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Abstract
Bordetella species contain highly contagious respiratory pathogens, including the human pathogen B. pertussis that causes whooping cough and B. bronchiseptica that causes respiratory infections in other animals. The current protein-based acellular pertussis vaccine conferred protection against severe infections but failed to prevent nasal colonization and subsequential transmission, leading to rising cases of pertussis in the vaccinated population. In this study, we discovered a novel oligosaccharide, bordetellae Colonization oligosaccharide (b-Cool), conserved in all Bordetella species. We characterized the unique structure of b-Cool by LC-MS and NMR and revealed its biological function as the first Bordetella glycan factor crucial for early nasal colonization. The b-Cool locus deletion mutant, Δb-Cool, showed an early nasal colonization defect compared to the wild type during in vivo mouse infection. To further study the role of b-Cool in colonization, we established mouse primary nasal epithelia (MNE) and mouse tracheobronchial epithelia (MTE) culture in the air-liquid interface (ALI). MNE and MTE infection studies revealed that b-Cool was crucial for nasal colonization but not lower airway colonization. Further studies identified that MNE secreted significantly more mucus than MTE. The addition of mucus to MTE during infection led to a colonization defect of Δb-Cool similar to what was observed in MNE, indicating that b-Cool possibly serves as a defense mechanism against host airway mucus.
We also revealed the complex interrelated UDP-sugar biosynthetic pathways of three glycans, b-Cool, transmission-related exopolysaccharide (tEPS), and lipopolysaccharide (LPS) O-antigen. Utilizing various mouse models, we dissected the contributions of the three complex glycans in transmission, infection, and virulence. All three glycans were found to be independently contributing to transmission and virulence. B-Cool was found to be the major factor mediating colonization and had the biggest contribution to transmission. O-antigen was found to be a major virulence factor and could induce an early IgM response during infection. Interestingly, B. bronchiseptica mutants deficient in O-antigen production acquired a transient advantage in infection due to the lack of anti-O-antigen immune response, providing a possible explanation for the loss of O-antigen in B. pertussis. Our results highlight the significant roles of Bordetella glycans in host-pathogen interactions.