Step 1 topics → Musculoskeletal, Skin, and Connective Tissue → Bacterial Skin Infections

Bacterial Skin Infections

USMLE Step 1 trap: Confuses impetigo as a purely staph infection, missing strep as a classic cause of non-bullous impetigo. Impetigo is caused by S. aureus (bullous form) and/or Streptococcus pyogenes (non-bullous/crusted form), with S. aureus now predominating in both.

Bacterial skin infections are a high-yield dermatology topic on USMLE Step 1, and the exam hits them from multiple angles: recognizing classic lesion descriptions, matching organisms to clinical settings, and distinguishing infections by anatomical depth. You need to know not just what each condition looks like, but why it looks that way — the exam loves giving you a clinical vignette and asking you to pick the organism or explain the mechanism. The four major entities here are impetigo, erysipelas/cellulitis, necrotizing fasciitis, and folliculitis, each with its own high-yield organism and treatment hook.

What makes this topic tricky is that students often oversimplify: they learn 'staph causes skin infections' and apply that rule everywhere, which fails them on hot-tub folliculitis (Pseudomonas), strep impetigo, and Type II necrotizing fasciitis. USMLE Step 1 specifically exploits this tendency by embedding context clues — a recent hot-tub exposure, a sharply raised border, or crepitus on palpation — that should redirect your thinking entirely. If you're not reading the stem for those environmental and morphological details, you'll default to S. aureus every time and miss a significant chunk of these questions.

The other major trap is conflating erysipelas and cellulitis, or assuming necrotizing fasciitis is always polymicrobial. These distinctions are not just academic — depth of infection and border characteristics are what the USMLE Step 1 uses to separate these diagnoses in a vignette. Build a mental model that ties lesion morphology → anatomical depth → organism → treatment, and you'll stop second-guessing yourself on this material.

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Common misconceptions

Common mistake
Wrong: Impetigo is caused only by Staphylococcus aureus.
Right: Impetigo is caused by S. aureus (bullous form) and/or Streptococcus pyogenes (non-bullous/crusted form), with S. aureus now predominating in both.
Impetigo is not a single-organism disease. The non-bullous (crusted) form — the classic honey-crusted lesion around the mouth — is classically caused by Streptococcus pyogenes (Group A Strep), while the bullous form is caused by S. aureus exfoliative toxins (exfoliatins A and B) that cleave desmoglein-1. S. aureus has become more prevalent in both forms, but the strep association with non-bullous impetigo remains testable, especially because strep impetigo can be complicated by post-streptococcal glomerulonephritis (but NOT rheumatic fever — another classic trap).
Common mistake
Wrong: Erysipelas and cellulitis are interchangeable terms for the same infection.
Right: Erysipelas involves the upper dermis and superficial lymphatics (sharply demarcated, raised border), while cellulitis involves the deeper dermis and subcutaneous tissue (poorly demarcated border).
These are not the same condition. The key discriminator is anatomical depth and the resulting border appearance: erysipelas infects the upper dermis and superficial lymphatics, producing a bright red, warm, raised plaque with a sharply demarcated edge — you can literally feel the border. Cellulitis goes deeper into the dermis and subcutaneous tissue, so the inflammatory boundary is blurred and the border is poorly demarcated. Both are commonly caused by Group A Strep (and S. aureus for cellulitis), but the morphological distinction is what the vignette will hinge on.
Common mistake
Wrong: Necrotizing fasciitis is always a polymicrobial infection.
Right: Type I necrotizing fasciitis is polymicrobial (mixed anaerobes/aerobes), but Type II is monomicrobial, classically caused by Group A Streptococcus (S. pyogenes).
There are two distinct subtypes of necrotizing fasciitis, and mixing them up costs points. Type I is the polymicrobial form — a mixed bag of anaerobes (Bacteroides, Clostridium) and aerobes (E. coli, Enterococcus) — and typically occurs in diabetics or immunocompromised patients, often in the perineal area (Fournier's gangrene). Type II is monomicrobial and is the one caused by Group A Streptococcus (S. pyogenes) — this is the 'flesh-eating bacteria' presentation you see in otherwise healthy people after minor trauma. When a vignette describes a healthy patient with rapidly spreading infection and severe pain after a skin breach, think Type II, think GAS.
Common mistake
Wrong: Folliculitis is always caused by Staphylococcus aureus.
Right: While S. aureus causes typical folliculitis, hot-tub folliculitis is caused by Pseudomonas aeruginosa, which thrives in inadequately chlorinated water.
The default assumption that S. aureus causes all folliculitis will betray you when the vignette mentions a hot tub, pool, or water exposure within the past 1-3 days. Pseudomonas aeruginosa thrives in warm, inadequately chlorinated water, and it causes a characteristic folliculitis that predominantly affects the trunk and areas covered by a bathing suit. The rash is pruritic, papulopustular, and self-limited in immunocompetent patients. The exposure history is the giveaway — if the stem mentions a hot tub, the answer is Pseudomonas, full stop.
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What the exam tests

  1. Impetigo: Given a lesion description (honey-crusted vs. fluid-filled bullae), identify the responsible organism (S. pyogenes vs. S. aureus) and the appropriate therapy (topical mupirocin for localized disease, oral antibiotics for widespread).
  2. Erysipelas vs. cellulitis: Distinguish the two infections based on depth of involvement and border characteristics — erysipelas has a sharply demarcated, raised border (upper dermis/superficial lymphatics) while cellulitis has a poorly defined border (deeper dermis/subcutaneous tissue) — and identify the classic causative organisms.
  3. Necrotizing fasciitis: Recognize the red flags (pain out of proportion to exam, rapid progression, crepitus, skin necrosis) and distinguish Type I (polymicrobial, mixed anaerobes and aerobes) from Type II (monomicrobial, Group A Streptococcus), with emergent surgical debridement as the cornerstone of treatment.
  4. Folliculitis: Identify S. aureus as the typical cause of folliculitis and Pseudomonas aeruginosa as the organism responsible for hot-tub folliculitis, using the clinical context (recent hot-tub or pool exposure, trunk/extremity distribution) as the discriminating clue.

Can you avoid these mistakes?

A 6-year-old presents with honey-crusted lesions around the mouth and nose. What is the classic causative organism of this form of impetigo, what serious post-infectious complication can occur, and what complication does NOT occur (unlike with pharyngeal strep)?
A 65-year-old diabetic man presents with a rapidly spreading, exquisitely painful erythema on his thigh with a foul-smelling discharge and crepitus on palpation. Imaging shows gas in the soft tissues. What is the diagnosis, what are the two major subtypes and their associated organisms, and what is the most critical next step in management?
You see two patients in the ED: Patient A has a bright red, warm, raised plaque on the cheek with a sharp, palpable border. Patient B has a red, warm, swollen lower leg with an indistinct, poorly demarcated border. What is each diagnosis, how do they differ in depth of infection, and which organism is classically associated with each?
A 22-year-old woman develops an itchy, papulopustular rash on her torso and buttocks two days after using a hotel hot tub. What is the causative organism, why is it found in this setting, and how does this differ from typical folliculitis in terms of organism and expected clinical course?

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