Pyogenic Infections

Pyogenic (pus-forming) bacteria cause infections characterised by suppuration: the accumulation of pus composed of dead and dying bacteria, neutrophils, and fibrin-rich inflammatory exudate. Abscesses represent localised collections of pus within a fibrin-lined cavity. The two most clinically important pyogenic gram-positive cocci are Staphylococcus aureus and Streptococcus pyogenes. Both are facultative anaerobes, both can cause bloodstream infection (sepsis), and both share a range of virulence factors, though they differ significantly in their typical infection patterns and antibiotic susceptibilities.

Staphylococcus aureus tends to cause localised infections such as abscesses. It produces coagulase — a key virulence factor that converts fibrinogen to fibrin, promoting clumping and immune evasion — and Panton-Valentine leukocidin (PVL), a toxin that forms pores in neutrophil membranes, killing these cells and releasing inflammatory contents that cause further tissue damage. S. aureus is catalase-positive and appears as clustered cocci (staphylae) on gram stain.

Streptococcus pyogenes causes spreading infections. Its principal virulence factor is the M protein, of which over 200 antigenic types exist. The M protein carries a negative charge that electrostatically repels phagocyte membranes, inhibits C3b deposition (preventing opsonisation), and acts as an adhesin by binding fibronectin. Antibodies against M protein can cross-react with cardiac tissue, causing rheumatic fever after S. pyogenes pharyngitis, which may progress to rheumatic heart disease with repeated episodes. S. pyogenes is catalase-negative and appears as chains (streptae) on gram stain.

Skin infections caused by both organisms include impetigo (non-bullous caused by either; bullous caused by S. aureus via epidermolytic toxin), folliculitis, furuncles, carbuncles, cellulitis, erysipelas (a superficial cellulitis with lymphatic involvement characteristic of S. pyogenes), and necrotising fasciitis (a rapidly spreading, high-mortality infection along the fascial plane). Treatment requires flucloxacillin for S. aureus (methicillin-stable penicillin) and penicillin for S. pyogenes; vancomycin is reserved for MRSA.