ABSTRACT Colibacillosis, caused by avian pathogenic Escherichia coli (APEC), is a disease of poultry that leads to high morbidity and mortality. Currently, 188 O groups are linked with bacterial antigenicity and pathogenicity in E. coli . Among APEC, serogroups O1, O2, and O78 are most often implicated in disease. However, our APEC collection from avians diagnosed with colibacillosis in Georgia identified several novel serogroups. This study evaluated the pathogenicity of 21 selected APEC strains representing 11 different serogroups and 14 different serotypes. These were assessed using an in vitro assay (the serum resistance assay), in vivo assays (three animal models: the embryo lethality assay ELA, chick challenge assay, and 3-week-old chicken challenge assay), and a yolk sac (YS) histopathological analysis. We hypothesized that some selected strains would exhibit higher virulence. Our study showed that 20 strains were able to resist chicken serum and proved to be virulent in embryonic eggs, while 17 APEC strains were classified as virulent in 1-day-old chicks. Interestingly, one APEC strain, GPEC 709 (O91:H7), proved to be very virulent in older birds. The YS structure was destroyed by APEC strains compared with the negative control group, although no significant differences among the other APEC strains were observed. Based on the assays used in this study, no single strain was consistently virulent across all assays, demonstrating that different APEC strains cause varying degrees of disease complications across the models tested. IMPORTANCE The pathogenicity of avian pathogenic Escherichia coli (APEC) in chickens has been ascribed primarily to serogroups O1, O2, and O78, with no previous studies exploring the virulence of newer, novel APEC strains that have been linked with recent poultry disease. Current commercial APEC vaccines only target serogroups O1 and O78, and cross-protection remains limited. Therefore, understanding how these novel APEC strains interact with the poultry host is critical. Moreover, better animal challenge model studies are needed to develop new preventive plans against these novel APEC serogroups. Our results show that while these novel APEC strains cause high mortality in embryonic chicks, one APEC strain caused high mortality in adult birds and was proven to be a primary pathogen. Also, these strains caused yolk sac destruction, the critical organ for embryo development, potentially leading to poor performance for the poultry production system. This study has used intensive analysis and holistic assays to elucidate these novel APEC strains’ virulence, thereby creating a better understanding of disease development.
Runcharoon et al. (Tue,) studied this question.