Microbial Diseases of the Skin and Eyes - Study Notes

Structure and Function of the Skin

  • Epidermis: Thin outer portion made of layers of epithelial cells

  • Keratin: Waterproofing protein coating the outer layer of the epidermis

  • Dermis: The inner, thick layer composed mainly of connective tissue

  • Perspiration: Provides moisture and nutrients for microbial growth

    • Salt in sweat inhibits microbial growth

    • Sebum from oil glands contains fatty acids that inhibit pathogens

Mucous Membranes

  • Moist tissues lining the inside of the body

  • Contain mucus-producing glands

    • Mucus is a thick fluid that lubricates and prevents tissue from drying

    • Often acidic, which prevents microbial growth

    • Some cells contain cilia, enhancing their protective function

Normal Microbiota of the Skin

  • Microbiota are resistant to drying and high salt concentrations

  • Predominantly gram-positive cocci such as

    • Staphylococci

    • Micrococci

  • Vigorous washing reduces numbers but doesn't eliminate them.

  • Higher populations found in moisture-prone areas; they metabolize sweat, contributing to body odor

    • Cutibacterium (Propionibacterium): Causes acne and inhabits hair follicles. Most common skin disease.

    • Malassezia furfur: A yeast that causes dandruff

Types of Microbial Diseases of the Skin

  • Vesicles: Small, fluid-filled lesions

  • Bullae: Larger vesicles over 1 centimeter

  • Macules: Flat, reddened lesions

  • Papules: Raised lesions

  • Pustules: Raised lesions filled with pus

  • Exanthem: Rash from a disease

  • Enanthem: Rash on mucous membranes from a disease

Staphylococcal Skin Infections

  • Staphylococci: Spherical gram-positive bacteria that form irregular clusters

    • Some produce coagulase, an enzyme that clots fibrin in blood

  • Staphylococcus epidermidis:

    • 90% of normal skin microbiota, a healthcare-associated pathogen that can produce biofilms on catheters.

  • Staphylococcus aureus:

    • Carried in the nasal passages of ~20% of the population

    • Can cause severe infections due to toxins and proteins that evade host defenses

Types of Staphylococcal Infections

  • Folliculitis: Infections of hair follicles

  • Sty: Folliculitis of an eyelash

  • Furuncle (Boil): Abscess; localized pus surrounded by inflamed tissue

  • Carbuncle: Spread of a furuncle causing tissue damage and inflammation

  • Impetigo: Nonbullous sores, spread by autoinoculation

  • Scalded Skin Syndrome:

    • Causes exfoliation due to toxin production

  • Toxic Shock Syndrome (TSS):

    • Characterized by fever, vomiting, shock, and organ failure from TSST-1 toxin

Streptococcal Skin Infections

  • Differentiated into groups A through T by antigenic cell wall carbohydrates

  • Group A Streptococci (GAS): Streptococcus pyogenes

    • Produces virulence factors such as

    • Streptolysins: Lyse red blood cells

    • M proteins: Enable phagocytic evasion and adherence to membranes

    • Hyaluronidase: Dissolves connective tissue

    • Streptokinases: Dissolves blood clots

Types of Streptococcal Infections

  • Erysipelas: Infection of the dermal layer leading to tissue destruction and sepsis

  • Necrotizing Fasciitis:

    • “Flesh-eating” disease caused by pyrogenic toxins

  • Streptococcal Toxic Shock Syndrome: More deadly than staphylococcal TSS

Infections by Pseudomonads

  • Pseudomonas aeruginosa

    • A gram-negative, aerobic rod known for producing Pyocyanin (blue-green pus)

    • Produces both exo- and endotoxins; often forms biofilms

    • Commonly causes Otitis externa and opportunistic infections in burn and cystic fibrosis patients

    • Known for antibiotic resistance

Microbial Diseases of the Eyes

  • Conjunctivitis: Inflammation of the conjunctiva, commonly caused by Haemophilus influenzae

  • Inclusion Conjunctivitis: Caused by Chlamydia trachomatis, transmitted during childbirth or swimming pool exposure

  • Ophthalmia Neonatorum:

    • Caused by Neisseria gonorrhoeae; can lead to blindness if untreated

  • Trachoma: Caused by serotypes of Chlamydia trachomatis, leading cause of blindness worldwide transmitted through hands or flies

Viral Diseases of the Skin

  • Warts: Small benign tumors caused by human papillomavirus (HPV), treated with cryotherapy

  • Smallpox: Caused by Orthropoxvirus; eradicated through vaccination

  • Chickenpox (Varicella): Caused by HSV-3; characterized by pus-filled vesicles, prevented by vaccine

  • Shingles (Herpes Zoster): Reactivation of latent varicella-zoster virus, causes painful lesions

  • Herpes Simplex:

    • HSV-1: Primarily oral transmission.

    • HSV-2: Primarily sexual transmission, causing genital herpes

  • Measles (Rubeola): Viral disease with cold-like symptoms and macular rash, prevented by MMR vaccine

  • German Measles (Rubella): Milder form with potential for causing congenital defects

Fungal Diseases of the Skin and Nails

  • Mycosis: Fungal infection of the body

  • Cutaneous Mycoses: Infections that colonize hair, nails, and outer epidermis; examples include

    • Tinea (

    • Tinea capitis: Scalp ringworm

    • Tinea pedis: Athlete’s foot

    • Tinea unguium: Nail ringworm

  • Subcutaneous Mycoses: More serious and penetrate deeper layers

  • Candidiasis: Overgrowth of Candida albicans, occurs when competing bacteria are suppressed

    • Thrush is a common oral infection of C. albicans

Arthropod Skin Diseases

  • Scabies: Caused by Sarcoptes scabiei, leading to intense itching and inflammation, treated with permethrin

  • Pediculosis (Lice): Included head lice (Pediculus humanus capitis) and body lice (P. corporis); treated with topical insecticides