Characterization of bacterial factors that promote resistance to complement-mediated opsonophagocytic killing and lysis

The complement system is critical for controlling diverse microbial infections. In this study, we have performed genomic screens in Streptococcus pneumoniae (the pneumococcus) and nontypable Haemophilus influenzae (NTHi) to identify novel genetic determinants involved in resistance to complement-mediated opsonophagocytic killing and lysis respectively. In the pneumococcus, we have further characterized how (1) surface-exposed exoglycosidases and (2) minimization of bacterial chain length promote resistance to opsonophagocytic killing via a complement evasion strategy. We demonstrate that the pneumococcal exoglycosidases, NanA, BgaA, and StrH, likely promote this resistance by deglycosylation of an undefined serum glycoprotein. Due to the stochastic nature of complement deposition, we show that shorter chains of the pneumococcus are less likely to have complement bound and have the capacity to bind less complement. Consistent with this, both the (1) surface-exposed exoglycosidases and (2) minimization of chain length promote pneumococcal virulence in a model of systemic infection in a complement-dependent manner. We also address how the host may subvert minimization of bacterial size as a virulence strategy via the agglutinating effect of antibody. We demonstrate that antibody can promote Fc-independent but complement-dependent opsonophagocytosis of the pneumococcus and lysis of Haemophilus influenzae by increasing the size of bacterial targets (i.e. via agglutination). Thus, increasing pathogen size, whether by natural changes in cellular morphology, or via the agglutinating affect of antibody, promotes complement-dependent clearance of these otherwise distinct pathogenic microbes. In NTHi, we have identified how expression of vacJ and yrbBDE promotes resistance to serum killing by excluding phospholipids from the outer leaflet of the outer membrane. This allows lipooligosaccharide (LOS) to pack tightly on the surface of the bacterial cell, which promotes resistance to serum killing in NTHi by masking LOS epitopes that can be bound by complement-activating natural antibodies found in human serum. This study highlights how basic changes in bacterial physiology (e.g. bacterial size or surface structure) can promote pathogenesis. As complement is important for controlling diverse microbial infections, the complement evasion strategies described in this study may be common among pathogenic microbes. Also, this study provides insight into a novel mechanism of antibody function