Essential Concepts in Introductory Microbiology

Microbial Genetics

Key Concepts

Microbial genetics explores the structure, function, and transmission of genetic material in microorganisms. Understanding these concepts is fundamental for studying microbial physiology, biotechnology, and evolution.

• Gene: A segment of DNA that codes for one functional product (protein or RNA).

• DNA Subunits: Made of nucleotides (sugar, phosphate, nitrogenous base).

• Replication Enzyme: DNA polymerase – proofreads and requires a template.

• Operon Model:

  • Corepressor: Binds to operator to shut off transcription.

  • Repressible gene: Normally "on," turned "off" when product accumulates.

  • Regulator gene: Codes for repressor protein.

  • Operator: Binding site for repressor.

• Griffith’s Experiment: Demonstrated transformation in Streptococcus pneumoniae.

• Contranscription: Translation occurs only in prokaryotes.

• Genetic Engineering: Direct manipulation of DNA in the lab.

• Biotechnology: Use of organisms’ biochemical processes to produce products (e.g., insulin).

Molecular Techniques

Key Techniques

Molecular techniques are essential for analyzing and manipulating microbial DNA, enabling identification, classification, and genetic modification.

• Gel electrophoresis: Separates DNA by size (DNA is negatively charged).

• PCR (Polymerase Chain Reaction): Amplifies DNA using synthetic DNA primers.

• Restriction enzymes: Cut DNA at specific sites.

• Southern blot: Produces visual DNA fingerprint.

• Microarray analysis: Detects gene expression, cancer subtype, etc.

• Transgenic organisms (GMOs): Contain foreign DNA (plants, bacteria, animals).

Microbial Nutrition and Growth

Nutritional Types

Microorganisms require nutrients for growth and metabolism. Their nutritional classification is based on their carbon and energy sources.

• Autotrophs: Use CO2 as carbon source.

• Heterotrophs: Use organic carbon.

• Phototrophs vs. Chemotrophs: Differ by energy source (light vs. chemicals).

• Saprobes: Feed on dead organisms.

• Parasites: Depend on living hosts.

Environmental Factors

• Osmotic pressure: Halophiles thrive in salt.

• Growth Phases: Lag → Log → Stationary → Death.

Transport Mechanisms

• Diffusion: Passive movement, no energy.

• Facilitated diffusion: Uses carrier proteins, no ATP.

• Active transport: Requires ATP.

• Endocytosis: Engulfing material using membrane and ATP.

Microbial Metabolism & Enzymes

Key Terms

Metabolism encompasses all chemical reactions in a cell, including energy production and biosynthesis. Enzymes are biological catalysts that regulate these reactions.

• Metabolism: All chemical reactions in a cell.

• Anabolism: Builds molecules; requires energy.

• Catabolism: Breaks down molecules; releases energy.

• Amphibolism: Integration of anabolic and catabolic pathways.

• Enzymes:

  • Biological catalysts: Usually proteins.

  • Apoenzyme: Protein part.

  • Cofactors: Metallic ions or vitamins.

• Denaturation: Loss of enzyme shape → loss of function.

• Inhibition:

  • Noncompetitive: End product binds at regulatory site, not active site.

• Phosphorylation: Substrate-level, oxidative, and photophosphorylation regenerate ATP.

Energy Pathways

• Glycolysis: Glucose → 2 pyruvate + 2 ATP + NADH. $\text{Glucose} \rightarrow 2\ \text{Pyruvate} + 2\ \text{ATP} + 2\ \text{NADH}$

• Krebs Cycle: Produces NADH, FADH2, CO2.

• Electron Transport Chain (ETC): Produces ATP and water.

• Fermentation: Regenerates NAD+; yields 2 ATP per glucose. $\text{Fermentation:}\ \text{Glucose} \rightarrow \text{2 ATP} + \text{Byproducts}$

Photosynthesis

Overview

Photosynthesis is the process by which autotrophic organisms convert light energy into chemical energy, producing organic compounds from CO2.

• Occurs in chloroplasts (eukaryotes) or membranes (prokaryotes).

• Light reactions: Capture energy.

• Calvin cycle: Produces glucose.

• Photosystems: Light-harvesting units.

Microbial Control

Physical Methods

Microbial control involves methods to destroy or inhibit microorganisms, ensuring safety in medical, laboratory, and food settings.

• Sterilization: Destroys all life forms (autoclave).

• Disinfection: Destroys vegetative pathogens on inanimate objects.

• Antisepsis: On living tissue.

• Sanitization/De-germing: Physical removal, not killing.

• Heat:

  • Moist heat: Usually more effective than dry heat.

  • TDP (Thermal Death Point): Lowest temp to kill in 10 min.

  • TDT (Thermal Death Time): Shortest time to kill at a given temp.

  • UHT (Ultra High Temperature): 134°C for 1-2 sec.

• Filtration: Removes microbes (HEPA filters for air).

• Surfactants: Disrupt membranes.

Chemical Agents

• Phenolics (e.g., Lysol, triclosan): Disrupt membranes.

• Halogens (iodine, chlorine): Denature proteins.

• Hydrogen peroxide: Antiseptic.

• Silver nitrate: Eye protection for newborns.

• Glutaraldehyde: Disinfectant, not antiseptic.

Antimicrobial Therapy

Key Principles

Antimicrobial therapy uses chemical agents to treat infections by targeting microbial cells while minimizing harm to host cells.

• Selective toxicity: Targets microbes, not host cells.

• Therapeutic Index (TI): High TI = safer drug. $\text{Therapeutic Index} = \frac{\text{Toxic Dose}}{\text{Therapeutic Dose}}$

• MIC (Minimum Inhibitory Concentration): Lowest concentration that inhibits growth.

• Drug Resistance:

  • Mechanisms include enzyme inactivation (e.g., β-lactamase), altered permeability, or modified binding sites.

• Superinfections: Often caused by broad-spectrum drugs.

• Biofilms: Increase resistance to antibiotics.

• Antibiotic misuse: (e.g., in cattle feed) promotes resistance.

Quick Review Tips

• Review enzyme terminology and inhibition types.

• Know examples of each control method (physical vs. chemical).

• Understand differences between operons, genetic engineering, and biotechnology.

• Practice distinguishing microbial types (autotroph, heterotroph, phototroph, chemotroph).

• Revisit metabolic diagrams: glycolysis → Krebs → ETC.