STEPTRACKERMICROBIOLOGY + ORTHOPEDICS + CARDIOLOGY | INFECTIONS BY SYSTEM | HIGH-YIELD REVIEW

Bone, Skin, Heart & Eye Infections

S aureus and streptococci dominate skin, bone and valves — the host and setting point to the exceptions.

FOCUS ON
Osteomyelitis: S aureus most common in all groups; Salmonella in sickle cell, Pseudomonas after nail puncture through a shoe or in IV drug users, Pasteurella after cat/dog bites, M tuberculosis in the spine (Pott), Kingella in toddlers; MRI + bone biopsy. Skin: impetigo (S aureus, GAS), erysipelas (GAS, raised sharp border), cellulitis (GAS, S aureus), abscess (S aureus/MRSA → drain), necrotizing fasciitis (GAS or polymicrobial; pain out of proportion, crepitus → surgery). Endocarditis: acute S aureus (IVDU, tricuspid), subacute viridans strep (dental, damaged valve), S gallolyticus (colon cancer), Enterococcus (GU/GI procedures), S epidermidis (prosthetic valve), HACEK. Eye: conjunctivitis (adenovirus, bacteria), keratitis (contact lenses: Pseudomonas, Acanthamoeba), endophthalmitis.
INFECTION
ORGANISMS · HOST
CLINICAL PICTURE
DIAGNOSIS · TREATMENT
Hematogenous osteomyelitis
S aureus; children (metaphysis of long bones), adults (vertebrae)
Fever, bone pain, refusal to bear weight; ↑ ESR/CRP
MRI most sensitive · biopsy/culture · IV antibiotics
Osteomyelitis — special hosts
Sickle cell → Salmonella · IVDU → Pseudomonas, S aureus · diabetic foot → polymicrobial · prosthesis → S epidermidis
Probe-to-bone in diabetic ulcer
Cover the expected organism
Septic arthritis
S aureus; N gonorrhoeae in sexually active young adults; Kingella in toddlers
Hot, swollen joint (knee); synovial WBC > 50,000, neutrophils
Aspirate before antibiotics · drainage
Impetigo
S aureus (bullous via exfoliative toxin), GAS
Honey-crusted lesions around nose/mouth in children
Topical mupirocin · post-strep GN (not rheumatic fever)
Erysipelas · cellulitis
GAS mainly (erysipelas) · GAS + S aureus (cellulitis)
Erysipelas: raised, sharply demarcated, bright red · cellulitis: deeper, ill-defined
Cephalexin; add MRSA coverage if purulent
Abscess · furuncle
S aureus (MRSA common)
Fluctuant tender nodule
Incision and drainage ± TMP-SMX/doxycycline
Necrotizing fasciitis
Type I polymicrobial (diabetes) · type II GAS ("flesh-eating") · Vibrio vulnificus, C perfringens
Pain out of proportion, rapid spread, bullae, crepitus, shock
Emergency debridement + broad antibiotics + clindamycin
Acute endocarditis
S aureus (IV drugs → tricuspid; also normal valves)
High fever, rapid valve destruction, septic emboli (lung if right-sided)
Blood cultures × 3 before antibiotics · echo · vancomycin
Subacute endocarditis
Viridans strep (dental work, damaged valve); Enterococcus (GU/GI procedures)
Low-grade fever, weight loss; Janeway lesions, Osler nodes, Roth spots, splinter hemorrhages
Small vegetations on abnormal valve
Endocarditis — special organisms
S gallolyticus (bovis) → colon cancer · S epidermidis → prosthetic valve · HACEK, Coxiella, Bartonella → culture-negative · Candida → IVDU, lines
Colonoscopy if S gallolyticus
Libman-Sacks (SLE) and marantic (cancer) are sterile
Conjunctivitis
Adenovirus (watery, preauricular node) · S pneumoniae, H flu, S aureus (purulent) · allergic (itchy)
Red eye without vision loss
Contact lens wearer → cover Pseudomonas
Keratitis
HSV (dendritic), Pseudomonas and Acanthamoeba (contact lenses), fungi (plant trauma)
Pain, photophobia, ↓ vision, corneal opacity
Urgent ophthalmology
Endophthalmitis · orbital cellulitis
Post-surgery or bacteremia (S aureus, Candida) · spread from ethmoid sinusitis
Severe pain, ↓ vision · proptosis, painful eye movements
Intravitreal antibiotics · IV antibiotics, CT

Mechanism — how endocarditis forms

Damaged endotheliumRheumatic, congenital, prosthetic, IV drugs
→
Sterile platelet-fibrin nidus
→
Transient bacteremia seeds itDextran, adhesins
→
Vegetation → emboli, immune complexes

S aureus can attack normal valves; viridans strep needs a previously damaged valve.

Skin infections by depth

LAYER
INFECTION
ORGANISM
Epidermis
Impetigo
S aureus, GAS
Upper dermis + lymphatics
Erysipelas
GAS
Deep dermis + fat
Cellulitis
GAS, S aureus
Fascia
Necrotizing fasciitis
GAS, polymicrobial
Muscle
Gas gangrene
C perfringens

How it's tested

COMMON CONFUSIONJaneway lesions (painless, palms/soles, septic emboli) vs Osler nodes (painful, finger pads, immune complexes).
THE TRAPNecrotizing fasciitis may look like mild cellulitis on the surface — pain out of proportion is the warning sign; don't wait for imaging to call surgery.
YOUR APPROACHSite and depth → host (sickle cell, IVDU, diabetic, prosthesis, contact lens) → organism → culture before antibiotics → drain or debride when needed.

If you see… think…

Most common cause of osteomyelitis → S aureus
Sickle cell child with osteomyelitis → Salmonella
Vertebral osteomyelitis with psoas abscess in an immigrant → TB (Pott disease)
Honey-colored crusts around a child's mouth → impetigo
Bright red, raised, sharply demarcated facial plaque → erysipelas (GAS)
Rapidly spreading pain, crepitus, shock → necrotizing fasciitis
IV drug user with tricuspid vegetation and lung nodules → S aureus endocarditis
Endocarditis after dental work on a rheumatic valve → viridans strep
S gallolyticus bacteremia → colonoscopy (colon cancer)
Contact lens wearer with corneal ulcer → Pseudomonas or Acanthamoeba