Bacterial Skin Infections, Immune System Disorders, and Vaccine Development: Microbiology Study Notes

Chapter 17 – Bacterial Skin Infections (Skin & Eye Infections)

Normal Skin Defenses

The skin provides multiple physical and chemical barriers to protect against microbial invasion. These defenses are crucial in preventing infections by various pathogens.

• Physical & Chemical Barriers:

  • Melanin (UV protection, antimicrobial)

  • Perspiration (acidic pH, high salt, lysozyme)

• Microbiome: Diverse, varies by body site; includes Staphylococcus, Pseudomonas, Janthinobacterium, and Malassezia.

• Infection Risk Factors: Age, occupation, hygiene, antibiotic use, cosmetics, hormones.

Staphylococcus Causes Infections

Staphylococcus species are common skin pathogens, responsible for a variety of infections ranging from mild to severe.

• General Features: Gram-positive cocci in clusters; catalase +, coagulase +, beta-hemolytic, mannitol fermenter.

• Virulence Factors: Coagulase, exotoxins, enzymes, biofilm formation.

• Diseases:

  • Impetigo: Pus-filled vesicles, honey-colored crusts; highly contagious; topical antibiotics.

  • Cellulitis: Infection of dermis/subcutaneous tissue → red, swollen, painful skin; fever; may spread to lymph nodes; MRSA risk.

  • Folliculitis: Infected hair follicles → boils/furuncles; prevented with good hygiene.

Streptococcus Pyogenes Infections

Streptococcus pyogenes is a Gram-positive bacterium that can cause a range of skin infections, some of which may be life-threatening.

• Characteristics: Gram-positive cocci in chains, catalase –, beta-hemolytic; produces many virulence factors.

• Diseases:

  • Impetigo, cellulitis, necrotizing fasciitis (flesh-eating disease), streptococcal toxic shock (toxin-mediated shock, rash, organ failure).

Pseudomonas Aeruginosa Infections

Pseudomonas aeruginosa is an opportunistic pathogen, notable for its antibiotic resistance and ability to cause severe infections in compromised hosts.

• Features: Gram-negative rod; opportunistic; highly antibiotic-resistant; forms biofilm.

• Diseases: Wound and burn infections (HAIs); blue-green pus (pyocyanin pigment); may cause systemic infection.

• Treatment: Wound debridement, antibiotics, silver-based dressings.

Gas Gangrene (Clostridium perfringens)

Clostridium perfringens is an anaerobic, spore-forming bacterium that can cause rapidly progressing tissue necrosis.

• Organism: Anaerobic, endospore-forming, Gram-positive rod.

• Pathogenesis: Deep wound infection → tissue necrosis and gas formation; can progress to shock, renal failure, death.

Chapter 13 – Immune System Disorders

Overview

Immune system disorders arise from abnormal immune responses, including overreactions (hypersensitivity) and weakened responses (immunodeficiency).

• Immune Dysfunctions:

  • Hypersensitivities: Overreactions to harmless antigens.

  • Autoimmune diseases: Attack on self-tissues.

  • Immunodeficiency: Weakened or absent responses.

• Hygiene Hypothesis: Reduced microbial exposure → poor immune training → increased allergies & autoimmunity.

Four Types of Hypersensitivity – “ACID”

Hypersensitivity reactions are classified into four types based on their mechanisms and clinical manifestations.

Type	Mechanism	Examples
I – Allergy / Anaphylaxis	IgE-mediated, mast-cell degranulation (histamine release)	Hay fever, asthma, anaphylaxis
II – Cytotoxic	IgG/IgM bind to cell-surface antigens → complement activation → cell lysis or receptor alteration	Transfusion reaction, HDN, Graves’, Myasthenia gravis
III – Immune Complex	Antigen-antibody complexes deposit in tissues → inflammation	Serum sickness, lupus, rheumatoid arthritis
IV – Delayed	T-cell-mediated, cytokine-driven inflammation	Contact dermatitis, TB test, transplant rejection

Type I – Allergy & Anaphylaxis

• Mechanism: Allergen exposure → IgE binds mast cells; subsequent exposure → allergen binds IgE → histamine release.

• Symptoms: Sneezing, wheezing, rash, hives, GI upset.

• Severe Reaction: Anaphylaxis: airway swelling, hypotension; treat with epinephrine (EpiPen).

• Common Allergens: Peanuts, milk, shellfish, pollen, etc.

Type II – Cytotoxic

• Mechanism: IgG/IgM bind to host cells → complement activation → cell lysis or receptor alteration.

• Cytotoxic Examples:

  • Transfusion reaction (ABO incompatibility)

  • Hemolytic disease of the newborn (HDN; Rh incompatibility)

  • Drug-induced anemia, Goodpasture’s, Rheumatic heart disease

• Non-cytotoxic Examples:

  • Graves’ disease – receptor overstimulation

  • Myasthenia gravis – receptor blockade

Type III – Immune Complex

• Mechanism: Antigen-antibody complexes deposit in tissues → inflammation.

• Autoimmune: Lupus, rheumatoid arthritis.

• Non-autoimmune: Serum sickness (antitoxin/antivenom).

• Symptoms: Fever, rash, joint pain, kidney inflammation.

Type IV – Delayed T-Cell-Mediated

• Mechanism: CD4+/CD8+ T cells → cytokine-mediated inflammation (24-72 h delay).

• Autoimmune Examples: Guillain-Barré, Hashimoto’s, Type 1 diabetes, Multiple sclerosis, Celiac disease.

• Non-autoimmune Examples:

  • Contact dermatitis (poison ivy, latex)

  • Tuberculin skin test (PPD)

  • Transplant rejection, graft-vs-host disease

• Treatment: Corticosteroids, cytokine blockers, immunosuppressants.

High-Yield Themes

• Mnemonic ACID for hypersensitivities.

• Differentiate antibody-mediated (II-III) vs T-cell-mediated (IV).

• Clinical links:

  • Rh incompatibility → HDN

  • Latex exposure → Type IV dermatitis or Type I allergy.

Early Vaccine Development

Historical Milestones

Vaccines have revolutionized public health by preventing infectious diseases. Their development is rooted in centuries of scientific progress.

• Variolation (China, >1000 years ago): Dried smallpox scabs inhaled → mild infection → immunity.

• Edward Jenner (1796): Cowpox inoculation protected against smallpox; origin of term vaccination (“vacca” = cow).

• Louis Pasteur (1800s): Developed vaccines for rabies and anthrax; advanced germ theory.

Impact of Vaccination

• Over 25 vaccine-preventable diseases

• Types of vaccines: live-attenuated, inactivated, subunit, toxoid, conjugate, mRNA (Additional info: mRNA vaccines are a recent innovation, e.g., COVID-19 vaccines)

Vaccine Hesitancy & Controversies

• 1800s: Early anti-vaccine movements (religious/personal rights).

• 1998: Fraudulent MMR-autism claim → decline in vaccination → measles resurgence (~1,300 US cases in 2019).

• Public Health Response: Mandatory vaccination for school attendance; personal-belief exemptions removed.

Immunization & Herd Immunity

Immunization protects individuals and communities by reducing the spread of infectious diseases.

• Artificial active immunity: Vaccines create memory B & T cells without causing disease.

• Herd immunity: High vaccination coverage protects non-immunized individuals (infants, immunocompromised).

• CDC Schedule: Routine childhood vaccines cover >15 pathogens; boosters enhance memory.

• Herd Immunity Threshold: 80–95% coverage (higher for measles & pertussis).

High-Yield Themes

• Know Jenner → smallpox, Pasteur → rabies/anthrax.

• Understand herd immunity and its public health role.

• Recognize vaccine misinformation consequences.

• Identify contraindications: pregnancy, immunosuppression.