Microbial Genetics

Central Dogma of Genetics

The central dogma describes the flow of genetic information within a cell: DNA is transcribed into messenger RNA (mRNA), which is then translated into proteins.

• Transcription: DNA → mRNA

• Translation: mRNA → Protein (amino acid sequence)

• Proteins: Carry out cellular functions and structure

• Example: Escherichia coli uses the central dogma to produce enzymes for lactose metabolism.

Prokaryotic vs Eukaryotic Genetic Processes

Genetic processes differ between prokaryotes and eukaryotes in terms of structure, location, and proofreading mechanisms.

• Chromosome Structure: Prokaryotes have circular chromosomes; eukaryotes have linear chromosomes.

• DNA Location: Prokaryotes: cytoplasm; Eukaryotes: nucleus.

• Proofreading: Eukaryotes generally have more extensive proofreading and mutation editing.

• Transcription/Translation: Prokaryotes can couple transcription and translation; eukaryotes separate these processes.

DNA Replication, Transcription, and Translation

These are the fundamental steps in gene expression and inheritance.

• DNA Replication: DNA is copied before cell division. Key steps: unwinding, primer synthesis, elongation, proofreading.

• Transcription: RNA polymerase synthesizes mRNA from DNA template.

• Translation: Ribosomes read mRNA and assemble amino acids into polypeptides.

Functions of Key Enzymes

• Ligase: Joins DNA fragments (especially Okazaki fragments on lagging strand).

• DNA Polymerase: Synthesizes new DNA strands; also proofreads.

• Topoisomerase: Relieves supercoiling during DNA replication.

• DNA Gyrase: A type of topoisomerase in prokaryotes; introduces negative supercoils.

• RNA Polymerase: Synthesizes RNA from DNA template during transcription.

Leading vs Lagging Strand Synthesis

• Leading Strand: Synthesized continuously in the direction of replication fork.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments, later joined by ligase.

Nucleotide Components

• Three Components: 1) Phosphate group, 2) Pentose sugar (deoxyribose or ribose), 3) Nitrogenous base (A, T, G, C, U)

Types of Mutations

• Silent Mutation: No change in amino acid sequence.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Introduces a stop codon, truncating protein.

• Frameshift Mutation: Insertion or deletion shifts reading frame, altering downstream sequence.

Plasmids

• Definition: Small, circular DNA molecules in prokaryotes.

• Advantages: Carry genes for antibiotic resistance, virulence, or metabolism.

Semiconservative Replication

• Definition: Each new DNA molecule contains one original and one new strand.

Types of RNA

• Messenger RNA (mRNA): Carries genetic code from DNA to ribosome.

• Transfer RNA (tRNA): Brings amino acids to ribosome during translation.

• Ribosomal RNA (rRNA): Structural and catalytic component of ribosomes.

Operons: Inducible vs Repressible

• Inducible Operon: Usually off; turned on by substrate (e.g., lac operon).

• Repressible Operon: Usually on; turned off by product (e.g., trp operon).

Mutations, Mutagens, Mutants

• Mutation: Change in DNA sequence.

• Mutagen: Agent that causes mutations (e.g., chemicals, radiation).

• Mutant: Organism with a mutation.

Analogs

• Definition: Chemical compounds similar to normal nucleotides; can cause mutations by being incorporated into DNA.

• Example: 5-bromouracil as a thymine analog.

Mutagen Examples and Mutation Repair

• Mutagen Example: UV light causes thymine dimers.

• Repair Mechanisms: Photoreactivation, excision repair, mismatch repair.

Horizontal Gene Transfer: Transformation, Transduction, Conjugation

• Transformation: Uptake of naked DNA from environment.

• Transduction: Transfer of DNA via bacteriophage.

• Conjugation: Direct transfer of DNA via pilus between bacteria.

Controlling Microbial Growth in the Environment

Definitions: Aseptic, Antiseptic, Disinfection, Sterilization, Pasteurization, Degerming

• Aseptic: Procedures to prevent contamination by microbes.

• Antiseptic: Chemicals used on living tissue to reduce microbes.

• Disinfection: Removal of pathogens from inanimate objects.

• Sterilization: Complete elimination of all microbes, including spores.

• Pasteurization: Heat treatment to reduce microbial load in food/drink.

• Degerming: Removal of microbes from surface by mechanical means.

Physical Antimicrobial Methods

• Dry Heat: Incineration or hot air; denatures proteins, oxidizes cell components.

• Moist Heat: Boiling, autoclaving; denatures proteins, disrupts membranes.

• Autoclaving: Steam under pressure; sterilizes by killing spores.

• Refrigeration: Slows microbial growth by lowering temperature.

• Desiccation: Drying; inhibits growth by removing water.

• Lyophilization: Freeze-drying; preserves microbes by removing water under vacuum.

Action of Antimicrobial Agents

• Cell Wall: Disruption leads to cell lysis.

• Cell Membrane: Damage causes leakage of cell contents.

• DNA: Damage prevents replication and transcription.

• Proteins: Denaturation halts cellular metabolism.

Ideal Microbe Characteristics

• Non-pathogenic

• Easy to culture

• Genetically stable

• Useful for research or industry

• Note: No microbe is truly ideal in all respects.

Resistance and Susceptibility

• Endospores: Highly resistant to physical and chemical agents.

• Vegetative Cells: More susceptible.

Factors Affecting Antimicrobial Methods

• Time: Longer exposure increases effectiveness.

• Temperature: Higher temperatures generally increase effectiveness.

• pH: Extreme pH can enhance or reduce effectiveness.

Osmotic Pressure

• Hypertonic Solutions: Cause water loss from cells, inhibiting growth.

• Preservation: Salt or sugar creates hypertonic environment, preserving food.

Chemical Method Example: Alcohols

• Alcohols: Ethanol and isopropanol are common disinfectants.

• Action: Denature proteins, disrupt membranes.

• Effective Against: Bacteria, fungi, enveloped viruses.

• Example: 70% ethanol used for skin antisepsis.

Controlling Microbial Growth in the Body: Antimicrobial Drugs

Definitions: Antibiotics, Semisynthetics, Synthetics, Antimicrobials

• Antibiotics: Natural compounds produced by microbes to inhibit others.

• Semisynthetics: Modified antibiotics for improved efficacy.

• Synthetics: Completely artificial compounds.

• Antimicrobials: General term for agents that kill or inhibit microbes.

Selective Toxicity

• Concept: Drug targets microbial structures/processes not found in host.

• Limitation: Eukaryotic and viral infections are harder to treat due to similarity to host cells or lack of unique targets.

Mechanisms of Antimicrobial Drugs

• Inhibition of Cell Wall Synthesis: Prevents peptidoglycan formation (e.g., penicillin).

• Inhibition of Protein Synthesis: Targets ribosomes (e.g., tetracycline).

• Disruption of Cytoplasmic Membranes: Damages membrane integrity (e.g., polymyxin).

• Inhibition of Metabolic Pathways: Blocks essential enzymes (e.g., sulfonamides).

• Inhibition of Nucleic Acid Synthesis: Prevents DNA/RNA replication (e.g., quinolones).

• Prevention of Attachment/Entry/Uncoating: Blocks viral infection (e.g., oseltamivir).

Broad Spectrum Drugs

• Problem: Can kill normal flora, leading to secondary infections and resistance.

Routes of Administration

• Oral

• Intravenous

• Topical

• Intramuscular

Therapeutic Index

• Definition: Ratio of toxic dose to effective dose.

• Formula:

• Higher Index: Safer drug.

Major Side Effects of Antimicrobial Drugs

• Allergic reactions

• Toxicity to organs

• Disruption of normal microbiota

Resistance Plasmids (R Plasmids)

• Definition: Plasmids carrying genes for antimicrobial resistance.

• Example: Staphylococcus aureus with R plasmid for methicillin resistance.

Mechanisms of Antimicrobial Resistance

• Enzymatic destruction of drug

• Alteration of drug target

• Decreased permeability

• Efflux pumps

• Bypass of metabolic pathway

• Overproduction of target

• Biofilm formation

Slowing Antimicrobial Resistance

• Use drugs only when necessary

• Complete prescribed course

• Use combination therapy

• Limit use in agriculture

Microbiology Laboratory Techniques

Ubiquity of Microorganisms

• Concept: Microbes are found everywhere.

• Experiment: Sampling various environments to demonstrate presence of microbes.

Aseptic Technique

• Purpose: Prevent contamination of cultures and environment.

• Methods: Flame sterilization, careful handling, use of sterile tools.

• Importance: Ensures reliable results and safety.

Pure vs Mixed Cultures

• Pure Culture: Contains one microbial species.

• Mixed Culture: Contains multiple species.

Streak Plate Methods

• Purpose: Isolate pure colonies from mixed samples.

• Types: T-streak, quadrant, zig-zag.

• Success: Requires proper technique to avoid contamination.

Selective and Differential Media

• Selective Media: Inhibits growth of certain microbes, allows others.

• Differential Media: Distinguishes microbes based on biochemical reactions.

• MacConkey Agar: Selective for Gram-negative, differential for lactose fermentation (pink = fermenter).

• EMB, PEA, MSA: Other examples of selective/differential media.

• TSA Plate: Used as a comparator; supports growth of most bacteria.

Disk Diffusion (Kirby-Bauer) Test

• Purpose: Assess effectiveness of antimicrobial agents.

• Zone of Inhibition: Clear area around disk indicates susceptibility.

• Interpretation: Larger zone = more effective agent.

Chemical Germicides and Disinfectants

• Multiple Concentrations: Test effectiveness at different strengths.

• Gram Positive vs Gram Negative: Test both to assess spectrum of activity.

• Variation: Not all germicides work equally on all microbes.

Additional info: Academic context and definitions were expanded for clarity and completeness. Tables were recreated and grouped logically for study purposes.