Microbial Diseases of the Skin and Eyes

Chapter 15 Summary

  • Principal Portals of Entry
      - Define the different pathways through which microbes gain entry into a host.

  • Definitions
      - ID50: Infectious dose for 50% of the population. A measure of virulence that indicates how many organisms are required to cause disease in half of the exposed hosts.
      - LD50: Lethal dose for 50% of the population. A measure of the potency of a toxin that indicates how much toxin is required to kill half of the test subjects.

  • Microbial Adherence to Host Cells
      - Explanation of how microbes attach to host tissues to establish infection, often utilizing structures like fimbriae, pili, and specialized proteins.

  • Pathogenicity Contributions of Capsules and Cell Wall Components
      - Capsules offer protection from phagocytosis.
      - Specific components of the cell wall can aid in adhesion, evasion of immune responses, and overall virulence.

  • Comparison of Enzymatic Effects
      - Coagulases: Enzymes that coagulate blood, aiding in combating host defense.
      - Kinases: Break down fibrin, allowing bacteria to spread from localized infections.
      - Hyaluronidase: Breaks down hyaluronic acid in connective tissue, facilitating the spread of pathogens.
      - Collagenase: Dismantles collagen in connective tissue, aiding in tissue invasion.

  • Antigenic Variation
      - Definition: The ability of pathogens to alter their surface antigens to evade host immune responses.
      - Example: Certain influenza viruses changing their surface proteins.

  • Role of Host Cell Cytoskeleton in Pathogen Entry
      - Explanation of mechanisms by which bacteria utilize host cell cytoskeletal components to enter cells.

  • Avoiding Phagocytosis
      - Identification of six mechanisms including:
        1. Encapsulation
        2. Production of surface proteins that inhibit phagocytosis
        3. Inhibition of phagosome-lysosome fusion
        4. Production of enzymes that destroy phagocytes
        5. Mimicking host molecules to evade detection
        6. Cell wall modifications that hinder adherence.

  • Function of Siderophores
      - Molecules produced by bacteria to scavenge iron from the host, an essential nutrient for microbial growth.

  • Direct Damage vs. Toxin Production
      - Direct damage: Pathogens disrupt normal cell function or cause cell death.
      - Toxins: Produce specific effects on host cells, potentially leading to more significant damage even at low doses.

  • Exotoxins vs. Endotoxins
      - Exotoxins: Soluble proteins released by living bacteria; highly toxic and specific in action (e.g., botulinum toxin).
      - Endotoxins: Lipopolysaccharides from the outer membrane of Gram-negative bacteria; less toxic than exotoxins and cause systemic effects (e.g., fever, septic shock).

  • Mechanisms of Action of Toxins
      - A-B toxins: Consist of an active (A) and binding (B) component; the A component inhibits protein synthesis within host cells.
      - Membrane-disrupting toxins: Form pores in host cell membranes, causing lysis and cell death.
      - Superantigens: Stimulate an excessive immune response and cytokine release.
      - Genotoxins: Damage DNA, potentially leading to mutations and malignancies.

  • Importance of the LAL Assay
      - Limulus Amebocyte Lysate assay detects bacterial endotoxins in medical devices and pharmaceuticals.

  • Roles of Plasmids and Lysogeny in Pathogenicity
      - Plasmids: Carry genes for antibiotic resistance and virulence factors, enhancing bacterial survival.
      - Lysogeny: Phage integration into bacterial genome can confer new pathogenic traits, including toxins.

  • Cytopathic Effects of Viral Infections
      - List of nine common effects including cell lysis, inclusion body formation, and apoptosis.

  • Symptoms in Other Pathogen Induced Diseases
      - Discussion of the causes of symptoms in fungal (e.g., mycoses), protozoan, helminthic, and algal diseases.

  • Portals of Entry vs. Exit
      - Distinction between how infectious agents enter and leave the host organism.

Chapter 21 - Microbial Diseases of the Skin and Eyes

Structure and Function of the Skin

  • Learning Objective: Describe the structure of the skin and mucous membranes and the ways pathogens can invade them.

  • Epidermis: Thin outer layer of skin, consisting of epithelial cells.

  • Keratin: A waterproofing protein that coats the outer layer of the epidermis, preventing water loss.

  • Dermis: The thicker inner layer composed mainly of connective tissue.

  • Defense Mechanisms:
      - Perspiration: Provides moisture and nutrients for growth, contains salt to inhibit microorganisms, and lysozyme to break down bacterial cell walls.
      - Sebum: Secreted by oil glands; contains fatty acids that inhibit pathogens.

Mucous Membranes

  • Line body cavities open to the exterior, consisting of tightly packed epithelial cells attached to the extracellular matrix.

  • Cells secrete mucus which traps pathogens, and some have cilia that help move mucus out.

  • Often acidic, which can discourage pathogen survival, and the eyes are cleaned by tears containing lysozyme.

Normal Microbiota of the Skin

  • Learning Objective: Provide examples of normal skin microbiota and their ecological roles.

  • Characteristics:
      - Resistant to drying and high salt concentration.
      - Large numbers of gram-positive cocci: including Staphylococci and Micrococci.
      - Microbiota density increases in moist areas; they metabolize sweat and produce body odor.
      - Diphtheroids (e.g., Cutibacterium acnes): Inhabit hair follicles; anaerobic and produce acids that maintain low pH; Corynebacterium xerosis: Aerobic and occupy skin surface.
      - Yeast (Malassezia furfur): Associated with dandruff.

Microbial Diseases of the Skin

  • Different types of skin lesions:
      - Vesicles: Small fluid-filled lesions.
      - Bullae: Vesicles larger than 1 cm in diameter.
      - Macules: Flat, reddened lesions.
      - Papules: Raised lesions.
      - Pustules: Raised lesions with pus.
      - Exanthem: Rash arising from a disease.
      - Enanthem: Rash on mucous membranes arising from a disease.

Staphylococcal Skin Infections

  • Characteristics:
      - Spherical gram-positive bacteria that form clusters.
      - Coagulase-positive/negative: Differentiate via the enzyme coagulase, which clots blood and aids in identification.
      - Staphylococcus epidermidis: Most common skin microbiota, healthcare-associated pathogen, forms biofilm on catheters.
      - Staphylococcus aureus: Carried in nasal passages, responsible for various diseases, avoids host defenses, and can produce toxic proteins.

  • Infections:
      - Folliculitis: Infections of hair follicles.
      - Impetigo: Crusting sores spread by autoinoculation.
      - Scalded Skin Syndrome: Causes exfoliation via Toxin B.
      - Toxic Shock Syndrome (TSS): Results from TSST-1, leading to severe systemic effects.

Streptococcal Skin Infections

  • Characteristics: Gram-positive cocci in chains; beta-hemolytic group A streptococci known as Streptococcus pyogenes.

  • Virulence factors include streptolysins and hyaluronidase.

  • Erysipelas: Infection of the dermal layer, can lead to sepsis.

  • Necrotizing fasciitis: Aggressive “flesh-eating” disease associated with streptococci toxins.

Infections by Pseudomonads

  • Pseudomonas aeruginosa: Gram-negative, causes opportunistic infections, resistant to many antibiotics, produces blue-green pus.

  • Conditions: Pseudomonas dermatitis from swimming pools and otitis externa.

Viral Diseases of the Skin

  • Warts: Small growths caused by papillomavirus.

  • Smallpox: Caused by orthopoxvirus, completely eradicated.

  • Chickenpox and Shingles: Virus remains latent and can reactivate.

  • Herpes Simplex: Spread via contact, latent infections trigger recurrent outbreaks.

Fungal Diseases of the Skin and Nails

  • Cutaneous Mycoses: Metabolize keratin in skin and nails, treated with topical drugs.

  • Subcutaneous Mycoses: More serious infections, usually soil-borne.

  • Candidiasis: Overgrowth of Candida, often occurs when normal microbiota are disrupted.

Microbial Diseases of the Eye

  • Conjunctivitis: Inflammation of conjunctiva; causes include Haemophilus influenzae.

  • Ophthalmia Neonatorum: Caused by Neisseria gonorrhoeae, can lead to blindness.

  • Trachoma: Leading cause of blindness caused by Chlamydia trachomatis.

Conclusion

  • Neglected Tropical Diseases: WHO goals for prevention and management strategies including enhanced sanitation and vector control.