Microbial Genetics, Pathogenicity, Immunity, Vaccines, and Microbial Diseases of the Skin: Study Notes

Microbial Genetics

Key Terms in Microbial Genetics

• Plasmid: A small, circular, double-stranded DNA molecule found in bacteria and some eukaryotes, independent of chromosomal DNA. Plasmids often carry genes for antibiotic resistance or other survival advantages.

• Haploid: A cell or organism with a single set of unpaired chromosomes. Most bacteria are haploid.

• Nucleotide: The basic building block of nucleic acids (DNA and RNA), consisting of a sugar, a phosphate group, and a nitrogenous base.

• Base Pairing: The specific hydrogen bonding between purines and pyrimidines in DNA and RNA. In DNA: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

• Mutation: A heritable change in the nucleotide sequence of DNA.

Central Dogma of Biology

The central dogma describes the flow of genetic information within a biological system:

• DNA → RNA → Protein

Prokaryotes vs. Eukaryotes:

• Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm, as there is no nuclear membrane.

• Eukaryotes: Transcription occurs in the nucleus, and translation occurs in the cytoplasm. mRNA processing (capping, polyadenylation, splicing) is required before translation.

Horizontal Gene Transfer Mechanisms

Horizontal gene transfer allows bacteria to acquire new genetic material from other organisms, not just from parent to offspring.

1. Transformation: Uptake of naked DNA from the environment by a competent cell.

2. Transduction: Transfer of DNA from one bacterium to another via bacteriophages (viruses that infect bacteria).

3. Conjugation: Direct transfer of DNA between two bacteria through a pilus (cell-to-cell contact), often involving plasmids.

Lysogenic Conversion

Lysogenic conversion occurs when a bacteriophage integrates its DNA into the host genome (prophage), potentially altering the phenotype of the host bacterium (e.g., toxin production).

Mutations and Their Outcomes

• Silent Mutation: Alters a nucleotide but does not change the amino acid sequence due to redundancy in the genetic code.

• Missense Mutation: Results in a different amino acid in the protein sequence.

• Nonsense Mutation: Converts a codon into a stop codon, leading to premature termination of translation.

• Frameshift Mutation: Insertion or deletion of nucleotides that shifts the reading frame, often resulting in a nonfunctional protein.

Selection of Mutants in the Laboratory

• Positive Selection: Detects mutant cells because they grow or appear different (e.g., antibiotic resistance).

• Negative Selection: Detects mutants that cannot grow under certain conditions (e.g., auxotrophs that cannot synthesize a required nutrient).

Operons: General and Specifics (lac and trp)

• Operon: A cluster of genes under the control of a single promoter, transcribed as a unit.

• lac Operon: Inducible operon; controls lactose metabolism in Escherichia coli. Activated in the presence of lactose.

• trp Operon: Repressible operon; controls tryptophan synthesis. Repressed when tryptophan is abundant.

Principles of Disease and Pathogenicity

Key Definitions

• Colonize: The establishment of a microbial population on a host surface without causing disease.

• Infection: Invasion and multiplication of microorganisms in host tissues, which may or may not cause disease.

• Disease: A deviation from health due to infection, resulting in signs and symptoms.

Human Microbiome and Microbial Antagonism

• Human Microbiome: The collection of all microorganisms living in association with the human body.

• Microbial Antagonism: The inhibition of pathogenic microbes by normal microbiota through competition for nutrients and production of inhibitory substances.

Opportunistic Infections

Opportunistic infections occur when normal microbiota or environmental microbes cause disease under certain conditions:

• When the host's immune system is compromised

• When microbes are introduced to unusual body sites

• When normal microbiota are disrupted (e.g., by antibiotics)

Steps to Cause Disease

1. Entry into the host (via portals of entry)

2. Adhesion to host tissues

3. Invasion and evasion of host defenses

4. Damage to host tissues

5. Exit from the host

Portals of Entry

• Endogenous: Originating from within the host (e.g., normal flora entering sterile sites).

• Exogenous: Originating from outside the host (e.g., inhalation, ingestion, wounds).

• Common Portals: Skin, respiratory tract, gastrointestinal tract, urogenital tract, placenta.

• TORCHES: Pathogens that can cross the placenta: Toxoplasma, Other (syphilis, varicella-zoster, parvovirus B19), Rubella, Cytomegalovirus, Herpes simplex virus, Enteroviruses, Syphilis.

• Infectious Dose (ID): The minimum number of microbes required to cause infection.

Adhesion

• Microbes use adhesins (surface molecules) to attach to host cells.

• Adhesion is essential for colonization and infection.

Pathogenicity vs. Virulence

• Pathogenicity: The ability of a microbe to cause disease.

• Virulence: The degree of pathogenicity; often measured by infectious dose or severity of disease.

Phagocytes and Leukocidins

• Phagocytes: White blood cells that engulf and destroy pathogens (e.g., neutrophils, macrophages).

• Leukocidins: Bacterial toxins that destroy phagocytes.

Exotoxins and Endotoxins

• Exotoxins: Proteins secreted by bacteria that cause specific damage to the host (e.g., diphtheria toxin).

• Endotoxins: Lipopolysaccharide (LPS) components of the outer membrane of Gram-negative bacteria, released upon cell death.

AB Toxin Mechanism

• AB toxins consist of two parts: A (active) and B (binding) subunits.

• B subunit binds to host cell; A subunit exerts toxic effect inside the cell.

Membrane Disrupting Toxins

• Cause cell lysis by disrupting the plasma membrane (e.g., hemolysins, leukocidins).

Superantigens

• Exotoxins that cause excessive activation of the immune system, leading to toxic shock (e.g., Staphylococcal toxic shock syndrome toxin).

Enzymes Involved in Pathogenicity

• Coagulase: Clots fibrinogen in blood, protecting bacteria from phagocytosis.

• Kinases: Digest fibrin clots, allowing spread of infection.

• Hyaluronidase: Hydrolyzes hyaluronic acid, aiding tissue penetration.

• Collagenase: Breaks down collagen, facilitating spread.

• IgA protease: Destroys IgA antibodies.

Innate and Adaptive Immunity

Innate vs. Adaptive Immunity

• Innate Immunity: Non-specific, immediate defense mechanisms present at birth (e.g., skin, phagocytes, inflammation).

• Adaptive Immunity: Specific, acquired defense mechanisms involving lymphocytes and antibodies; develops after exposure to antigens.

Lines of Defense

• First Line: Physical and chemical barriers (skin, mucous membranes, secretions).

• Second Line: Internal defenses (phagocytes, inflammation, fever, complement system).

• Third Line: Adaptive immune responses (B cells, T cells, antibodies).

Steps of Phagocytosis

1. Chemotaxis and adherence of microbe to phagocyte

2. Ingestion of microbe by phagocyte

3. Formation of phagosome

4. Fusion of phagosome with lysosome (phagolysosome)

5. Digestion of ingested microbe

6. Discharge of waste materials

Inflammation, Fever, and Complement

• Inflammation: Localized response to infection or injury, characterized by redness, heat, swelling, and pain.

• Fever: Systemic increase in body temperature, inhibits pathogen growth and enhances immune responses.

• Complement System: A group of serum proteins that enhance phagocytosis, lyse pathogens, and promote inflammation.

Why Blood is Hostile to Pathogens

• Contains immune cells, antibodies, complement proteins, and iron-binding proteins that inhibit microbial growth.

Immune Cells in Blood

• Neutrophils

• Lymphocytes (B cells, T cells)

• Monocytes/Macrophages

• Eosinophils

Antigen vs. Antibody

• Antigen: Any substance that elicits an immune response (usually foreign proteins or polysaccharides).

• Antibody: A protein produced by B cells that specifically binds to an antigen.

Antibody-Mediated Destruction of Pathogens

• Neutralization

• Opsonization (enhanced phagocytosis)

• Activation of complement

• Agglutination

Vaccines and Immunity

Specificity and Memory

• Specificity: The immune system targets specific antigens.

• Memory: The immune system "remembers" previous encounters, leading to a faster, stronger response upon re-exposure.

Primary vs. Secondary Immune Response

• Primary Response: The initial immune response to an antigen; slower and less robust.

• Secondary Response: Subsequent exposure leads to a faster and more effective response due to memory cells.

Types of Immunity

Type	Definition	Example
Natural Active	Immunity from natural exposure to antigen	Recovery from infection
Natural Passive	Transfer of antibodies from mother to child	Placental IgG, breast milk IgA
Artificial Active	Immunity from vaccination	MMR vaccine
Artificial Passive	Injection of antibodies	Antitoxin administration

Vaccines

• Definition: Preparations containing antigens that stimulate an immune response to confer protection against specific diseases.

• Types of Vaccines:

  • Live attenuated (weakened pathogens)

  • Inactivated (killed pathogens)

  • Subunit (purified components)

  • Toxoid (inactivated toxins)

  • Conjugate (linked to carrier proteins)

  • mRNA vaccines (genetic instructions for antigen)

• What Happens After Injection: The immune system recognizes the antigen, mounts a response, and forms memory cells.

• Who Can Get Vaccines: Most healthy individuals; some vaccines are contraindicated in immunocompromised or pregnant individuals (depends on vaccine type).

• Immunity: Vaccination leads to active, long-lasting immunity.

Common Vaccine Myths (Vaccine Hesitancy)

• Vaccines cause autism (debunked by extensive research)

• Vaccines contain harmful toxins (ingredients are present in safe amounts)

• Natural infection is better than vaccination (natural infection carries higher risks)

Microbial Diseases of the Skin

Main Bacteria, Viruses, and Fungi Causing Skin Disease

• Bacteria: Staphylococcus aureus, Streptococcus pyogenes, Pseudomonas aeruginosa

• Viruses: Varicella-zoster virus (chickenpox), Herpes simplex virus, Human papillomavirus (warts)

• Fungi: Dermatophytes (e.g., Trichophyton species), Malassezia

Virulence Factors in Skin Infections

• Enzymes (e.g., coagulase, hyaluronidase)

• Toxins (e.g., exfoliative toxin in S. aureus)

• Capsules (inhibit phagocytosis)

Clinical Presentation and Epidemiology

• Skin diseases can present as abscesses, cellulitis, impetigo, rashes, or ulcers.

• Susceptible populations include children, immunocompromised individuals, and those with skin injuries.

• Appearance is determined by the pathogen and host response (e.g., pus formation, blistering, necrosis).

Diagnosis Based on Symptoms and Lab Results

• Staphylococcus aureus: Abscesses, golden-yellow colonies, positive coagulase test.

• Streptococcus pyogenes: Erysipelas, impetigo, beta-hemolytic colonies on blood agar.

• Pseudomonas aeruginosa: Green pigment, fruity odor, common in burn wounds.

• Varicella-zoster virus: Vesicular rash, "dew drop on a rose petal" appearance.

• Dermatophytes: Ring-shaped lesions, positive KOH prep for hyphae.

Additional info: Where details were not explicit in the original notes, standard textbook explanations and examples were provided for completeness and clarity.