The membrane attack complex (C5b-9) forms pores to lyse microbes. Which organism is particularly susceptible to MAC-mediated lysis?

The membrane attack complex (C5b-9) forms pores to lyse microbes. Which organism is particularly susceptible to MAC-mediated lysis?

• A. Staphylococcus aureus
• B. Streptococcus pneumoniae
• C. Neisseria
• D. Escherichia coli

Answer: (C)

The key idea is that the membrane attack complex makes pores in the cell membrane to lyse microbes, and this mechanism is especially effective against Neisseria species. Neisseria are thin-walled Gram-negative diplococci, so their membranes are particularly susceptible to pore formation by C5b-9, leading to rapid lysis. This is clinically important because people with deficiencies in MAC components (C5–C9) are especially prone to disseminated Neisseria infections. In contrast, Gram-positive organisms like Staphylococcus aureus and Streptococcus pneumoniae have thick peptidoglycan layers and other barriers that resist MAC, and while some Gram-negatives can be affected, Neisseria are the classic susceptible group.

The key idea is that the membrane attack complex makes pores in the cell membrane to lyse microbes, and this mechanism is especially effective against Neisseria species. Neisseria are thin-walled Gram-negative diplococci, so their membranes are particularly susceptible to pore formation by C5b-9, leading to rapid lysis. This is clinically important because people with deficiencies in MAC components (C5–C9) are especially prone to disseminated Neisseria infections. In contrast, Gram-positive organisms like Staphylococcus aureus and Streptococcus pneumoniae have thick peptidoglycan layers and other barriers that resist MAC, and while some Gram-negatives can be affected, Neisseria are the classic susceptible group.