It is true regarding normal microbial flora present on skin and mucous membrane that

It is difficult to permanently eradicate by antimicrobial agents

It is absent in the stomach due to acidic pH

It establishes in the body only after the neonatal period

The flora in the small bronchi is similar to that of the trachea

An 18-year-old man has facial and upper back lesions that have waxed and waned for the past 6 years. On physical examination, there are 0.3- to 0.9-cm comedones, erythematous papules, nodules, and pustules most numerous on the lower face and posterior upper trunk. Other family members have been affected by this condition at a similar age. The lesions worsen during a 5-day cruise to the Adriatic. Which of the following organisms is most likely to play a key role in the pathogenesis of these lesions?

Group A β-hemolytic streptococcus

Which of the following is a false statement about the respective fungal infections?

Superficial layers of skin are involved

Candida albicans causes skin infection

Epidermophyton doesn't involve hair

All of the following statements about Staphylococcus aureus are true, except:

Most common source of infection is cross infection from infected people.

About 30% of general population is healthy nasal carriers.

Methicillin resistance is chromosomally mediated.

Epidermolysin and TSS toxin are superantigens.

Skin Microbiome for FMGE: High-Yield Microbiology Lessons

Skin Microbiome - Your Skin's BFFs

Definition: Diverse community of microorganisms (bacteria, fungi, viruses, mites) on the skin.
Key Inhabitants:
- Bacteria: Staphylococcus epidermidis, Cutibacterium acnes, Corynebacterium spp.
- Fungi: Malassezia spp.
Regional Variation: Site-dependent (sebaceous, moist, dry areas).
- Sebaceous (oily): ↑ Lipophilic bacteria (e.g., C. acnes).
- Moist (axilla): ↑ Gram-positives (e.g., Staphylococcus, Corynebacterium).
- Dry (forearm): Mixed, lower density.
Functions:
- Pathogen displacement (colonization resistance).
- Immune system education.
- Nutrient metabolism & production of antimicrobials.

⭐ Staphylococcus epidermidis produces antimicrobial peptides (AMPs) that selectively inhibit S. aureus and can modulate host immune responses, contributing to skin homeostasis.

Skin Microbiome - Meet the Micro-Crew

Diverse residents: bacteria, fungi, viruses, mites. Site-specific: sebaceous (oily), moist, dry.

Microbe	Predominant Type(s)	Key Site(s)	Clinical Relevance / Role
Cutibacterium	C. acnes (Gram +ve anaerobe)	Sebaceous (face, chest, back)	Acne vulgaris, sebum metabolism, biofilm formation
Staphylococcus	S. epidermidis (Gram +ve cocci)	Ubiquitous	Commensal, barrier protection, opportunistic (biofilms on devices)
Corynebacterium	Lipophilic spp. (Gram +ve rods)	Moist (axilla, groin, toe webs)	Body odor (via lipid metabolism), erythrasma
Malassezia	Lipophilic yeasts (M. globosa, M. furfur)	Sebaceous areas	Pityriasis versicolor, seborrheic dermatitis, dandruff
Demodex	D. folliculorum, D. brevis (mites)	Hair follicles, sebaceous glands	Commensal; high density linked to rosacea, blepharitis

⭐ Propionibacterium acnes (now Cutibacterium acnes) is a major trigger for inflammatory acne due to its lipase activity on sebum and PAMPs stimulating TLR2.

Skin Microbiome - The Skinfluencers

Dynamic ecosystem: bacteria, fungi, viruses, mites on skin.
Key Genera: 📌 CoPS (Corynebacterium, Propionibacterium/Cutibacterium, Staphylococcus); Malassezia (fungus).
Site Variation:
- Sebaceous (oily): Lipophilic Cutibacterium spp.
- Moist (axilla, groin): Staphylococcus spp., Corynebacterium spp.
- Dry (forearm): Mixed, lower density, higher diversity.
Host Factors: Age, sex, genetics, immune system, hygiene.
Environmental Factors: Climate (humidity, UV), occupation, antibiotics, cosmetics.
Dysbiosis (imbalance) linked to: Acne vulgaris, atopic dermatitis, psoriasis.

⭐ Staphylococcus epidermidis, a major skin commensal, produces antimicrobial peptides (AMPs) like phenol-soluble modulins (PSMs) that selectively inhibit S. aureus colonization and biofilm formation, contributing to skin homeostasis.

Skin Microbiome - Microbial Mayhem

Dysbiosis: Imbalance in normal skin microbial flora leading to or exacerbating skin diseases.
- Key Triggers: Host genetics, immune status, hygiene practices, antibiotic use, environmental factors.

Disease	Key Microbial Dysbiosis & Impact
Acne Vulgaris	↑ Cutibacterium acnes (esp. virulent strains), ↓ diversity; inflammation, comedogenesis.
Atopic Dermatitis (AD)	↑ Staphylococcus aureus (esp. during flares, produces toxins), ↓ S. epidermidis (some protective strains), ↓ diversity; exacerbates barrier dysfunction.
Psoriasis	Altered Firmicutes/Actinobacteria ratio, ↑ Malassezia spp., ↑ Streptococcus spp. (guttate trigger); immune dysregulation.
Rosacea	Controversial; potential roles for Demodex folliculorum mites & associated Bacillus oleronius; ↑ some Staphylococcus epidermidis strains; neurovascular dysregulation.
Seborrheic Dermatitis	↑ Malassezia spp. (esp. M. globosa, M. restricta); metabolize sebum to irritant fatty acids.

High‑Yield Points - ⚡ Biggest Takeaways

Dominant phyla: Actinobacteria (Cutibacterium), Firmicutes (Staphylococcus), Proteobacteria, Bacteroidetes.

S. epidermidis: Main commensal, makes AMPs; opportunistic.

C. acnes: Lipophilic, in sebaceous glands; causes acne.

Malassezia spp.: Top fungal commensal (yeast); linked to pityriasis versicolor, seborrheic dermatitis.

Site variation: Distinct flora in sebaceous, moist, dry skin.

Dysbiosis: Linked to atopic dermatitis, psoriasis, acne, poor wound healing.

Key influences: Host genetics, age, hygiene, acidic pH.

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Skin Microbiome