Essential Concepts in Microbiology: Genetics, Techniques, Metabolism, and Control

Microbial Genetics

Key Concepts

Microbial genetics explores the structure, function, and regulation of genetic material in microorganisms. Understanding these concepts is fundamental to microbiology.

• 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 – synthesizes new DNA strands; process is semiconservative and requires a template.

• Operon Model: A group of genes regulated together.

  • Inducible operon: Normally off – turned on when product accumulates.

  • Repressible operon: 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.

• Central Dogma: Flow of genetic information: DNA → RNA → Protein.

• Transcription: Synthesis of RNA from DNA template (occurs only in prokaryotes in the cytoplasm).

• Translation: Synthesis of protein from mRNA template.

• 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.

• 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 can be classified by their carbon and energy sources.

• Autotrophs: Use CO2 as a carbon source.

• Heterotrophs: Use organic carbon.

• Phototrophs: Use light as an energy source.

• Lithotrophs/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 required.

• 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 those that build up or break down molecules.

• 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.

  • Cofactor: Metallic ions or vitamins.

  • Coenzyme: Apoenzyme + cofactor.

• 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.

• Krebs Cycle: Produces NADH, FADH2, CO2.

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

• Fermentation: Regenerates NAD+; yields 2 ATP per glucose.

Photosynthesis

Photosynthesis is the process by which light energy is converted into chemical energy, primarily in plants, algae, and some bacteria.

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

• Light reactions: Capture energy.

• Calvin cycle: Produces glucose.

• Photosystems: Light-harvesting units.

Microbial Control

Physical Methods

• 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: Most (moist) heat is 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

• Selective toxicity: Targets microbes, not host cells.

• Therapeutic Index (TI): High TI = safer drug.

• 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 (e.g., C. difficile).

• 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.