Microbial Infections of the Skin and Eyes

Qualities of the Skin and Mucous Membranes That Protect Against Bacterial Invasion

The skin and mucous membranes serve as the body's primary barriers against microbial invasion, employing both physical and chemical defenses.

• Skin:

  • Keratin: A tough, waterproof protein forming a barrier to pathogens.

  • Dry Surface: Inhibits microbial growth due to lack of moisture.

  • Sebum: Contains fatty acids that inhibit the growth of some pathogens.

  • Lysozyme: An enzyme that breaks down bacterial cell walls, especially effective against Gram-positive bacteria.

  • Salt in Sweat: Creates a hypertonic environment that inhibits microbial growth.

  • Antimicrobial Peptides: Small proteins that disrupt microbial membranes.

• Mucous Membranes:

  • Mucus: Traps microbes and particles.

  • Cilia: Hair-like structures that move mucus and trapped microbes out of the body.

  • Acidic Environment: Many mucosal surfaces are acidic, inhibiting microbial growth.

  • Tears: Contain lysozyme, providing additional antimicrobial action.

Normal Skin Microbiota

The skin hosts a diverse microbiota that is adapted to its harsh environment and can prevent colonization by pathogens.

• Resistant to drying and high salt concentrations.

• Dominated by Gram-positive cocci such as Staphylococcus and Micrococcus.

• Moist areas have higher microbial populations.

• Gram-positive pleomorphic rods: e.g., Cutibacterium (formerly Propionibacterium).

• Yeasts: e.g., Malassezia furfur.

Terms Describing Microbial Diseases of the Skin

Specific terms are used to describe the appearance and nature of skin lesions caused by microbial infections.

• Vesicles: Small, fluid-filled lesions.

• Bullae: Vesicles larger than 1 cm in diameter.

• Macules: Flat, reddened lesions.

• Papules: Raised lesions.

• Pustules: Raised lesions containing pus.

• Exanthem: Skin rash arising from a disease.

• Enanthem: Rash on mucous membranes arising from a disease.

Staphylococcal Pathogenicity Factors and Diseases

Staphylococcus species possess several virulence factors that enable them to cause a range of skin diseases.

• Pathogenicity Factors:

  • Coagulase: Clots fibrin to protect bacteria from immune cells.

  • Toxins: Damage host tissues and cells.

  • Enzymes: Destroy tissues and facilitate spread.

  • Immune Evasion: Mechanisms to avoid phagocytosis and immune detection.

  • Destruction of Phagocytes: Kill immune cells.

• Diseases Caused:

  • Folliculitis (infection of hair follicles)

  • Sty (infection of eyelash follicle)

  • Furuncle (boil)

  • Carbuncle (cluster of boils)

  • Impetigo (crusting sores, often in children)

  • Scalded skin syndrome

  • Toxic shock syndrome

Streptococcal Pathogenicity Factors and Diseases

Streptococcus pyogenes is a major cause of skin and soft tissue infections, utilizing several virulence factors.

• Pathogenicity Factors:

  • Streptolysins: Destroy red blood cells (hemolysis).

  • M Proteins: Inhibit phagocytosis and immune recognition.

  • Hyaluronidase: Breaks down connective tissue, aiding spread.

  • Streptokinases: Dissolve blood clots.

  • Pyrogenic Toxins: Act as superantigens, causing systemic effects.

• Diseases Caused:

  • Erysipelas (acute infection of upper dermis)

  • Necrotizing fasciitis ("flesh-eating disease")

  • Streptococcal toxic shock syndrome

Pseudomonas aeruginosa Pathogenicity and Diseases

Pseudomonas aeruginosa is an opportunistic pathogen notable for its resistance and ability to form biofilms.

• Pathogenicity Factors:

  • Exotoxins and endotoxins

  • Pyocyanin pigment (produces blue-green pus)

  • Biofilm formation

  • Antibiotic resistance

• Diseases Caused:

  • Pseudomonas dermatitis ("hot tub rash")

  • Otitis externa ("swimmer’s ear")

  • Opportunistic infections in burns and cystic fibrosis patients

Acne: Most Common Skin Disease

Acne is the most prevalent skin disease in humans, primarily caused by the bacterium Cutibacterium acnes (formerly Propionibacterium acnes).

• Causative Agent: Cutibacterium acnes

Viral Diseases of the Skin

Several viruses cause characteristic skin diseases, often with distinct modes of transmission and symptoms.

Fungal Diseases (Mycoses) of the Skin

Mycoses are classified based on the depth of infection in the skin.

• Cutaneous Mycoses: Affect outer skin, hair, and nails. Example: Tinea (ringworm).

• Subcutaneous Mycoses: Penetrate deeper into tissues. Example: Sporotrichosis caused by Sporothrix schenckii.

Parasitic Skin Diseases: Scabies and Pediculosis

Parasitic infestations of the skin are common and have distinct causative agents, symptoms, and treatments.

Microbial Diseases of the Eye

Several bacteria and viruses can infect the eye, leading to conjunctivitis and other conditions.

• Pink Eye (Conjunctivitis): Caused by Haemophilus influenzae and adenoviruses.

• Ophthalmia Neonatorum: Caused by Neisseria gonorrhoeae, typically acquired during birth.

• Trachoma:

  • Causative Agent: Chlamydia trachomatis

  • Transmission: Hand contact and flies

  • Significance: Leading cause of blindness worldwide