A diabetic patient has a deep wound infection of the right leg with rapidly spreading necrosis, crepitus on palpation, and 'dishwater' foul-smelling exudate. X-ray shows gas in soft tissues. Gram stain of exudate shows large gram-positive bacilli with blunt ends (boxcar appearance), few inflammatory cells, and no spores seen on routine staining. What is the virulence factor primarily responsible for local tissue destruction?
- A Hyaluronidase (spreading factor) of Streptococcus pyogenes
- B Coagulase of Staphylococcus aureus promoting abscess formation
- C Collagenase of Bacteroides fragilis degrading connective tissue
- D Alpha toxin (phospholipase C / lecithinase) of Clostridium perfringens — cleaves phosphatidylcholine in cell membranes causing haemolysis, platelet destruction, and myonecrosis ✓
Explanation
Gas gangrene (clostridial myonecrosis) is most commonly caused by Clostridium perfringens (Type A). The major virulence factor is alpha toxin (lecithinase, phospholipase C), which hydrolyses phosphatidylcholine and sphingomyelin in eukaryotic cell membranes, causing membrane disruption, haemolysis, platelet aggregation, capillary thrombosis, and rapid myonecrosis. C. perfringens spores are not seen on routine Gram stain (subterminal spores, rarely sporulate in tissue). The 'boxcar' Gram-positive rods in tissue with absent PMNs (toxin kills WBCs) is pathognomonic of clostridial infection.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.