Factor H Is Bound by Outer Membrane-Displayed Carbohydrate Metabolism Enzymes of Extraintestinal Pathogenic Escherichia coli and Contributes to Opsonophagocytosis Resistance in Bacteria

The GE296_RS03820 and GE296_RS03830 genes are involved in capsular polysaccharide biosynthesis in Riemerella anatipestifer

Riemerella anatipestifer is a pathogenic bacterium that causes duck serositis and meningitis, leading to significant harm to the duck industry. To escape from the host immune system, the meningitis-causing bacteria must survive and multiply in the bloodstream, relying on specific virulence factors such as capsules. Therefore, it is essential to study the genes involved in capsule biosynthesis in R. anatipestifer. In this study, we successfully constructed gene deletion mutants Δ3820 and Δ3830, targeting the GE296_RS03820 and GE296_RS03830 genes, respectively, using the RA-LZ01 strain as the parental strain. The growth kinetics analysis revealed that these two genes contribute to bacterial growth. Transmission and scanning electron microscopy (TEM and SEM) and silver staining showed that Δ3820 and Δ3830 produced the altered capsules and compounds of capsular polysaccharides (CPSs). Serum resistance test showed the mutants also exhibited reduced C3b deposition and decreased resistance serum killing. In vivo, Δ3820 and Δ3830 exhibited markedly declining capacity to cross the blood-brain barrier, compared to RA-LZ01. These findings indicate that the GE296_RS03820 and GE296_RS03830 genes are involved in CPSs biosynthesis and play a key role in the pathogenicity of R. anatipestifer. Furthermore, Δ3820 and Δ3830 mutants presented a tendency toward higher survival rates from RA-LZ01 challenge in vivo. Additionally, sera from ducklings immunized with the mutants showed cross-immunoreactivity with different serotypes of R. anatipestifer, including 1, 2, 7 and 10. Western blot and SDS-PAGE assays revealed that the altered CPSs of Δ3820 and Δ3830 resulted in the exposure of some conserved proteins playing the key role in the cross-immunoreactivity. Our study clearly demonstrated that the GE296_RS03820 and GE296_RS03830 genes are involved in CPS biosynthesis in R. anatipestifer and the capsule is a target for attenuation in vaccine development.

Extraintestinal Pathogenic Escherichia coli Utilizes Surface-Located Elongation Factor G to Acquire Iron from Holo-Transferrin

Extraintestinal pathogenic Escherichia coli (ExPEC) can cause systemic infections in both humans and animals. As an essential nutrient, iron is strictly sequestered by the host. Circumventing iron sequestration is a determinant factor for ExPEC infection. However, the ExPEC iron acquisition mechanism, particularly the mechanism of transferrin (TF) acquisition, remains unclear. This study reports that iron-saturated holo-TF can be utilized by ExPEC to promote its growth in culture medium and survival in macrophages. ExPEC specifically bound to holo-TF instead of iron-free apo-TF via the surface located elongation factor G (EFG) in both culture medium and macrophages. As a moonlighting protein, EFG specifically bound holo-TF and also released iron in TF. These two functions were performed by different domains of EFG, in which the N-terminal domains were responsible for holo-TF binding and the C-terminal domains were responsible for iron release. The functions of EFG and its domains have also been further confirmed by surface-display vectors. The surface overexpression of EFG bound significantly more holo-TF in macrophages and significantly improved bacterial intracellular survival ability. Our findings reveal a novel iron acquisition mechanism involving EFG, which suggests novel research avenues into the molecular mechanism of ExPEC resistance to nutritional immunity. IMPORTANCE Extraintestinal pathogenic Escherichia coli (ExPEC) is an important pathogen causing systemic infections in humans and animals. The competition for iron between ExPEC and the host is a determinant for ExPEC to establish a successful infection. Here, we sought to elucidate the role of transferrin (TF) in the interaction between ExPEC and the host. Our results revealed that holo-TF could be utilized by ExPEC to enhance its growth in culture medium and survival in macrophages. Furthermore, the role of elongation factor G (EFG), a novel holo-TF-binding and TF-iron release protein, was confirmed in this study. Our work provides insights into the iron acquisition mechanism of ExPEC, deepens understanding of the interaction between holo-TF and pathogens, and broadens further researches into the molecular mechanism of ExPEC pathogenicity.