Determining the interactions between serum proteins of the complement system and outer membrane proteins in avian pathogenic Escherichia coli

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Eschericia coli is a well-known member of the intestinal flora of mammals and birds. However, there exist pathogenic strains capable of causing disease. One strain comes from the O-serogroup of E. coli, APEC O2 (+/+), and causes millions of dollars of global losses annually. The elucidation of the mechanisms of complement avoidance in pathogenic strains could potentially provide vital information to understanding bacterial pathogenicity and assist in the future development of a vaccine. The bacterial strains used were APEC O2, an iss+/bor+ strain, and DH5α, a iss-/bor+ strain. Mutant strains were created from a knockout of iss in APEC O2 (+/+) to create APEC O2∆iss (-/+), and a knockout of the gene bor to create DH5α∆bor (-/-). In order to determine how Iss and Bor assist each other in surface exclusion tactics and in serum resistance, each strain was subjected to a complement consumption assay, which measured the amount of complement consumed by each strain, the bactericidal assay, which determined the amount of cell death in response to complement, ELISA, which measured the amount of terminal complement complex remaining after exposure of bacteria to serum, and immunofluorescent microscope visualization, which visually showed the bound C5b-9 terminal complement complex to the outer membrane of the bacterial strains. The results of this study determined that the role the protein Bor has on assisting Iss with serum resistance may not be as significant as previously thought. There may also be something other than the gene bor assisting iss in the prevention of cell death due to the attachment of membrane attack complex (MAC) on the cell wall. This study expands on our previous understanding of how proteins of the outer bacterial cell membrane cooperate in order to provide resistance to complement proteins in the immune system.