Microbiology Lab Concepts and Core Topics: Metabolism, Genetics, and Biotechnology

Lab Concepts

Streak Plating

Streak plating is a fundamental microbiological technique used to isolate pure colonies from a mixed culture. It involves spreading bacteria across the surface of an agar plate in a pattern that thins out the sample and separates individual cells.

• Purpose: To obtain isolated colonies for further study or identification.

• Common Errors: Not flaming the loop between quadrants, using too much inoculum, or not streaking enough can lead to confluent growth instead of isolated colonies.

• Troubleshooting: Ensure proper sterilization of the loop, use a light touch, and streak in multiple directions to maximize isolation.

DNA Extractions

DNA extraction is the process of isolating DNA from cells or tissues for downstream applications such as PCR, cloning, or sequencing.

• Centrifuge Safety and Balancing: Always balance tubes of equal weight opposite each other in the centrifuge to prevent damage or accidents.

• Centrifuge Purpose: To separate components based on density, such as pelleting cell debris while leaving DNA in the supernatant.

• Supernatant vs Pellet: The supernatant is the liquid above the solid residue (pellet) after centrifugation. The pellet contains heavier particles, while the supernatant contains soluble molecules, including DNA after cell debris is pelleted.

Key Materials and Functions for Strawberry DNA Extraction

• Strawberries: High DNA yield due to being octoploid (eight sets of chromosomes).

• Dish Soap (Detergent): Disrupts cell and nuclear membranes by dissolving phospholipids, releasing DNA.

• Salt (NaCl): Neutralizes DNA's negative charge, allowing it to clump and separate from proteins.

• Ice-Cold Ethanol/Isopropyl Alcohol (70-90%): Precipitates DNA, making it visible as white fibers.

• Plastic Bag/Mash: Mechanically breaks down cell walls.

• Filter (Coffee filter/cheesecloth): Removes cell debris, leaving DNA in the filtrate.

• Stirrer (Wooden skewer/hook): Used to spool and collect the precipitated DNA.

Comparison to Other Methods: Bead bashing physically disrupts cells; DNA wash removes contaminants; column filtration binds DNA to a matrix for purification.

Additional info: In molecular biology labs, similar principles apply to bacterial DNA extraction, with variations in reagents and mechanical disruption methods.

CH 5: Microbial Metabolism

Catabolism and Anabolism

Microbial metabolism encompasses all chemical reactions within a microbe, divided into two main types: catabolism and anabolism.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. These are exergonic and often involve hydrolytic reactions (addition of water to break bonds).

• Anabolism: The synthesis of complex molecules from simpler ones, consuming energy. These are endergonic and involve dehydration synthesis (removal of water to form bonds).

• ATP: Catabolic reactions release ATP; anabolic reactions consume ATP.

Enzymes

Enzymes are biological catalysts, usually proteins ending in -ase, that speed up chemical reactions without being consumed.

• Role in Pathways: Enzymes lower activation energy, allowing metabolic pathways to proceed efficiently.

• Substrate: The molecule upon which an enzyme acts.

• Active Site: The region on the enzyme where the substrate binds.

• Products: Common products include ATP, acids, alcohols, gases, and other metabolites.

Factors Affecting Enzyme Activity

• pH: Each enzyme has an optimal pH; deviations can denature the enzyme.

• Temperature: High temperatures can denature enzymes; low temperatures slow activity.

• Substrate Concentration: Increased substrate increases reaction rate until saturation.

• Inhibitors: Competitive inhibitors block the active site; noncompetitive inhibitors bind elsewhere, altering enzyme shape.

CH 8: Microbial Genetics

Central Dogma and -Omics Analyses

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

• Genomics: Study of the entire genome (DNA).

• Transcriptomics: Study of RNA transcripts (gene expression).

• Proteomics: Study of the entire set of proteins.

• Mutations: Changes at any step can affect subsequent steps and overall phenotype.

Gene Expression and Regulation

• Transcription: Synthesis of RNA from DNA; gene expression.

• Constitutively Expressed: Genes that are always transcribed and translated.

• Operon: A group of genes regulated together.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Operator: DNA segment where a repressor can bind to block transcription.

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

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

• Role of Enzymes and Substrates: Induction and repression depend on the presence or absence of specific substrates or end products.

Enzymes in Genetic Processes

• Enzymes are essential for DNA replication (e.g., DNA polymerase), transcription (RNA polymerase), and translation (ribosomes, tRNA synthetases).

RNA Differences: Prokaryotes vs Eukaryotes

• Prokaryotes: mRNA is often polycistronic (encodes multiple proteins), no introns, transcription and translation are coupled.

• Eukaryotes: mRNA is monocistronic, contains introns (spliced out), transcription in nucleus, translation in cytoplasm.

Ribosomes, Amino Acids, and Codons

• Ribosomes: Sites of protein synthesis; 70S in prokaryotes, 80S in eukaryotes.

• Amino Acids: Building blocks of proteins; 20 standard amino acids.

• Codon: A sequence of three nucleotides in mRNA that specifies an amino acid. Not all codons are the same; some are start or stop signals.

Gene Transfer and Mobile Genetic Elements

• Vertical Gene Transfer: Genes passed from parent to offspring.

• Horizontal Gene Transfer: Genes transferred between organisms, not by descent.

• Mobile Genetic Elements (MGEs): DNA segments that can move within or between genomes (e.g., plasmids, transposons).

• Plasmids: Small, circular DNA molecules; conjugative plasmids transfer via conjugation; R factors confer antibiotic resistance.

• Transposons: "Jumping genes" that move within the genome; can disrupt genes (pro: genetic diversity; con: mutations).

• Phages: Viruses that infect bacteria; lytic cycle destroys host, lysogenic cycle integrates into host genome.

Mechanisms of Horizontal Gene Transfer

• Conjugation: Direct transfer of DNA via cell-to-cell contact (e.g., F plasmid).

• Transformation: Uptake of free DNA from the environment (demonstrated by Griffith's experiment).

• Transduction: Transfer of DNA by bacteriophages.

CH 9: Biotechnology and DNA Technology

Key Concepts

• Vector: A DNA molecule used to carry foreign genetic material into a host cell (e.g., plasmids, viruses).

• Biotechnology: The use of living organisms or their products to modify or improve human health and the environment.

• rDNA Technology: Recombinant DNA technology involves combining DNA from different sources to create genetically modified organisms or products (e.g., insulin, growth hormone).

• Recombination: The exchange of genetic material between different DNA molecules, resulting in new genetic combinations.

PCR (Polymerase Chain Reaction)

PCR is a technique to amplify specific DNA sequences using cycles of heating and cooling.

• Heating: Denatures (separates) DNA strands.

• Cooling: Allows primers to anneal and DNA polymerase to extend new strands.

$\text{PCR Cycle: Denaturation} \rightarrow \text{Annealing} \rightarrow \text{Extension}$

Cloning

• Purpose: To produce multiple copies of a gene or protein for research, medicine, or industry.

DNA Insertion Methods

• Transformation: Uptake of naked DNA by cells.

• Electroporation: Use of electrical pulses to introduce DNA into cells.

• Protoplast Fusion: Fusion of cells without cell walls to combine genetic material.

• Microinjection: Direct injection of DNA into cells using a fine needle.

Sequencing Methods

• Amplicon Sequencing: Targets specific DNA regions for sequencing (e.g., 16S rRNA gene).

• Shotgun Sequencing: Randomly fragments DNA and sequences all pieces; useful for whole-genome sequencing and metagenomics.

• Nanotechnology: Used to enhance sequencing accuracy and throughput.

Method	Main Purpose	Example Application
Streak Plating	Isolate pure colonies	Identifying bacterial species
DNA Extraction	Isolate DNA from cells	PCR, cloning, sequencing
PCR	Amplify DNA	Genetic testing, forensics
Cloning	Produce gene/protein copies	Insulin production
Transformation	Introduce foreign DNA	Genetic engineering

Additional info: These core concepts are foundational for laboratory and research work in microbiology, biotechnology, and molecular genetics.