Microbial Diseases of the Skin and Eyes

1. Skin Microbiota and Skin Barriers

The skin microbiota is the resident community of microorganisms that normally colonize the skin, including Staphylococcus epidermidis, Corynebacterium species, Cutibacterium acnes (formerly Propionibacterium acnes), and the yeast Malassezia. The skin barriers are both physical and chemical: a dry, keratinized outer layer; continual shedding of dead cells; a slightly acidic pH; salt from sweat; sebum (oily secretions); and antimicrobial peptides. Together these make skin a tough environment for most microbes.

2. Colonization vs. Infection

Colonization means microorganisms are present and multiplying on a body surface without causing harm — a normal, often lifelong state. Infection means microbes have invaded tissues or multiplied to the point of causing damage and symptoms. The distinction matters clinically: colonizing bacteria are expected and usually harmless, while infection requires the microbe to breach a barrier or exploit a weakness. Many skin infections are caused by a person's own normal flora becoming opportunistic.

3. Bacterial, Viral, and Fungal Skin Infections

• Bacterial: Staphylococcus aureus and Streptococcus pyogenes cause impetigo, folliculitis, boils (abscesses), cellulitis, and erysipelas. Methicillin-resistant Staphylococcus aureus (MRSA) is a resistant variant of concern. Cutibacterium acnes contributes to acne, and Pseudomonas aeruginosa can cause "hot-tub" folliculitis.
• Viral: herpes simplex virus causes cold sores; varicella-zoster virus causes chickenpox and later shingles; human papillomaviruses cause warts; molluscum contagiosum virus causes small skin bumps; and several viruses produce rash illnesses.
• Fungal: dermatophytes cause ringworm (tinea) of the skin, scalp, feet, and nails; Candida species cause infections of moist skin folds; Malassezia is associated with certain scaly rashes.

4. Eye Infections

The eye is protected by tears, eyelids, and lysozyme, but infections still occur. Conjunctivitis ("pinkeye") is inflammation of the conjunctiva, the membrane lining the eyelid and covering the white of the eye. It may be bacterial (for example Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, or Chlamydia trachomatis), viral (commonly adenoviruses, which are highly contagious), or, in newborns, caused by organisms acquired during birth. Keratitis is infection of the cornea and is a special concern for contact-lens wearers, with risks including Pseudomonas aeruginosa and the amoeba Acanthamoeba.

How it works

1. Normal skin microbiota colonize the surface without harm.
2. A break in the skin barrier — a cut, abrasion, or moisture damage — creates an opening.
3. Resident or introduced microbes multiply in the now-vulnerable tissue, producing inflammation.
4. Symptoms (redness, warmth, swelling, pain, or discharge) signal infection.
5. Spread occurs by contact, fomites, or autoinoculation unless hygiene and barrier care interrupt it.

Common confusions

Memory aids

For red-flag eye symptoms, remember "P-V-L" — Pain, Vision change, Light sensitivity. Any of these in an eye infection means seek care promptly rather than wait.

Quick review

Topic Recap

The skin's barriers and resident microbiota normally keep microbes in a harmless colonized state, but a break in the barrier or an opportunity can turn normal flora into infectious agents. Bacterial, viral, and fungal skin infections and eye infections such as conjunctivitis spread mainly by contact and fomites and are prevented by hygiene and vaccination. Recognizing the colonization-to-infection transition and red-flag signs — especially vision-threatening eye symptoms — supports safe, timely care and appropriate referral.

Knowledge Check

1. What is the difference between colonization and infection?
2. Which two bacterial genera most commonly cause bacterial skin infections?
3. Why is adenovirus conjunctivitis especially likely to spread?
4. Name two physical or chemical barriers of the skin.
5. What eye symptoms should prompt immediate referral?

Answers and Rationales

1. Colonization is the harmless presence of microbes on a surface; infection is invasion or overgrowth that causes tissue damage and symptoms.
2. *Staphylococcus* (especially S. aureus) and *Streptococcus* (especially S. pyogenes).
3. Adenoviruses are highly contagious and spread easily by contact with contaminated hands or objects, so conjunctivitis outbreaks can spread quickly in shared settings.
4. Any two of: dry keratinized outer layer, shedding of dead cells, acidic pH, salt from sweat, sebum, or antimicrobial peptides.
5. Eye pain, reduced or blurred vision, and photophobia (light sensitivity) — these are red flags that warrant prompt clinical evaluation.