Q.23 Gram-positive bacteria are generally resistant to complement-mediated lysis because
(A) thick peptidoglycan layer prevents insertion of membrane attack complex into the inner membrane
(B) Gram-positive bacteria import the membrane attack complex and inactivate it
(C) membrane attack complex is degraded by the proteases produced by the Gram-positive bacteria
(D) Gram-positive bacteria cannot activate the complement pathway
Gram-positive bacteria often evade the host’s immune defenses, particularly complement-mediated lysis, a key bacterial killing mechanism. This multiple-choice question (Q.23) tests understanding of bacterial cell wall structure and complement system interactions in microbiology and immunology.
Correct Answer: (A) thick peptidoglycan layer prevents insertion of membrane attack complex into the inner membrane
The complement system’s membrane attack complex (MAC), formed during the terminal pathway, creates pores in bacterial membranes to cause lysis. Gram-positive bacteria feature a thick peptidoglycan layer (20-80 nm) in their cell wall, acting as a physical barrier. This multilayered mesh blocks MAC from reaching and inserting into the cytoplasmic membrane, preventing cell lysis. Studies, like those in Nature Reviews Microbiology, highlight how this structural feature confers resistance, unlike Gram-negative bacteria with thin peptidoglycan and exposed outer membranes.
Detailed Explanation of All Options
Option (A): Thick peptidoglycan layer prevents insertion of membrane attack complex into the inner membrane
This is correct, as explained above. The peptidoglycan’s rigidity and thickness sterically hinder MAC assembly and penetration, a classic example of structural immunity in Gram-positives like Staphylococcus aureus.
Option (B): Gram-positive bacteria import the membrane attack complex and inactivate it
Incorrect. Gram-positive bacteria lack mechanisms to actively import MAC, a large transmembrane complex (~600 kDa). Import would require specific transporters, which aren’t documented. Instead, resistance is passive via the cell wall barrier.
Option (C): Membrane attack complex is degraded by the proteases produced by the Gram-positive bacteria
Incorrect. While some Gram-positives produce proteases (e.g., Streptococcus pyogenes), these target host proteins, not MAC directly. MAC degradation isn’t a primary resistance strategy; evidence from complement assays shows intact MAC on Gram-positive surfaces without lysis due to inaccessibility.
Option (D): Gram-positive bacteria cannot activate the complement pathway
Incorrect. Gram-positives can activate complement via classical, lectin, or alternative pathways through surface molecules like lipoteichoic acid. Activation occurs, but downstream lysis fails because MAC can’t breach the peptidoglycan shield.
Why This Matters in Biotechnology and Microbiology Research
Understanding gram-positive complement resistance informs antibiotic development, vaccine design, and phage therapy. For bioengineers modeling microbial growth kinetics or enzyme assays, this highlights cell wall roles in pathogenesis. SEO tip for scientific writers: Target phrases like “gram-positive bacteria complement resistance” to rank in academic searches on PubMed or Google Scholar.
