages, relative to ST-23 and ST-117, suggesting pronounced phenotypic differences between clonal groups. However, in HD11 cell assays, no difference in magnitude of elicited immune response was observed between lineages, indicating differing intracellular survival was not a result of immune response modulation. <i>In vivo</i> virulence in the <i>Galleria mellonella</i> 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 for other lineages. Collectively, this suggests that diverse APEC genotypes have distinct phenotypic profiles <i>in vitro</i> and <i>in vivo</i>. These results highlight the need for intervention strategies that can simultaneously target a broad range of pathogenic lineages.