Terminal complement complexes with or without C9 potentiate antimicrobial activity against Neisseria gonorrhoeae

ABSTRACT The complement cascade is a front-line defense against pathogens. Complement activation generates the membrane attack complex (MAC), a 10–11 nm diameter pore formed by complement proteins C5b through C8 and polymerized C9. The MAC embeds within the outer membrane of Gram-negative bacteria and displays bactericidal activity. In the absence of C9, C5b-C8 complexes can form 2–4 nm pores on membranes, but their relevance to microbial control is poorly understood. Deficiencies in terminal complement components uniquely predispose individuals to infections by pathogenic Neisseria, including N. gonorrhoeae (Gc). Increasing antibiotic resistance in Gc makes new therapeutic strategies a priority. Here, we demonstrate that MAC formed by complement activity in human serum disrupts the Gc outer and inner membranes, potentiating the activity of antimicrobials against Gc and re-sensitizing multidrug-resistant Gc to antibiotics. C9-depleted serum also exerts bactericidal activity against Gc and, unlike other Gram-negative bacteria, disrupts both the outer and inner membranes. C5b-C8 complex formation potentiates Gc sensitivity to azithromycin and ceftriaxone, but not lysozyme or nisin. These findings expand our mechanistic understanding of complement lytic activity, suggest a size limitation for terminal complement-mediated enhancement of antimicrobials against Gc, and suggest that complement manipulation can be used to combat drug-resistant gonorrhea.IMPORTANCEThe complement cascade is a front-line arm of the innate immune system against pathogens. Complement activation results in membrane attack complex (MAC) pores forming on the outer membrane of Gram-negative bacteria, resulting in bacterial death. Individuals who cannot generate MAC are specifically susceptible to infection by pathogenic Neisseria species including N. gonorrhoeae (Gc). High rates of gonorrhea, its complications like infertility, and high-frequency resistance to multiple antibiotics make it important to identify new approaches to combat Gc. Beyond direct anti-Gc activity, we found that the MAC increases the ability of antibiotics and antimicrobial proteins to kill Gc and re-sensitizes multidrug-resistant bacteria to antibiotics. The most terminal component, C9, is needed to potentiate the anti-Gc activity of lysozyme and nisin, but azithromycin and ceftriaxone activity is potentiated regardless of C9. These findings highlight the unique effects of MAC on Gc and suggest novel translational avenues to combat drug-resistant gonorrhea.