BACTERIA · Gram-positive cocci

Staphylococcus aureus

Staphylococcus aureus — Gram stain (grape-like clusters)

Stain: Gram stain (1000x)Morphology: Gram-positive cocci in irregular grape-like clustersYield: HIGHDifficulty: EASY

Staphylococcus aureus microscopic image — Gram-positive cocci, Gram stain (1000x)

Image: Wikimedia Commons · File:Staphylococcus_aureus_Gram.jpg · Y Tambe (CC BY-SA 3.0)

Key facts

**Pathogenesis**: Protein A binds Fc of IgG (blocks opsonization); coagulase walls off abscesses; TSST-1 is a superantigen causing massive cytokine release. **Diagnostic clue**: Catalase-positive (vs Streptococci), coagulase-positive (vs S. epidermidis), beta-hemolytic golden colonies on blood agar, ferments mannitol on MSA. **Virulence**: Exfoliative toxin (SSSS), enterotoxins (preformed → rapid-onset food poisoning), Panton–Valentine leukocidin (necrotizing pneumonia/skin).

Boards buzzwords

grape-like clusters
golden colonies
coagulase-positive
catalase-positive
beta-hemolytic
Protein A

Associated diseases

Skin/soft tissue infections, abscesses
Acute bacterial endocarditis (tricuspid in IVDU)
Osteomyelitis (#1 cause)
Toxic shock syndrome
Food poisoning (preformed toxin)
MRSA pneumonia

Treatment

MSSA → nafcillin/oxacillin/cefazolin; MRSA → vancomycin, daptomycin, linezolid, or ceftaroline; drain abscesses; decolonize with mupirocin + chlorhexidine for recurrence

Related organisms

Listeria monocytogenes

Gram-positive rod (facultative intracellular)

Mycobacterium tuberculosis

Acid-fast bacillus (mycolic acid cell wall)

Nocardia spp.

Gram-positive filamentous, partially acid-fast

Treponema pallidum

Spirochete

Mycobacterium leprae

Acid-fast bacillus (obligate intracellular)

Neisseria meningitidis

Gram-negative diplococci