Role of avian pathogenic Escherichia coli virulence factors in bacterial interaction with chicken heterophils and macrophages.

Avian pathogenic Escherichia coli (APEC) cause extraintestinal disease in avian species via respiratory tract infection. Virulence factors associated with APEC include type 1 and P fimbriae, curli, aerobactin, lipopolysaccharide (LPS), K1 capsular antigen, temperature-sensitive hemagglutinin (Tsh), and an uncharacterized pathogen-specific chromosomal region (the 0-min region). The role of these virulence factors in bacterial interaction with phagocytes was investigated by using mutants of three APEC strains, each belonging to one of the most predominant serogroups O1, O2, and O78. Bacterial cell interaction with avian phagocytes was tested with primary cultures of chicken heterophils and macrophages. The presence of type 1 fimbriae and, in contrast, the absence of P fimbriae, K1 capsule, O78 antigen, and the 0-min region promoted bacterial association with chicken heterophils and macrophages. The presence of type 1 and P fimbriae, O78 antigen, and the 0-min region seemed to protect bacteria against the bactericidal effect of phagocytes, especially heterophils. The tested virulence factors seemed to have a limited role in intracellular survival for up to 48 h in macrophages. Generally, opsonized and nonopsonized bacteria were eliminated to the same extent, but in some cases, unopsonized bacteria were eliminated to a greater extent than opsonized bacteria. These results confirm the important role of type 1 fimbriae in promotion of initial phagocytosis, but nevertheless indicate a role for type 1 fimbriae in the protection of bacteria from subsequent killing, at least in heterophils. The results also indicate a role for K1 capsule, O78 antigen, P fimbriae, and the 0-min region in initial avoidance of phagocytosis, but demonstrate an additional role for O78 antigen, P fimbriae, and the 0-min region in subsequent protection against the bactericidal effects of phagocytes after bacterial association has occurred.