Neisseria meningitidis (Gram-negative cocci) life threatening cause of 
sepsis and bacterial meningitis.
Classically, individuals who lack terminal complement proteins (C5-C9) cannot form the membrane attack complex (MAC), which is needed for bacterial lysis of this encapsulated pathogen (can lead to a 1000- to 10,000-fold increased risk of meningococcal disease). More information here.
REFERENCES
- Figueroa, J.E., and P. Densen. 1991. Infectious diseases associated with complement deficiencies. Clin. Microbiol. Rev. 4:359–395.
- Infectious diseases associated with complement deficiencies. Figueroa JE, Densen P. Clin Microbiol Rev. 1991 Jul; 4(3):359-95.
- Rosa D.D., Pasqualotto A.C., de Quadros M., Prezzi S.H. Deficiency of the eighth component of complement associated with recurrent meningococcal meningitis—case report and literature review. Braz J Infect Dis. 2004;8(August (4)):328–330.