---

Staphylococcus aureus - Harrison's 22E (2025) - NEET SS MCQ Notes

---

1. MICROBIOLOGY & TAXONOMY (Harrison Ch. 152)

• S. aureus = coagulase positive among staphylococci
• Catalase differentiates Staph (positive) from Strep (negative)
• "Non-S. aureus staphylococci (NSaS)" is preferred over "coagulase-negative staphylococci" in modern taxonomy

---

2. EPIDEMIOLOGY

• 20-40% of healthy individuals are persistent nasal carriers; up to 50-80% are intermittent carriers
• Anterior nares = principal reservoir
• Carrier rate is higher in: healthcare workers, IV drug users, patients with eczema or insulin-dependent diabetes
• Spread: mainly direct contact and droplets; airborne less common
• MRSA is the most common healthcare-associated infection in neonatal ICUs

---

3. PATHOGENESIS - Virulence Factors

Surface Proteins (Colonization & Invasion)

Toxins

Immune Evasion

• Protein A binds IgG Fc → blocks opsonophagocytosis
• Capsule antiphagocytic
• Catalase: breaks down H₂O₂ (protects from oxidative killing)
• Coagulase: fibrin barrier around bacteria
• Biofilm formation: critical in prosthetic device infections

---

4. CLINICAL SYNDROMES

A. Skin and Soft Tissue Infections (SSTIs)

• Most common manifestation
• Folliculitis - superficial; furunculosis (boils) - deep; carbuncles - interconnected furuncles
• Impetigo - primarily GAS, but S. aureus also causes (especially bullous impetigo via exfoliative toxin)
• Cellulitis, mastitis, myositis, pyomyositis
• CA-MRSA most common cause of community-acquired SSTIs

B. Food Poisoning

• One of the most common foodborne outbreaks in the USA
• Cause: heat-stable enterotoxins (preformed in food)
• Onset: 1-6 hours after ingestion (very rapid - key MCQ!)
• Symptoms: nausea + vomiting dominant; diarrhea, hypotension, dehydration may occur
• No fever (distinguishes from infection-mediated diarrhea)
• No secondary spread
• Resolves in 8-10 hours
• Food: custards, potato salad, processed meats
• Treatment: purely supportive
• Diagnosis: demonstrate bacteria or enterotoxin in implicated food

C. Toxic Shock Syndrome (TSS)

• GI: vomiting or diarrhea at onset
• Muscular: severe myalgia or CPK ≥2x ULN
• Mucous membranes: vaginal/oropharyngeal/conjunctival hyperemia
• Renal: BUN or Cr ≥2x ULN, or pyuria without UTI
• Hepatic: bilirubin or transaminases ≥2x ULN
• Hematologic: platelets <100,000/mL

• >90% of menstrual TSS = caused by TSST-1
• Nonmenstrual TSS = mostly caused by enterotoxin B
• TSST-1 is a superantigen - binds MHC II directly without antigen processing → massive cytokine release
• TSS onset: 2-3 days after menstruation starts
• Linked to highly absorbent tampons (early 1980s outbreak)
• Nonmenstrual TSS: surgical/postpartum wounds, especially packed wounds

D. Bacteremia & Endocarditis

• S. aureus is the most common cause of infective endocarditis in IV drug users (tricuspid valve)
• Right-sided endocarditis → septic pulmonary emboli → lung abscesses
• Risk factors for complicated bacteremia: prosthetic valves, intracardiac devices, persistent bacteremia >72h
• Hematogenous seeding → osteomyelitis, septic arthritis, epidural abscess, meningitis
• S. aureus bacteremia mortality remains 20-30%

E. Pneumonia

• Hospital-acquired pneumonia and ventilator-associated pneumonia
• Necrotizing pneumonia: associated with PVL-producing CA-MRSA (rapid cavitation, hemorrhagic)
• Septic emboli from right-sided endocarditis

F. Scalded Skin Syndrome (SSSS / Ritter's Disease)

• Caused by exfoliative toxins A & B (serine proteases targeting desmoglein-1)
• Cleavage at stratum granulosum (superficial - distinguishes from TEN where cleavage is at dermoepidermal junction)
• Predominantly affects neonates and young children
• Nikolsky sign positive
• Bullae contain sterile fluid (toxin-mediated, not direct infection of skin)
• Treatment: anti-staphylococcal antibiotics (e.g., nafcillin or oxacillin)

G. Osteomyelitis

• S. aureus = most common cause of hematogenous osteomyelitis in all age groups
• In children: most common site = metaphysis of long bones (rich blood supply)
• Post-trauma/surgical: MRSA important pathogen
• Also: septic arthritis, psoas abscess, epidural abscess

H. CNS Infections

• Brain abscesses post-head trauma or neurosurgery: frequently MRSA, S. epidermidis, Enterobacteriaceae, Pseudomonas, Clostridium
• Spinal epidural abscess: S. aureus most common cause

I. Peritoneal Dialysis Infections

• S. aureus more common in nasal carriers
• Most common pathogen in CAPD-related peritonitis

---

5. RESISTANCE - MRSA

Mechanism of MRSA Resistance

• mecA gene (on SCCmec - staphylococcal cassette chromosome mec)
• Encodes PBP2a (PBP2') - altered penicillin-binding protein with low affinity for all β-lactams
• Renders all β-lactams ineffective (except ceftaroline and ceftobiprole - 5th gen cephalosporins active against MRSA)

HA-MRSA vs CA-MRSA

Risk factors for MRSA acquisition

• Healthcare exposure, prolonged hospitalization, ICU stay
• IV drug use, prison, unstable housing
• Long-term care facility residence
• Indwelling devices, prior antibiotics

---

6. TREATMENT

MSSA (Methicillin-Sensitive S. aureus)

MRSA

• Daptomycin - do NOT use in pneumonia (inactivated by surfactant)
• Linezolid - preferred for MRSA pneumonia over vancomycin (better lung penetration)
• Ceftaroline - only β-lactam with MRSA activity; 5th gen cephalosporin
• Ceftobiprole - approved 2023 for S. aureus bacteremia (N Engl J Med 2023)
• Rifampin - never as monotherapy; used adjunctively in prosthetic device infections
• Vancomycin is bacteriostatic against enterococci but bactericidal against S. aureus

VISA / VRSA

• VISA (vancomycin-intermediate S. aureus): MIC 4-8 μg/mL
• VRSA (vancomycin-resistant S. aureus): MIC ≥16 μg/mL
• VRSA: acquired vanA gene from VRE (horizontal transfer)
• Mechanism: thickened cell wall with increased D-Ala-D-Lac substitution

---

7. DECOLONIZATION & PREVENTION

• Intranasal mupirocin + chlorhexidine washes (individuals + household contacts)
• Bleach baths (½ cup bleach in half-filled bathtub, 15 min, 3x/week) for extensive skin disease
• Universal decolonization in ICU > screening + contact precautions alone
• Pre-surgical decolonization reduces SSI from S. aureus
• Post-hospital discharge decolonization reduces short-term MRSA risk by 30%
• Vaccines: multiple trials (capsular polysaccharide-conjugate, clumping factor antibody) - all failed in clinical trials to date

---

8. COAGULASE-NEGATIVE STAPHYLOCOCCI (NSaS) - High-Yield Facts

• Less virulent than S. aureus
• Important in prosthetic device infections (prosthetic valves, joint replacements, catheters)
• S. epidermidis: most common CoNS pathogen
• S. lugdunensis and S. schleiferi: cause more serious infections than other NSaS (native valve endocarditis, osteomyelitis) - behave more like S. aureus
• CoNS is the most common cause of prosthetic valve endocarditis (early <12 months)
• Biofilm formation is key virulence factor in CoNS

---

9. RAPID-FIRE MCQ BULLETS

1. Gram stain: gram-positive cocci in clusters
2. Differentiator from Strep: catalase positive
3. Differentiator from other Staph: coagulase positive
4. Blood agar: golden β-hemolytic colonies
5. Ferments: mannitol (unique among common Staph)
6. Food poisoning onset: 1-6 h (fastest onset of all food poisonings)
7. Food poisoning toxin: heat-stable enterotoxin (toxin survives cooking)
8. TSS in >90% menstrual cases: TSST-1
9. TSS nonmenstrual: enterotoxin B
10. TSST-1 mechanism: superantigen (binds Vβ region of TCR + MHC II)
11. SSSS split level: stratum granulosum
12. SSSS toxin: exfoliative toxin A or B (desmoglein-1 target)
13. SSSS vs TEN: bullae in SSSS contain sterile fluid; TEN = drug reaction
14. Most common cause of osteomyelitis (all ages): S. aureus
15. Most common cause of IE in IVDU: S. aureus (tricuspid valve)
16. Most common cause of CAPD peritonitis: S. aureus (in nasal carriers)
17. MRSA resistance gene: mecA → PBP2a (low β-lactam affinity)
18. CA-MRSA dominant strain USA: USA300 (PVL-positive)
19. PVL toxin: necrotizing pneumonia + furunculosis
20. Daptomycin: DON'T use in lung (surfactant inactivation)
21. Linezolid: preferred for MRSA pneumonia
22. Ceftaroline: only β-lactam active against MRSA
23. Vaccine: none successful as of 2025
24. Decolonization: mupirocin (intranasal) + chlorhexidine (body wash)
25. VRSA: acquires vanA from VRE
26. NSaS with worst infections: S. lugdunensis (native valve endocarditis)
27. Prosthetic device infections: S. epidermidis (CoNS)
28. Protein A: binds Fc region of IgG (immune evasion)
29. Biofilm: critical for prosthetic device and CIED infections
30. Bundling interventions: reduce nosocomial S. aureus infections on IV catheters

---