Microbial Genetics, Control of Microbial Growth, and Antimicrobial Drugs: Study Guide

Microbial Genetics

The Central Dogma of Genetics

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

• DNA is transcribed into mRNA.

• mRNA is translated into amino acids, forming polypeptides and ultimately proteins.

• Proteins carry out cellular functions.

Example: In Escherichia coli, the lac operon is transcribed into mRNA, which is then translated into enzymes that metabolize lactose.

Prokaryotic vs Eukaryotic Genetic Processes

• Chromosome and DNA Shape: Prokaryotes typically have circular chromosomes; eukaryotes have linear chromosomes.

• Location: Prokaryotic processes occur in the cytoplasm; eukaryotic transcription occurs in the nucleus, translation in the cytoplasm.

• Proofreading and Mutation Editing: Eukaryotes generally have more extensive proofreading and repair mechanisms.

Example: Eukaryotic DNA polymerase has higher fidelity due to proofreading activity.

Steps in DNA Replication, Transcription, and Translation

• DNA Replication: DNA is copied to produce two identical molecules. Key steps: initiation, elongation, termination.

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: mRNA is decoded by ribosomes to synthesize proteins.

Additional info: Each process involves specific enzymes and regulatory mechanisms.

Functions of Key Enzymes

• Ligase: Joins DNA fragments together.

• DNA Polymerase: Synthesizes new DNA strands.

• Topoisomerase: Relieves supercoiling during DNA replication.

• DNA Gyrase: Introduces negative supercoils (mainly in prokaryotes).

• RNA Polymerase: Synthesizes RNA from DNA template.

Leading vs Lagging Strand Synthesis

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

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

Components of a Nucleotide

• Phosphate group

• Five-carbon sugar (deoxyribose in DNA, ribose in RNA)

• Nitrogenous base (adenine, thymine, cytosine, guanine, uracil)

Types of Mutations

• Silent Mutation: No change in amino acid sequence.

• Missense Mutation: Changes one amino acid.

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

• Frameshift Mutation: Insertion or deletion shifts the reading frame.

Plasmids

• Definition: Small, circular DNA molecules found in prokaryotes.

• Advantages: Can carry genes for antibiotic resistance, virulence, or metabolic functions.

Semiconservative Replication

• Each new DNA molecule consists of one old strand and one new strand.

Equation:

$\text{DNA}_{\text{parent}} \rightarrow \text{DNA}_{\text{daughter}}$

Types of RNA

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes.

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

• Ribosomal RNA (rRNA): Forms the core of ribosome's structure and catalyzes protein synthesis.

Operons: Inducible vs Repressible

• Inducible Operon: Usually off; can be turned on by an inducer (e.g., lac operon).

• Repressible Operon: Usually on; can be turned off by a repressor (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 be incorporated into DNA/RNA, causing mutations.

• Example: 5-bromouracil is a thymine analog.

Mutagen Examples and Mutation Repair

• Mutagen Example: UV light causes thymine dimers.

• Repair Mechanisms: Photoreactivation, excision repair.

Horizontal Gene Transfer: Transformation, Transduction, Conjugation

• Transformation: Uptake of naked DNA from environment.

• Transduction: Transfer of DNA via bacteriophages.

• Conjugation: Transfer of DNA via direct cell-to-cell contact (often plasmid-mediated).

Controlling Microbial Growth in the Environment

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

• Aseptic: Procedures that prevent contamination by pathogens.

• Antiseptic: Chemicals used on living tissue to reduce microbial load.

• Disinfection: Removal of most pathogens from inanimate objects.

• Sterilization: Complete destruction of all microorganisms, including spores.

• Pasteurization: Heat treatment to reduce microbial load in liquids.

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

Physical Antimicrobial Methods

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

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

• Autoclaving: Uses pressurized steam; effective for sterilization.

• Refrigeration: Slows microbial growth by lowering temperature.

• Desiccation: Removes water; inhibits metabolism.

• Lyophilization: Freeze-drying; preserves microbes for long-term storage.

Additional info: Most methods target cell walls, membranes, or nucleic acids.

Action of Antimicrobial Agents

• Cell Wall: Disruption leads to cell lysis.

• Cell Membrane: Loss of integrity causes leakage of cell contents.

• DNA: Damage prevents replication and transcription.

• Proteins: Denaturation inhibits cellular functions.

Ideal Microbe Characteristics

• Non-pathogenic

• Easy to grow

• Genetically stable

• Useful for industrial or research purposes

Additional info: 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, Temperature, pH

• Time: Longer exposure increases effectiveness.

• Temperature: Higher temperatures generally increase effectiveness.

• pH: Extreme pH can enhance or reduce effectiveness.

Additional info: Microbial ubiquity means these factors must be optimized for each situation.

Osmotic Pressure

• Hypertonic Solutions: Cause water to leave cells, leading to plasmolysis.

• Use of Salt/Sugar: Preserves food by creating hypertonic environments.

Chemical Method Example: Alcohols

• Alcohols: (e.g., ethanol, isopropanol) denature proteins and disrupt cell membranes.

• Effective Against: Bacteria, fungi, enveloped viruses.

• Example: Hand sanitizers use 70% ethanol.

Controlling Microbial Growth in the Body: Antimicrobial Drugs

Definitions: Antibiotics, Semisynthetics, Synthetics, Antimicrobials

• Antibiotics: Naturally produced by microorganisms.

• Semisynthetics: Chemically modified antibiotics.

• Synthetics: Completely synthesized in the lab.

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

Selective Toxicity

• Concept: Drugs should target microbial cells without harming host cells.

• Limitations: Eukaryotic microbes and viruses share more similarities with host cells, making selective toxicity harder.

Mechanisms of Antimicrobial Drugs

• Inhibition of Cell Wall Synthesis: Targets peptidoglycan (bacteria) or chitin (fungi).

• Inhibition of Protein Synthesis: Targets ribosomes.

• Disruption of Cytoplasmic Membranes: Causes cell leakage.

• Inhibition of Metabolic Pathways: Blocks essential enzymes.

• Inhibition of Nucleic Acid Synthesis: Prevents DNA/RNA replication.

• Prevention of Attachment/Entry/Uncoating: Blocks viral infection.

Broad Spectrum Drugs

• Problematic: Can kill normal microbiota, leading to secondary infections.

Routes of Administration

• Oral: Convenient, but variable absorption.

• Intravenous: Rapid, high concentration.

• Topical: Localized effect.

Therapeutic Index

• Definition: Ratio of toxic dose to effective dose.

Equation:

$\text{Therapeutic Index} = \frac{\text{Toxic Dose}}{\text{Effective Dose}}$

Major Side Effects of Antimicrobial Drugs

• Allergic reactions

• Toxicity to organs

• Disruption of normal microbiota

Plasmids and Resistance (R Plasmids)

• R Plasmids: Carry genes for antimicrobial resistance.

• Spread: Can be transferred via conjugation.

Mechanisms of Antimicrobial Resistance

• Enzymatic destruction of drug

• Alteration of drug target

• Decreased uptake

• Increased efflux

• Bypass of metabolic pathway

• Overproduction of target

• Formation of biofilms

Slowing Antimicrobial Resistance

• Use drugs only when necessary

• Complete prescribed courses

• Use combination therapy

• Limit use in agriculture

Summary Table: Key Definitions and Concepts

Term	Definition	Example/Application
Aseptic	Preventing contamination by pathogens	Surgical procedures
Antiseptic	Chemical used on living tissue	Hand sanitizer
Disinfection	Removal of most pathogens from objects	Bleach on surfaces
Sterilization	Complete destruction of all microbes	Autoclaving
Pasteurization	Heat treatment to reduce microbes	Milk processing
Degerming	Mechanical removal of microbes	Hand washing
Plasmid	Small, circular DNA molecule	Antibiotic resistance genes
R Plasmid	Plasmid with resistance genes	Spread of resistance in hospitals
Antibiotic	Natural antimicrobial agent	Penicillin
Semisynthetic	Modified antibiotic	Amoxicillin
Synthetic	Lab-made antimicrobial	Sulfa drugs