Staphylococcus aureus

Gram-positive cocci in grape-like CLUSTERS (staphylo = "bunch of grapes"). Non-motile, non-spore-forming, facultative anaerobe.

🔗 The king IS a cluster of grapes — Staph = clusters; Strep = chains.

Golden-yellow colonies (aureus = "gold"). The carotenoid staphyloxanthin is an antioxidant virulence factor that neutralizes the neutrophil oxidative burst and is antiphagocytic.

🔗 Gold crown + golden grapes = aureus / staphyloxanthin.

S. aureus asymptomatically COLONIZES the anterior nares (nose) in ~30% of people — the main reservoir and source of nosocomial spread and self-infection.

🔗 The throne is a giant nose — S. aureus lives up your nose.

GRAM-POSITIVE — the thick peptidoglycan wall retains crystal violet and stains deep purple.

🔗 Royal purple robe = Gram-positive (purple) stain.

COAGULASE-POSITIVE — activates prothrombin → fibrin clot coats the organism and shields it from phagocytosis. This single test separates S. aureus from all other staph.

🔗 He pours blood that CLOTS on the floor → coagulase +.

CATALASE-POSITIVE (bubbles with H₂O₂) — distinguishes Staphylococcus from Streptococcus. Catalase-positive organisms cause invasive infection in Chronic Granulomatous Disease (CGD).

🔗 Bubbling glass = catalase splits H₂O₂ into O₂ bubbles.

Protein A binds the Fc region of IgG, flipping the antibody backwards → blocks opsonization, complement fixation, and phagocytosis.

🔗 Shield knocking away Y-shaped antibodies = Protein A binds IgG Fc.

β-HEMOLYTIC — completely lyses RBCs, leaving a clear zone around colonies on blood agar.

🔗 Clear ring cleared in the red blood pool = beta-hemolysis.

Ferments MANNITOL → acid turns mannitol salt agar (and the phenol-red indicator) YELLOW. The high salt selects for staph.

🔗 Yellow patch in the pool = mannitol fermentation turns agar yellow.

Exfoliative toxins A/B cleave desmoglein-1 → Staphylococcal Scalded Skin Syndrome: diffuse sloughing, Nikolsky +, spares mucosa, in infants/young children.

🔗 Skin peeling off in sheets = exfoliative toxin / SSSS.

Most common cause of skin/soft-tissue infection: abscesses, boils/furuncles, folliculitis, cellulitis, and honey-crusted IMPETIGO.

🔗 Man covered in boils + honey crust = skin/soft-tissue infection.

TSST-1 SUPERANTIGEN cross-links MHC II to the TCR Vβ region outside the groove → polyclonal T-cell activation → cytokine storm → toxic shock: fever, diffuse rash, desquamation, hypotension. Classic with tampons/nasal packing.

🔗 Guard shocked while a goblin jams a tampon in = toxic shock (TSST-1).

Heat-stable PREFORMED enterotoxin → rapid food poisoning 1–6 h after eating, prominent VOMITING, short course. Classic foods: mayonnaise, custards, ham left out. Reheating does not help.

🔗 Force-fed mayo/ham then vomiting fast = preformed enterotoxin.

MSSA treatment: anti-staphylococcal penicillins — NAFCILLIN / oxacillin / dicloxacillin, or cefazolin (penicillinase-resistant).

🔗 Blue medic-knight syringing the plain goblins = nafcillin kills MSSA.

ACUTE bacterial endocarditis — rapidly destructive, large valve VEGETATIONS; #1 cause of acute infective endocarditis; can hit previously normal valves.

🔗 Heart with cauliflower growths = acute endocarditis vegetations.

In IV drug users, S. aureus seeds the RIGHT-sided TRICUSPID valve (#1 organism).

🔗 Needle stabbing the heart's right side = tricuspid endocarditis in IVDU.

Pneumonia — classically POST-INFLUENZA or nosocomial; patchy/multilobar infiltrates with abscess/cavitation and EMPYEMA; MRSA can be necrotizing (PVL).

🔗 Goblins storming flu-weakened lungs = post-viral S. aureus pneumonia.

#1 cause of acute SEPTIC ARTHRITIS in adults (after N. gonorrhoeae in young sexually active patients) — hot, swollen, painful joint.

🔗 Red swollen knee = septic arthritis.

#1 cause of OSTEOMYELITIS overall (hematogenous; metaphysis of long bones in kids, vertebral in adults/IVDU).

🔗 Goblin gnawing a cracked bone = osteomyelitis.

MRSA: the mecA gene (on SCCmec) encodes altered penicillin-binding protein PBP2a → resistant to ALL β-lactams except ceftaroline.

🔗 Armored boss the goblins can't hurt = methicillin-resistant S. aureus.

Altered PBP2a has low β-lactam affinity, so penicillins/cephalosporins can no longer bind the cell-wall transpeptidase → β-lactam resistance.

🔗 Warped unbreakable padlock = altered PBP2a, β-lactams can't bind.

MRSA treatment: VANCOMYCIN (or linezolid, daptomycin, ceftaroline). VISA/VRSA via vanA → D-Ala-D-Lac wall terminus lowers vancomycin binding.

🔗 A VAN crashing the armored boss = VANcomycin for MRSA.