Mechanisms of Microbial Pathogenicity

Factors Required for Microbial Disease

To cause disease, a microorganism must complete several steps to overcome host defenses and establish infection.

• Portals of Entry: Microbes must enter the host through specific sites, such as mucous membranes, skin, or via the parenteral route (e.g., injections, bites).

• Adherence: Microbes attach to host tissues using adhesins/ligands that bind to host cell receptors.

• Penetration/Evasion of Host Defenses: Pathogens resist removal and immune responses, such as phagocytosis.

• Damage to Host Cells: Pathogens cause direct damage or produce toxins.

• Portals of Exit: Microbes leave the host to infect new hosts, often via the same routes as entry.

Portals of Entry and Exit

• Mucous Membranes: Respiratory tract (most common), gastrointestinal tract, genitourinary tract, conjunctiva.

• Skin: Usually impenetrable except through hair follicles, sweat glands, or by boring organisms (e.g., hookworms).

• Parenteral Route: Direct deposition beneath skin/membranes (e.g., injections, cuts, bites, surgery).

• Portals of Exit: Respiratory tract (coughing/sneezing), GI tract (feces/saliva), genitourinary tract, skin, blood (needles, biting insects).

Infectious and Lethal Doses

• ID50 (Infectious Dose): Number of microbes required to cause infection in 50% of test animals.

• LD50 (Lethal Dose): Number of microbes required to kill 50% of test animals.

• Distinction: ID50 measures infectivity; LD50 measures lethality/toxicity.

Adherence to Host Cells

Adherence is mediated by interactions between microbial adhesins/ligands (e.g., LPS, pili, capsules) and host cell receptors (carbohydrates or peptides).

Microbial Structures and Enzymes Contributing to Pathogenicity

• Capsules: Prevent phagocytosis, increasing virulence.

• Cell Wall Components: M protein (attachment, resists phagocytosis), Opa protein (inhibits T helper cells), mycolic acid (resists digestion).

• Enzymes (Exoenzymes): Leukocidins (destroy WBCs), coagulases (form clots), hyaluronidase (breaks down connective tissue), invasins (rearrange host actin for entry).

Host Tissue Damage

• Direct Damage: Pathogen metabolism and multiplication kill host cells.

• Toxin Production: Microbes produce toxins causing symptoms such as fever, diarrhea, shock, or neurological effects.

Exotoxins vs. Endotoxins

Exotoxins and endotoxins are two major classes of bacterial toxins with distinct properties.

Types of Exotoxins

• A-B Toxins (Type III): Two-part toxins; A (active) destroys cell machinery, B (binding) attaches to host. Examples: Botulinum, Tetanus, Vibrio enterotoxin.

• Membrane-Disrupting Toxins (Type II): Lyse host cells by forming protein channels. Examples: Leukocidins, hemolysins.

• Superantigens (Type I): Cause excessive cytokine release, leading to fever, shock, and death. Example: TSST-1.

Endotoxins: Fever, Shock, and Detection

• Fever: Endotoxins stimulate macrophages to release cytokines (IL-1, TNF-α), which induce fever via the hypothalamus.

• Shock: TNF-α increases capillary permeability, causing septic shock.

• LAL Assay: Limulus amoebocyte lysate test detects endotoxins using horseshoe crab blood, forming a gel-clot in presence of endotoxin.

Pathogenic Properties of Viruses

• Cytopathic Effects (CPE): Include inhibition of macromolecular synthesis, formation of inclusion bodies, syncytia (giant cells), antigenic/chromosomal changes, and transformation (loss of contact inhibition).

• Examples: Rabies (inclusion bodies), HIV/Measles (syncytia).

Principles of Pathogenesis and Epidemiology

Core Definitions

• Pathogenesis: Study of disease development.

• Infection: Growth of microorganisms in/on the body.

• Disease: Condition resulting from infection, especially when microbes are in abnormal locations.

• Pathology: Study of disease (not explicitly defined in the excerpt).

Microbiota and Symbiotic Relationships

• Normal Microbiota: Permanent, beneficial residents of the body; provide nutrients and protection.

• Transient Microbiota: Temporary residents; may become opportunistic pathogens.

• Commensalism: One organism benefits, the other is unaffected.

• Mutualism: Both organisms benefit.

• Parasitism: One benefits at the other's expense, often causing disease.

Classification and Occurrence of Diseases

• Communicable Disease: Spread from host to host.

• Noncommunicable Disease: Not spread between hosts.

• Incidence: Number of new cases in a time period.

• Prevalence: Total cases (old and new) at a specific time.

• Sporadic Disease: Occurs occasionally.

• Endemic Disease: Constantly present in a region.

• Epidemic Disease: Many cases in a short time in a region.

• Pandemic Disease: Worldwide epidemic.

Extent of Host Involvement

• Local Infection: Confined to a specific area.

• Systemic Infection: Microbes/toxins spread throughout the body.

• Focal Infection: Systemic infection that began locally.

• Bacteremia: Bacteria in the blood.

• Septicemia: Bacterial growth in the blood.

• Toxemia/Viremia: Toxins/viruses in the blood (not explicitly defined in excerpt).

• Primary Infection: Initial acute infection.

• Secondary Infection: Caused by opportunists after primary infection weakens host.

• Subclinical Disease: No noticeable illness; carrier state possible.

Stages of Disease

1. Incubation Period: Time between infection and first symptoms.

2. Prodromal Period: Mild, non-specific symptoms begin.

3. Period of Illness: Most severe symptoms; peak intensity (acme).

4. Period of Decline: Symptoms subside as pathogen is controlled.

5. Period of Convalescence: Recovery and return to health.

Reservoirs and Transmission

• Reservoir of Infection: Source of pathogen (living or non-living).

• Human Reservoirs: Carriers (e.g., HIV, Typhoid).

• Animal Reservoirs: Zoonoses (e.g., Rabies, Salmonella).

• Non-living Reservoirs: Soil, water, food (e.g., botulism, tetanus).

• Transmission Methods:

  • Contact: Direct, indirect (fomites), droplet (within 1 meter).

  • Vehicle: Water, food, air (over 1 meter).

  • Vectors: Mechanical (on body), biological (inside vector).

  • Vertical: Mother to offspring (transplacental, perinatal, breastfeeding).

  • Horizontal: Person-to-person (contact, vehicle, vector).

Nosocomial (Healthcare-Associated) Infections

• Definition: Infections acquired in healthcare settings.

• Prevalence: Affect ~2 million/year in the US; ~90,000 deaths.

• Compromised Host: Weakened resistance (disease, therapy, surgery).

• Transmission: Surgery, catheters, staff-patient contact.

• Prevention: Universal precautions, handwashing, instrument cleaning, disposable supplies, infection control committees.

Emerging Infectious Diseases (EID)

• Contributing Factors: New strains, antibiotic misuse, climate change, travel, ecological disruption, public health failures.

• Examples: Hantavirus, West Nile, Ebola.

Molecular Foundations of Biotechnology and Recombinant DNA Technology

Core Definitions

• Biotechnology: Use of organisms or cell components to produce products (e.g., antibiotics, enzymes).

• Recombinant DNA (rDNA) Technology: Insertion/modification of genes to produce desired proteins.

• Genome: Entire genetic content of an organism.

• Selection: Culturing microbes that naturally produce a desired product.

• Site-Directed Mutagenesis: Changing specific DNA sequences to alter proteins.

Tools of Recombinant DNA

• Restriction Enzymes: Cut DNA at specific sequences, often producing sticky ends for joining DNA fragments.

• DNA Ligase: Joins DNA fragments to create recombinant molecules.

• Vectors: DNA molecules (plasmids, viruses) used to transfer genes into host cells.

• Marker Genes: Allow identification of cells that have taken up the vector (e.g., antibiotic resistance).

Key Laboratory Techniques

• Polymerase Chain Reaction (PCR): Amplifies DNA segments.

  • Denaturation: 94°C, separates DNA strands.

  • Annealing: 60°C, primers bind to target.

  • Extension: 72°C, DNA polymerase synthesizes new DNA.

• Complementary DNA (cDNA): Synthesized from mRNA using reverse transcriptase; lacks introns, suitable for bacterial expression.

• Blue-White Screening: Identifies bacteria with recombinant plasmids using X-gal and ampicillin; white colonies contain foreign DNA.

DNA Analysis and Identification

• DNA Fingerprinting: Identifies organisms by unique DNA patterns.

• Southern Blotting: Detects specific DNA sequences using gel electrophoresis and labeled probes.

• DNA Probes: Short, labeled DNA sequences that hybridize to target genes.

• Microarrays: Measure expression of thousands of genes simultaneously.

New Technologies and Applications

• CRISPR-Cas9: Genome editing tool using guide RNA and Cas9 protein to cut DNA at specific sites.

• Therapeutic Applications: Production of human enzymes, subunit vaccines, DNA vaccines, gene therapy.

• Forensic Microbiology: Uses real-time PCR and RT-PCR for rapid identification of pathogens.

Fundamentals of Microbial Genetics and Gene Expression

Definitions and Distinctions

• Genetics: Study of heredity and genetic information flow.

• Gene: DNA segment coding for a protein.

• Genetic Code: Triplet codons specify amino acids.

• Gene Expression: DNA directs protein synthesis via transcription and translation.

DNA Replication

• Origin of Replication: Site where replication begins, forming a bubble.

• Helicase: Unwinds DNA at the fork.

• Single-Strand Binding Protein: Stabilizes unwound DNA.

• Topoisomerase: Relieves overwinding ahead of fork.

• Primase: Synthesizes RNA primer for DNA polymerase.

• DNA Polymerase: Adds nucleotides to 3' end; DNA pol III (synthesis), DNA pol I (replaces primers).

• Leading Strand: Synthesized continuously toward fork.

• Lagging Strand: Synthesized in Okazaki fragments away from fork.

• DNA Ligase: Joins Okazaki fragments.

Transcription and Translation

• Transcription: RNA polymerase synthesizes RNA from DNA template, starting at promoter and ending at terminator.

• Translation: Ribosomes use mRNA to assemble proteins.

  • mRNA: Carries codons.

  • tRNA: Brings amino acids; anticodon pairs with mRNA codon.

  • rRNA: Forms ribosome structure; A, P, E sites for tRNA binding.

  • Start Codon: AUG (methionine).

  • Stop Codons: Signal end of translation.

Genetic Code and RNA Processing

• Degenerate Code: 64 codons for 20 amino acids; redundancy exists.

• RNA Processing:

  • Eukaryotes: 5' cap, 3' poly-A tail, splicing (removal of introns).

  • Prokaryotes: No cap/tail or splicing; transcription and translation are coupled.

Regulation of Gene Expression

• Repression (trp operon): End product (tryptophan) binds repressor, blocking transcription (anabolic pathways).

• Induction (lac operon): Inducer (allolactose) inactivates repressor, allowing transcription (catabolic pathways).

• Catabolite Repression: Low glucose increases cAMP, which activates CAP to enhance lac operon transcription.

Mutations and Mutagens

• Mutation: Change in genetic material.

• Mutagen: Agent causing mutations.

• Types of Mutations:

  • Point Mutations: Single base change (silent, missense, nonsense).

  • Frameshift Mutation: Insertion/deletion shifts reading frame.

Genetic Recombination and Transfer

• Transformation: Uptake of naked DNA from environment.

• Transduction: DNA transfer via bacteriophage; generalized (random genes), specialized (genes near prophage).

• Conjugation: DNA transfer via pili (mating bridge) between bacteria.

• Vertical Transfer: Parent to offspring.

• Horizontal Transfer: Between cells of same generation.

Plasmids and Transposons

• Plasmids: Small, circular DNA; F plasmid (conjugation), R plasmid (antibiotic resistance).

• Transposons: Mobile genetic elements; can move between chromosomes and plasmids, spreading resistance genes.

Additional info: Where definitions or details were not explicit in the source, standard academic context was provided for completeness (e.g., definitions of pathology, toxemia, viremia, and the coupling of transcription/translation in prokaryotes).