Microbial Diseases of the Skin and Eyes: Comprehensive Study Notes

Microbes in Our Lives: The Good and the Bad

Introduction to Microbial Roles

Microorganisms play diverse roles in the environment and human health, with both beneficial and harmful effects.

• Pathogenic microbes: Only a small fraction cause disease in humans.

• Decomposition: Microbes decompose organic waste, recycling nutrients.

• Photosynthesis: Some microbes generate oxygen, supporting life on Earth.

• Industrial production: Microbes produce chemicals (ethanol, acetone, vitamins) and fermented foods (vinegar, cheese, bread).

• Biotechnology: Microbes are used to manufacture products (e.g., cellulase), treat diseases (e.g., insulin), and produce antibiotics (e.g., penicillin).

Structure and Function of the Skin

Skin Anatomy and Protective Features

The skin acts as a physical and chemical barrier to microbial invasion.

• Epidermis: Thin, outer layer composed of epithelial cells; provides protection.

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

• Dermis: Inner, thicker layer composed mainly of connective tissue.

Skin Defenses Against Microbes

• Perspiration: Provides moisture and nutrients, but contains salt that inhibits microbial growth.

• Lysozyme: Enzyme that breaks down bacterial cell walls.

• Antimicrobial peptides: Small proteins that inhibit microbial growth.

• Sebum: Oil gland secretion containing fatty acids that inhibit pathogens.

Normal Microbiota of the Skin

Characteristics and Composition

The skin's normal microbiota is adapted to survive harsh conditions and contributes to health and disease.

• Resistance: Microbiota are resistant to drying and high salt concentrations.

• Gram-positive cocci: Dominant groups include Staphylococci and Micrococci.

• Moisture areas: Higher populations; microbes metabolize sweat, contributing to body odor.

Staphylococcal Skin Infections

Staphylococcus epidermidis

• Ninety percent of normal skin microbiota.

• Healthcare-associated pathogen; produces biofilm on catheters.

• Coagulase-negative.

Staphylococcus aureus

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

• Golden-yellow colonies; coagulase-positive.

• Produces toxins, may cause sepsis, and avoids host defenses by killing phagocytic cells.

• MRSA strains are resistant to antibiotics.

Types of Staphylococcal Infections

• Folliculitis: Infection of hair follicles.

• Sty: Folliculitis of an eyelash.

• Furuncle (boil): Abscess with pus surrounded by inflamed tissue.

• Carbuncle: Deep tissue inflammation from spreading furuncle.

• Impetigo: Crusting sores, spread by autoinoculation.

Severe Staphylococcal Syndromes

• Scalded skin syndrome: Bullous impetigo; Toxin B causes exfoliation.

• Pemphigus neonatorum: Impetigo of the newborn.

• Toxic shock syndrome (TSS): Fever, vomiting, shock, organ failure due to TSST-1 toxin.

Streptococcal Skin Infections

General Features

• Gram-positive cocci in chains; produce hemolysins that lyse red blood cells.

• Beta-hemolytic streptococci often cause disease; differentiated into groups A-T by cell wall carbohydrates.

Group A Streptococci (GAS): Streptococcus pyogenes

• Eighty immunological types.

• Virulence factors:

  • Streptolysins: Lyse RBCs.

  • M proteins: Aid adherence and immune evasion.

  • Hyaluronidase: Dissolves connective tissue.

  • Streptokinases: Dissolve blood clots.

Diseases Caused by GAS

• Erysipelas: Infection of dermal layer; causes tissue destruction and sepsis.

• Necrotizing fasciitis: "Flesh-eating" disease; Exotoxin A acts as a superantigen.

• Streptococcal toxic shock syndrome: Similar to staphylococcal TSS.

Infections by Pseudomonads

Pseudomonas aeruginosa

• Gram-negative, aerobic rod.

• Produces pyocyanin (blue-green pus), exo- and endotoxins; grows in biofilms.

• Pseudomonas dermatitis: Self-limiting rash from swimming pools.

• Otitis externa: "Swimmer's ear."

• Opportunistic in burn patients; resistant to many antibiotics.

Viral Diseases of the Skin

General Features

• Many are transmitted via respiratory routes and are systemic.

• Often cause problems in children and developing fetuses.

Warts

• Papillomas: Small skin growths caused by papillomavirus (over 50 types).

• Some types cause skin and cervical cancers.

• Treated with cryotherapy, electrodesiccation, or salicylic acid.

Chickenpox (Varicella) and Shingles (Herpes Zoster)

• Chickenpox: Caused by herpesvirus varicella-zoster (human herpesvirus 3); transmitted via respiratory route; causes pus-filled vesicles.

• Reye's syndrome: Severe complication; vomiting and brain dysfunction. Aspirin increases risk.

• Virus becomes latent in central nerve ganglia; prevented by live attenuated vaccine.

• Breakthrough varicella: Can occur in vaccinated individuals.

• Shingles: Reactivation of latent virus; moves along peripheral nerves to skin, often due to stress or lowered immunity. Limited to one side of body; causes postherpetic neuralgia. Prevented by zoster vaccine; antivirals may lessen symptoms.

Herpes Simplex

• HSV-1: Spread by oral/respiratory routes; remains latent in trigeminal nerve ganglia. Outbreaks triggered by sun, stress, hormones.

• HSV-2: Spread sexually; remains latent in sacral nerve ganglia.

• Causes cold sores/fever blisters (not canker sores).

• Herpes gladiatorum: Vesicles on skin.

• Herpetic whitlow: Vesicles on fingers.

• Herpes encephalitis: Virus spreads to brain; treated with acyclovir.

Measles (Rubeola)

• Transmitted by respiratory route; causes cold-like symptoms and macular rash.

• Koplik's spots: Red spots on oral mucosa opposite molars.

• Encephalitis in 1/1000 cases; subacute sclerosing panencephalitis (rare, years after recovery).

• Prevented by MMR vaccine; not given to children under 1 year.

Rubella (German Measles)

• Rubella virus; causes macular rash and light fever.

• Transmitted via respiratory route; 2-3 week incubation.

• Congenital rubella syndrome: Fetal damage, deafness, heart defects, mental retardation; 15% mortality.

• Prevented by MMR vaccine; not recommended for pregnant women.

Other Viral Rashes

• Fifth disease (erythema infectiosum): Human parvovirus B19; mild flu-like symptoms, "slapped-cheek" rash.

• Roseola: Human herpesviruses 6 and 7; high fever, body rash, recovery in 1-2 days.

• Hand-foot-mouth disease: Enteroviruses; spread via mucous/saliva (children); fever, sore throat, rash on hands, feet, mouth, tongue.

Fungal Diseases of the Skin and Nails

Cutaneous Mycoses

• Mycosis: Fungal infection of the body.

• Colonize hair, nails, outer epidermis; metabolize keratin.

• Dermatomycoses: Tineas/ringworm.

  • Tinea capitis: Scalp ringworm.

  • Tinea cruris: Jock itch.

  • Tinea pedis: Athlete's foot.

  • Tinea unguium: Ringworm of nails.

• Genera involved: Trichophyton, Microsporum, Epidermophyton.

• Treatment: Topical drugs (miconazole, clotrimazole).

Subcutaneous Mycoses

• More serious; penetrate stratum corneum.

• Usually caused by soil fungi.

• Sporotrichosis: Caused by Sporothrix schenkii; enters wound, forms small ulcer.

Candidiasis

• Overgrowth of Candida albicans (yeast); forms pseudohyphae, resists phagocytosis.

• Occurs in skin, mucous membranes of genitourinary tract and mouth.

• Thrush: Oral cavity infection.

• Results from antibiotic use or pH changes; fulminating disease in immunosuppressed.

Microbial Diseases of the Eye

Inflammation of the Eye Membranes: Conjunctivitis

• Inflammation of conjunctiva; also called red eye or pinkeye.

• Commonly caused by Haemophilus influenzae or adenoviruses.

• Can be caused by pseudomonads (unsanitary contact lenses).

Ophthalmia

• Caused by Neisseria gonorrhoeae; large amount of pus, ulceration of cornea, may lead to blindness.

• Transmitted to newborn during birth; prevented by antibiotic treatment.

Inclusion Conjunctivitis

• Caused by Chlamydia trachomatis; obligate intracellular parasite.

• Transmitted to newborn during birth; spread via swimming pool water.

• Treated with tetracycline.

Trachoma

• Caused by some serotypes of Chlamydia trachomatis; leading cause of blindness worldwide.

• Transmitted via hand contact or flies.

• Infection causes scarring, corneal abrasion, blindness; secondary infections may contribute.

• Treated with oral azithromycin.

Summary Tables: Major Microbial Diseases of Skin and Eyes

Table 1: Selected Viral and Fungal Diseases

Disease	Pathogen	Portal of Entry	Symptoms	Method of Transmission	Treatment
Measles (Rubeola)	Measles virus	Respiratory tract	Macular rash, Koplik's spots	Aerosol	No treatment; prevented by vaccine
Rubella	Rubella virus	Respiratory tract	Macular rash, light fever	Aerosol	No treatment; prevented by vaccine
Fifth Disease	Human parvovirus B19	Respiratory tract	"Slapped-cheek" facial rash	Aerosol	No treatment
Roseola	Human herpesvirus 6, 7	Respiratory tract	High fever, body rash	Aerosol	No treatment
Hand-Foot-Mouth Disease	Enteroviruses	Mouth	Fever, sore throat, rash on hands, feet, mouth, tongue	Aerosol, direct contact	No treatment
Candidiasis	Candida albicans	Skin, mucous membranes	Red, irritated skin or mucosa	Direct contact, endogenous infection	Topical antifungals

Table 2: Selected Bacterial and Viral Diseases

Disease	Pathogen	Portal of Entry	Symptoms	Method of Transmission	Treatment
Impetigo	Staphylococcus, Streptococcus	Skin	Vesicles on skin	Direct contact, fomites	Topical antibiotics
Smallpox	Variola virus	Respiratory tract	Pustules on skin	Aerosol	None
Monkeypox	Monkeypox virus	Respiratory tract	Pustules on skin	Direct contact, aerosol	None
Chickenpox	Varicella-zoster virus	Respiratory tract	Vesicles on face, trunk	Aerosol	Acyclovir, prevention by vaccine
Shingles	Varicella-zoster virus	Endogenous (latent infection)	Vesicles on one side of body	Recurrence of latent infection	Acyclovir, prevention by vaccine
Herpes Simplex	Herpes simplex virus	Skin, mucous membranes	Vesicles around mouth, other areas	Direct contact, recurring latent infection	Acyclovir

Table 3: Selected Bacterial, Fungal, and Parasitic Diseases

Disease	Pathogen	Portal of Entry	Symptoms	Method of Transmission	Treatment
Folliculitis	Staphylococcus aureus	Hair follicle	Infection of hair follicle	Direct contact	Drainage, topical antibiotics
Toxic Shock Syndrome	Staphylococcus aureus	Surgical/nasal wounds	Fever, rash, shock	Endogenous infection	Antibiotics, IV fluids
Necrotizing Fasciitis	Streptococcus pyogenes	Skin abrasions	Extensive soft tissue destruction	Direct contact	Surgical tissue removal, broad-spectrum antibiotics
Erysipelas	Streptococcus pyogenes	Skin	Red patches on skin, fever	Endogenous infection	Cephalosporin
Pseudomonas Dermatitis	Pseudomonas aeruginosa	Skin abrasions	Superficial rash	Swimming water, hot tubs	Self-limiting
Otitis Externa	Pseudomonas aeruginosa	Ear canal	Superficial infection of external ear canal	Swimming water	Topical antibiotics
Ringworm	Microsporum, Trichophyton, Epidermophyton	Skin	Skin lesions of varying appearance	Direct contact, fomites	Topical antifungals
Sporotrichosis	Sporothrix schenkii	Skin	Ulcer at site of infection	Soil	Potassium iodide solution
Scabies	Sarcoptes scabiei (mite)	Skin	Papules, itching	Direct contact	Gamma benzene hexachloride, permethrin
Buruli Ulcer	Mycobacterium ulcerans	Skin	Deep, ulcerating wound	Contaminated water	Antimycobacterial drugs