Microbial Infections of the Skin: Bacterial, Viral, and Fungal Diseases

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Skin Infections

Overview

Skin infections are caused by a variety of microorganisms, including bacteria, viruses, and fungi. These infections can range from mild to life-threatening and often present with characteristic lesions or rashes. Understanding the causative agents, pathogenesis, clinical manifestations, and treatments is essential for effective management and prevention.

Bacterial Infections of the Skin

Hair Follicle Infections

Hair follicle infections are most commonly caused by Staphylococcus aureus. This bacterium possesses several virulence factors that contribute to its pathogenicity.

Coagulase: An exoenzyme that promotes fibrin clot formation, protecting bacteria from immune cells.
Protein A: Binds to the Fc region of antibodies, inhibiting opsonization and phagocytosis.
Hyaluronidase and Collagenase: Degrade connective tissue components, facilitating bacterial spread.
Leukocidin: Destroys leukocytes, especially neutrophils.
α-toxin: Forms pores in host cell membranes, leading to cell lysis.

Staphylococcus aureus infecting hair follicle Abscess formation in hair follicle infection

Additional Virulence Factors

Some S. aureus strains produce:

Exfoliatin: An exotoxin causing separation of skin layers, leading to scalded skin syndrome, especially in newborns. Transmission occurs via direct or indirect contact and is a concern in hospital nurseries. Prevention includes the use of hexachlorophene.
Toxic Shock Syndrome Toxin (TSST-1): Acts as a superantigen, causing toxic shock syndrome with symptoms such as fever, rash, vomiting, shock, organ failure, and potentially death. Often associated with tampon use.
Enterotoxins: Produced in foods stored at room temperature, leading to food poisoning.

Scalded skin syndrome on hand

Impetigo

Impetigo is a superficial skin infection most commonly caused by Streptococcus pyogenes and sometimes by Staphylococcus aureus.

Requires a break in the skin (cuts, bites, scratches) for entry.
Characterized by inflammation of the epidermis, thin-walled blisters that rupture and ooze plasma, facilitating spread.
Transmitted by direct or indirect contact.
Treated with antibiotics.

Severe impetigo around the mouth Impetigo lesions on the chin

Acne Vulgaris

Acne vulgaris is an inflammatory condition caused by Propionibacterium acnes, a normal skin microbiota member.

Grows anaerobically in hair follicles, utilizing sebum as a nutrient.
Overproduction of sebum can block follicles, leading to bacterial overgrowth and inflammation.
Treatment options include antibiotics, benzoyl peroxide, and isoretinoin (Accutane), which reduces sebum production but has significant side effects.

Inflammatory acne on the chin

Lyme Disease

Lyme disease is caused by the spirochete Borrelia burgdorferi.

Reservoirs: Deer and mice.
Vector: Ticks transmit the bacterium to humans.
Symptoms progress in stages:
- First stage: Bullseye rash at the bite site, fever, muscle pain, headache (lasts ~2 weeks).
- Second stage: Weeks to months later, arthritis, facial paralysis, irregular heart rhythms.
- Third stage: Months to years later, chronic arthritis, neurological changes (memory/mood disturbances, sleep issues).
Early treatment with antibiotics is effective; later stages are likely due to autoimmunity and do not respond to antibiotics.

Borrelia burgdorferi spirochete under microscope Bullseye rash of Lyme disease

Viral Infections of the Skin

Varicella (Chickenpox and Shingles)

Caused by the Varicella-Zoster Virus (VZV).
Highly contagious, transmitted by respiratory droplets.
Incubation: 10–21 days, followed by a characteristic itchy, raised rash.
VZV becomes latent in nerve cells; reactivation causes shingles (herpes zoster), presenting as painful lesions.
Treatment: Acyclovir.
Prevention: Attenuated whole-agent vaccine (part of MMRV).

Chickenpox rash on torso Shingles recurrence along a dermatome

Rubeola (Measles)

Caused by the measles virus; highly contagious via respiratory droplets.
Incubation: 10–12 days.
Symptoms: Cold-like symptoms, Koplik spots (oral mucosa), macular rash.
Complications: Viral pneumonia, encephalitis, brain damage, subacute sclerosing panencephalitis (SSPE).
No specific treatment; prevention via attenuated vaccine (MMR/MMRV).
Humans are the only reservoir; eradication is possible.

Koplik spots in measles Measles rash on child

Rubella (German Measles)

Caused by the Rubella virus; milder than measles.
Symptoms: Faint pink rash.
Serious complications if infection occurs during pregnancy: congenital rubella syndrome (birth defects such as cataracts, deafness, heart defects, mental deficits, stillbirth).
No treatment; prevention via attenuated vaccine (MMR/MMRV).

Rubella rash on arm

Human Papillomavirus (HPV)

Causes warts (benign tumors) on skin and mucous membranes.
Transmitted by direct or indirect contact; different strains cause different types of warts (plantar, dermal, genital).
Some strains are oncogenic (cervical, mouth, throat cancers).
Treatment: Removal of infected cells (freezing, salicylic acid, surgical removal).
Prevention: HPV vaccine (protects against 9 strains, including types 16 & 18 associated with cervical cancer).

Fungal Infections of the Skin

Cutaneous Mycoses

Fungi grow on hair, nails, and outer skin layers, using keratin as a nutrient.
Examples: Ringworm (ring-shaped rash), Athlete’s foot, Jock itch.
Common in moist areas of the body.
Treatment: Topical azole creams for skin; oral griseofulvin for hair/nail infections.

Ringworm on arm Athlete's foot between toes

Candidiasis

Caused by Candida albicans (yeast).
Manifests as thrush (oral) or vaginitis.
Opportunistic pathogen; infection occurs when normal microbiota are disrupted (e.g., after antibiotics).
Treatment: Topical azoles or nystatin mouthwash.

Oral candidiasis (thrush)

Summary Table: Major Microbial Skin Infections

Infection	Agent	Key Features	Treatment/Prevention
Hair follicle infections	Staphylococcus aureus	Pus, abscess, virulence factors	Antibiotics, hygiene
Impetigo	Streptococcus pyogenes, S. aureus	Blisters, oozing plasma	Antibiotics
Acne vulgaris	Propionibacterium acnes	Pustules, inflammation	Antibiotics, benzoyl peroxide, isoretinoin
Lyme disease	Borrelia burgdorferi	Bullseye rash, arthritis, neurological symptoms	Early antibiotics
Chickenpox/Shingles	Varicella-Zoster Virus	Itchy rash, latent infection	Acyclovir, vaccine
Measles (Rubeola)	Measles virus	Koplik spots, macular rash	Vaccine
Rubella	Rubella virus	Faint pink rash, congenital syndrome	Vaccine
HPV (warts)	Human papillomavirus	Warts, oncogenic strains	Removal, vaccine
Ringworm, Athlete’s foot	Dermatophyte fungi	Ring-shaped rash, moist areas	Topical/oral antifungals
Candidiasis	Candida albicans	Thrush, vaginitis	Topical/oral antifungals