STEPTRACKERSKIN | PATHOLOGY + MICROBIOLOGY + PHARMACOLOGY | HIGH-YIELD REVIEW

Skin Infections

Bacteria by depth, the viral bumps, the fungi of skin and nails, and the mite that itches at night.

FOCUS ON
Depth sorts bacterial infections: impetigo (epidermis, honey crusts) → erysipelas (upper dermis, sharp edges, strep) → cellulitis (deep dermis) → necrotizing fasciitis (fascia — pain out of proportion, crepitus, surgery now). Viral: warts (HPV), molluscum (poxvirus). Fungal: dermatophytes (ring, KOH hyphae), Malassezia (tinea versicolor). Scabies: burrows, nocturnal itch.
INFECTION
ORGANISM · MECHANISM
FEATURES
CLUE · TREATMENT
Impetigo
S aureus (bullous: exfoliative toxin), S pyogenes
Children; face; honey-colored crusts; very contagious
Topical mupirocin · strep → PSGN (not rheumatic fever)
Erysipelas
S pyogenes in upper dermis and lymphatics
Bright red, raised, sharply demarcated plaque on face or legs; fever
Penicillin
Cellulitis
S pyogenes, S aureus in deep dermis/subcutis; entry through breaks
Warm, red, swollen, poorly demarcated; lymphangitic streaks
Cat/dog bite → Pasteurella · seawater → Vibrio vulnificus · freshwater → Aeromonas
Abscess, furuncle, carbuncle
S aureus (incl. MRSA) in hair follicles
Fluctuant tender collection; furuncle = one follicle, carbuncle = several
Incision and drainage · hot tub folliculitis → Pseudomonas
Necrotizing fasciitis
Type I: polymicrobial (diabetics, perineum) · type II: S pyogenes (± S aureus); toxins spread along fascia
Pain out of proportion, rapid spread, bullae, crepitus, skin anesthesia, septic shock
Emergency debridement + broad antibiotics (clindamycin)
Gas gangrene
Clostridium perfringens α-toxin (lecithinase) in deep wounds
Crepitus, myonecrosis, bronze skin, hemolysis
Debridement, penicillin + clindamycin
Warts (verrucae)
HPV infects keratinocytes
Common (HPV 2, 4), plantar (1), genital (6, 11); koilocytes, papillomatosis
Cryotherapy, salicylic acid
Molluscum contagiosum
Poxvirus
Pearly umbilicated papules; children, sexual contact; extensive in HIV; molluscum bodies (eosinophilic cytoplasmic inclusions)
Self-limited
Dermatophytes (tinea)
Trichophyton, Microsporum, Epidermophyton digest keratin
Corporis (scaly ring, central clearing), capitis (kids, kerion), pedis, cruris, unguium (onychomycosis)
KOH: branching hyphae · topical azole/terbinafine; oral for scalp and nails
Tinea (pityriasis) versicolor
Malassezia makes azelaic acid (inhibits melanocytes)
Hypo- or hyperpigmented scaly patches after heat and humidity
"Spaghetti and meatballs" · selenium sulfide, azoles
Cutaneous candidiasis
Candida albicans in moist folds
Beefy red intertrigo with satellite lesions; diaper rash; diabetes, obesity
Topical azoles, nystatin
Scabies
Sarcoptes scabiei burrows in stratum corneum; type IV reaction
Intense nocturnal itch; burrows in finger webs, wrists, genitals; household spread; crusted in immunocompromised
Permethrin, oral ivermectin · treat contacts

Mechanism — depth decides the disease

EpidermisImpetigo
→
Upper dermis + lymphaticsErysipelas
→
Deep dermis + fatCellulitis
→
FasciaNecrotizing fasciitis
→
MuscleMyonecrosis

The deeper the infection, the less the surface shows — necrotizing fasciitis may look mild while the fascia dies.

Exposure → organism

EXPOSURE
ORGANISM
Cat or dog bite
Pasteurella multocida
Human bite / fist fight
Eikenella, anaerobes, strep
Oyster/seawater + liver disease
Vibrio vulnificus
Fresh water, leeches
Aeromonas hydrophila
Fish tank
Mycobacterium marinum
Rose thorns, gardening
Sporothrix schenckii
Hot tub
Pseudomonas aeruginosa

How it's tested

COMMON CONFUSIONErysipelas has a sharp raised edge (superficial, strep); cellulitis has a blurry edge (deeper, strep or staph).
THE TRAPSevere pain with little visible change, plus fever and rising creatinine kinase or crepitus, is necrotizing fasciitis — imaging must not delay surgery.
YOUR APPROACHDepth → appearance → exposure history → organism → treatment (topical, oral, drainage, surgery) → complications (PSGN, toxic shock).

If you see… think…

Child with honey-crusted lesions around the nose → impetigo
Cola-colored urine weeks after impetigo → post-streptococcal GN
Bright red, sharply raised plaque on the cheek → erysipelas
Cellulitis after a cat bite → Pasteurella multocida
Cirrhotic with bullous cellulitis after eating oysters → Vibrio vulnificus
Diabetic with severe perineal pain, crepitus and sepsis → necrotizing fasciitis (Fournier)
Umbilicated pearly papules in a child → molluscum contagiosum
Scaly ring with central clearing, KOH hyphae → tinea corporis
Pale patches on the back after a humid summer → tinea versicolor (Malassezia)
Whole family itching at night, finger-web burrows → scabies — permethrin