Avian pathogenic Escherichia coli (APEC) and human extraintestinal pathogenic E. coli (ExPEC) cause various diseases in humans and animals and cannot be clearly distinguished by molecular epidemiology and genome content. We characterized traits of eight representative human ExPEC and APEC variants to either support the zoonotic potential or indicate factors involved in host specificity. These strains were very similar regarding phylogeny, virulence gene content and allelic variation of adhesins. Host- or serogroup-specific differences in type 1-, P-, S/F1C-fimbriae, curli, flagella, colicin and aerobactin expression or in vivo virulence were not found. Serogroup-dependent differences in genome content may depend on the phylogenetic background. To identify traits involved in host specificity, we performed transcriptome analysis of human ExPEC IHE3034 and APEC BEN374 in response to human (37 degrees C) or avian (41 degrees C) body temperature. Both isolates displayed similar transcriptional profiles at both temperatures. Transcript levels of motility/chemotaxis genes were repressed at 41 degrees C. The hdeAB and cadA genes involved in acid stress resistance, although often induced at 41 degrees C, could not be correlated with host specificity. Beside strain-specific effects, the common behavior of both strains at human or avian body temperature supports the idea of a potential zoonotic risk of certain human ExPEC and APEC variants.