A 21-year-old man comes to the physician because of painful, firm, dark bumps on his neck and jawline. He has no history of serious illness and takes no medications. His brother had a similar rash. A photograph of the rash is shown. Which of the following is the most likely underlying mechanism of this patient's condition?

Interfollicular penetration of the skin by distal end of hair

Trichophyton infection of the superficial hair follicle

Bacterial infection of the superficial or deep hair follicle

Follicular obstruction with subsequent duct rupture

Cutibacterium acnes colonization of the pilosebaceous unit

A 27-year-old female presents to her primary care physician because she is concerned about lighter colored patches on her skin. She recently went sunbathing and noticed that these areas also did not tan. Her doctor explains that she has a fungal infection of the skin that damages melanocytes by producing acids. She is prescribed selenium sulfide and told to follow-up in one month. Which of the following describes the appearance of the most likely infectious organism under microscopy?

"Spaghetti and meatballs" fungus

A hospital implements a bundle to reduce catheter-associated bloodstream infections. Components include: chlorhexidine bathing, antibiotic-impregnated catheters, antiseptic catheter site dressings, and daily line necessity assessment. After implementation, bloodstream infections with coagulase-negative staphylococci decrease by 60%, but Candida bloodstream infections increase by 40%. Evaluate the microbiological mechanisms underlying these divergent outcomes and synthesize an optimal prevention strategy.

Multiple interventions disrupted skin flora creating ecological niche for Candida; modify bundle to preserve some commensal bacteria while maintaining antisepsis

Antibiotic-impregnated catheters select for resistant Candida; use non-antibiotic catheters

The bundle successfully reduced bacterial infections, revealing underlying fungal infections; add antifungal prophylaxis

Chlorhexidine bathing eliminates bacterial skin flora but promotes fungal colonization; discontinue chlorhexidine

Candida increase represents surveillance bias from increased culturing; no change needed

Skin microbiome for USMLE Step 3: High-Yield Microbiology Lessons

Skin Microbiome - Our Living Shield

Primary Colonizers: Predominantly Gram-positive bacteria, tolerant of dry, high-salt conditions.
- Sebaceous (oily) areas: Propionibacterium (e.g., P. acnes), Staphylococcus spp.
- Moist areas (axilla, groin): Staphylococcus spp., Corynebacterium spp.
- Dry areas (forearm): Mixed flora, lower density.
Protective Functions:
- Competes with pathogens for nutrients & binding sites.
- Produces antimicrobial peptides (AMPs).
- Modulates the host immune response.

⭐ High-Yield: Staphylococcus epidermidis, a key resident, produces bacteriocins that inhibit S. aureus colonization and can help prevent skin infections.

Skin microbiome composition across different body sites

Key Inhabitants - The Usual Suspects

The skin is dominated by a few key players, primarily bacteria and fungi, adapted to its dry, slightly acidic, and lipid-rich environment.

Organism Group	Key Species	Characteristics & Clinical Relevance
Bacteria (Gram +)	Staphylococcus epidermidis	Coagulase-negative. Most abundant skin commensal. Forms biofilms on catheters and prosthetic devices.
	Cutibacterium acnes	Anaerobic rod found in sebaceous glands. Associated with acne vulgaris by hydrolyzing triglycerides.
	Corynebacterium spp.	Diphtheroids; aerobic rods. Contribute to body odor.
	Staphylococcus aureus	Coagulase-positive. Colonizes nares and moist areas. Can be a pathogen (e.g., MRSA).
Fungi (Yeast)	Malassezia spp.	Lipophilic yeast. Most common fungus. Causes pityriasis (tinea) versicolor ("spaghetti and meatballs" on KOH prep).
Arthropods	Demodex folliculorum	Microscopic mite in hair follicles. Usually asymptomatic.

⭐ Exam Favorite: Staphylococcus epidermidis is a notorious contaminant of blood cultures and a primary cause of endocarditis on prosthetic heart valves and infections of other indwelling devices due to its potent biofilm-forming ability.

📌 Mnemonic: "Skin's Common Microbes": Staph, Corynebacterium/Cutibacterium, Malassezia.

Clinical Correlations - When Good Microbes Go Bad

Dysbiosis, an imbalance in normal flora, can lead to opportunistic infections, especially in immunocompromised individuals or when skin barriers are breached.

Staphylococcus epidermidis
- Forms biofilms on prosthetic devices (catheters, heart valves).
- A leading cause of nosocomial infections.
Cutibacterium acnes (formerly Propionibacterium)
- Associated with acne vulgaris pathogenesis.
- Thrives in lipid-rich sebaceous glands.
Staphylococcus aureus
- Can be transient or persistent flora.
- Causes folliculitis, abscesses, and impetigo.

⭐ S. epidermidis, a common skin commensal, is a major cause of nosocomial infections on prosthetic devices and catheters due to its ability to form biofilms.

S. epidermidis biofilm on catheter

High‑Yield Points - ⚡ Biggest Takeaways

Staphylococcus epidermidis is the most dominant inhabitant of the skin; it is coagulase-negative.

The skin is primarily colonized by Gram-positive bacteria that are tolerant of dry, high-salt conditions.

Cutibacterium acnes (formerly Propionibacterium) thrives in sebaceous glands and is implicated in acne vulgaris.

Normal flora provides colonization resistance, preventing overgrowth of pathogens like S. aureus.

Breaches in the skin barrier (e.g., catheters, wounds) can cause opportunistic infections by resident flora.

Moist areas like the axilla and groin harbor a higher density and diversity of microbes.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play

Skin microbiome