Microbial Genetics

DNA Structure and Function

DNA (deoxyribonucleic acid) is the hereditary material in all cellular organisms. Its structure and replication are central to microbial genetics.

• DNA Structure: Double helix composed of two antiparallel strands held together by complementary base pairing (A–T, G–C).

• Nucleotide Components: Each nucleotide contains a deoxyribose sugar, phosphate group, and nitrogenous base.

• Antiparallel Strands: One strand runs 5' to 3', the other 3' to 5'.

• Gene: A segment of DNA that encodes a functional product, usually a protein.

Example: The gene for β-galactosidase in Escherichia coli encodes an enzyme that breaks down lactose.

Central Dogma: Replication, Transcription, and Translation

The flow of genetic information follows the central dogma: DNA → RNA → Protein.

• Replication: Copying DNA to produce identical DNA molecules. Key enzyme: DNA polymerase.

• Transcription: Synthesis of RNA from a DNA template. Key enzyme: RNA polymerase.

• Translation: Synthesis of proteins using mRNA as a template. Involves mRNA, tRNA (with anticodons), and ribosomes.

Example: The coding DNA strand sequence 5'-ATG-3' is transcribed to mRNA 5'-AUG-3', which is translated to the amino acid methionine.

Enzymes in Genetic Processes

• DNA Polymerase: Synthesizes new DNA strands.

• DNA Ligase: Joins DNA fragments (e.g., Okazaki fragments).

• DNA Gyrase: Relieves supercoiling during replication (target of some antibiotics).

• RNA Polymerase: Synthesizes RNA from DNA template.

• Transposase: Catalyzes movement of transposable elements (jumping genes).

Bacterial Genetic Variation

• Mutation: Heritable change in DNA sequence.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA between bacteria via cell-to-cell contact (often involves plasmids).

Generalized Transduction: Any bacterial gene may be transferred by a phage due to accidental packaging of host DNA.

Gene Regulation: Operons

• Inducible Operons: Gene clusters regulated together; transcription is turned on by an inducer.

• Key Elements: Promoter (RNA polymerase binding site), operator (repressor binding site), repressor (blocks transcription), inducer (removes repressor).

Example: The lac operon in E. coli is induced by lactose, allowing genes for lactose metabolism to be expressed.

Viruses, Viroids, and Prions

Nature and Structure of Viruses

Viruses are acellular infectious agents that require host cells for replication.

• Components: Genome (DNA or RNA), capsid (protein coat), and sometimes an envelope (lipid membrane derived from host).

• Viral Morphologies: Helical, polyhedral (icosahedral), complex (e.g., bacteriophages).

Viral Replication Cycle

• Attachment: Virus binds to host cell surface.

• Entry: Virus or genome enters host cell.

• Uncoating: Viral genome is released (if applicable).

• Biosynthesis: Viral genome is replicated and viral proteins are synthesized.

• Assembly/Maturation: New virions are assembled.

• Release: Virions exit the cell by lysis (cell bursts) or budding (acquires envelope).

Example: Influenza virus buds from the host cell, acquiring an envelope in the process.

Lysogeny, Latency, and Antigenic Shift

• Lysogeny: Phage DNA integrates into bacterial chromosome (prophage); can be reactivated to enter lytic cycle.

• Latency: Animal viruses (e.g., herpes simplex) can persist in host cells without producing new virions, reactivating later.

• Antigenic Shift: Segmented viral genomes (e.g., influenza) can reassort when multiple strains infect the same cell, leading to new combinations.

Viroids and Prions

• Viroids: Small, circular RNA molecules that infect plants; lack protein coat.

• Prions: Infectious proteins that cause neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

Principles of Disease and Epidemiology

Host–Microbe Relationships

• Symbiosis: Close association between two species.

• Types:

  • Mutualism: Both benefit.

  • Commensalism: One benefits, other unaffected (may become opportunistic pathogen).

  • Parasitism: One benefits at the other's expense.

• Normal/Resident Microbiota: Permanent microbes that protect against pathogens.

• Transient Microbiota: Temporary; may be removed by competition or immune response.

Koch’s Postulates

• Framework for linking specific microbes to specific diseases.

• Four steps: (1) Microbe found in diseased but not healthy hosts; (2) Isolate and grow in pure culture; (3) Cause disease in healthy host; (4) Re-isolate same microbe.

Transmission and Epidemiological Terms

• Transmission Routes: Respiratory droplet, airborne, foodborne, direct contact, vector-borne.

• Healthcare-Associated Infections (HAIs): Infections acquired in healthcare settings; often involve opportunistic organisms and antimicrobial resistance.

• Herd Immunity: When enough individuals are immune (e.g., via vaccination), disease transmission is reduced in the population.

Innate Immunity

First-Line Defenses

• Physical Barriers: Intact skin, mucous membranes, mucociliary clearance.

• Chemical Barriers: Lysozyme (breaks down bacterial cell walls), acidic secretions (e.g., stomach acid).

Note: Deep wounds bypass surface barriers, requiring internal defenses.

Cellular Defenses

• Neutrophils: Rapid phagocytosis of microbes.

• Monocytes/Macrophages: Phagocytosis and antigen presentation.

• Eosinophils: Defense against helminths (parasitic worms).

• Basophils: Release inflammatory mediators (e.g., histamine).

• Natural Killer (NK) Cells: Kill infected or abnormal host cells.

Pattern Recognition and Complement System

• Toll-like Receptors (TLRs): Recognize pathogen-associated molecular patterns (PAMPs) such as LPS, flagellin, and peptidoglycan.

• Complement System: Cascade of proteins activated by antigen–antibody complexes (classical pathway); leads to opsonization, inflammation, and membrane attack complex (MAC) formation.

Inflammation and Other Responses

• Inflammation: Mediator release → vasodilation/increased permeability → leukocyte recruitment (margination and diapedesis) → phagocytosis → tissue repair.

• Interferons: Antiviral signaling proteins.

• Fever: Systemic response; can inhibit pathogens but extreme hyperthermia is dangerous.

Adaptive Immunity

Types and Acquisition of Immunity

• Humoral Immunity: Mediated by B cells and antibodies.

• Cell-Mediated Immunity: Mediated by T cells (helper, cytotoxic, regulatory).

• Active Immunity: Immune response generated by exposure to antigen (natural infection or vaccination).

• Passive Immunity: Transfer of antibodies (e.g., maternal antibodies, antiserum).

• Natural vs. Artificial: Natural (infection or maternal transfer); artificial (vaccination or antibody injection).

Antigens, Epitopes, and Antibodies

• Antigen: Substance that elicits an immune response.

• Epitope: Specific region of antigen recognized by antibody or T-cell receptor.

• Antibody Structure: Y-shaped molecule with two heavy and two light chains, variable (antigen-binding) and constant (Fc) regions, stabilized by disulfide bonds.

Immunoglobulin Classes

Interpretation Note: IgM can suggest a recent infection, but clinical context is essential.

T Cells and Immune Responses

• Helper T Cells (TH): Secrete cytokines to activate other immune cells; TH1 cells promote cellular responses.

• CD8+ Cytotoxic T Lymphocytes (CTLs): Kill infected or abnormal cells.

• Regulatory T Cells: Suppress immune responses to prevent autoimmunity.

Primary vs. Secondary Immune Response

• Primary Response: Initial exposure; lag phase before antibody production; mainly IgM, then IgG.

• Secondary Response: Faster, stronger response upon re-exposure; mainly IgG due to immunological memory.

Example: Vaccination primes the immune system for a rapid secondary response upon real infection.

Final Preparation Checklist

• Explain genetic enzyme functions and gene-transfer mechanisms.

• Practice codon/anticodon and DNA-to-RNA sequence problems.

• Identify virus morphology and order replication events.

• Compare latency, lysogeny, lysis, and budding.

• Define epidemiological terms and identify transmission routes.

• Explain innate immune cells, TLRs, complement, and inflammation.

• Identify antibody classes and regions in diagrams.

• Compare immunity acquisition and primary/secondary immune responses.

Study Tip: Review labeled diagrams of viral structure, genetic code, antibody structure, and immune response graphs for visual reinforcement.