Microbial Genetics, Growth, and Control: Study Guide Notes

Microbial Genetics, Growth, and Control

Microbial Genetics

Microbial genetics explores how microorganisms inherit traits, replicate genetic material, and express genes. Understanding these processes is fundamental to microbiology.

• Conjugation: The transfer of genetic material between bacterial cells via direct contact, often through a pilus. This process allows for horizontal gene transfer and genetic diversity.

• Transduction: The transfer of DNA from one bacterium to another via a bacteriophage (virus). This mechanism can introduce new genes, including antibiotic resistance.

• Transformation: The uptake of free DNA from the environment by a bacterial cell. This process can result in new traits if the DNA is incorporated into the genome.

• Replication: The process by which DNA is copied before cell division. It ensures genetic information is passed to daughter cells.

• Transcription: The synthesis of mRNA from a DNA template. This is the first step in gene expression.

• Translation: The process by which ribosomes synthesize proteins using mRNA as a template.

• Binary Fission: A method of asexual reproduction in bacteria where a single cell divides into two identical daughter cells. Steps include DNA replication, cell elongation, septum formation, and cell separation.

• Fredrick Griffith's Experiment: Demonstrated transformation in bacteria. Griffith showed that non-virulent Streptococcus pneumoniae could become virulent when exposed to heat-killed virulent cells, indicating the transfer of genetic material.

Example: If given a DNA sequence, transcription produces an mRNA strand (replace T with U), and translation uses the genetic code to determine the amino acid sequence.

Additional info: The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein.

Microbial Growth and Nutrition

Microbial growth refers to the increase in cell number, not cell size. Growth requirements include nutrients, environmental conditions, and energy sources.

• Growth Curve: Bacterial populations grow in distinct phases: lag, log (exponential), stationary, and death. Each phase reflects changes in cell number and metabolic activity.

• Growth Requirements: Microorganisms need carbon, nitrogen, phosphorus, sulfur, trace elements, and appropriate temperature, pH, and oxygen levels to survive.

• Types of Media:

  • Defined (synthetic) media: Exact chemical composition is known.

  • Complex media: Contains extracts and digests of natural products; composition is not precisely known.

  • Selective media: Favors growth of specific microbes while inhibiting others.

  • Differential media: Distinguishes between organisms based on metabolic reactions.

• Ways to Store Bacteria:

  • Short-term: Refrigeration, agar slants.

  • Long-term: Deep freezing, lyophilization (freeze-drying).

• Measuring Microbial Growth:

  • Direct microscopic count: Counting cells under a microscope.

  • Viable plate count: Counting colonies formed on agar plates.

  • Spectrophotometry: Measuring turbidity (cloudiness) of a culture.

  • Membrane filtration: Filtering and counting colonies from a known volume.

Example: Drawing and labeling a growth curve shows the four phases and their characteristics.

Microbial Metabolism

Microbial metabolism includes all chemical reactions in a cell, especially those involved in energy production and biosynthesis.

• Carbohydrate Metabolism: Microbes break down carbohydrates (like glucose) for energy via glycolysis, the Krebs cycle, and electron transport chain.

• Key Equations:

  • Glycolysis: $\text{Glucose} + 2\text{ADP} + 2\text{P}_i + 2\text{NAD}^+ \rightarrow 2\text{Pyruvate} + 2\text{ATP} + 2\text{NADH} + 2\text{H}_2\text{O}$

  • Krebs Cycle: $\text{Acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i \rightarrow 2\text{CO}_2 + 3\text{NADH} + \text{FADH}_2 + \text{ATP}$

Example: Fermentation allows microbes to generate ATP without oxygen, producing products like lactic acid or ethanol.

Recombinant DNA Technology (Biotechnology)

Biotechnology uses living organisms or their systems to develop products. Recombinant DNA technology involves manipulating genetic material for practical applications.

• Definition: The use of biological systems, organisms, or derivatives to make or modify products or processes for specific use.

• Why We Use It: To produce medicines (e.g., insulin), improve crops, and develop diagnostic tools.

• Examples: Genetically engineered bacteria producing human insulin, gene therapy, and PCR for DNA amplification.

• Techniques and Processes: Restriction enzymes, cloning vectors, PCR, gel electrophoresis, and DNA sequencing.

Additional info: Recombinant DNA technology is central to modern microbiology and medicine.

Controlling Microbial Growth

Controlling microbial growth is essential in clinical, laboratory, and industrial settings. Methods are classified as physical or chemical.

• Physical Methods:

  • Heat: Autoclaving, pasteurization, dry heat sterilization.

  • Filtration: Removes microbes from liquids or air.

  • Radiation: UV or ionizing radiation to damage DNA.

• Chemical Methods:

  • Disinfectants: Chlorine, alcohols, phenolics.

  • Antiseptics: Used on living tissue (e.g., iodine, hydrogen peroxide).

  • Antibiotics: Target specific microbial processes.

Example: Autoclaving uses pressurized steam to sterilize equipment; alcohol disinfects surfaces.

DNA, mRNA, and Protein Synthesis

Understanding the flow of genetic information is crucial for interpreting DNA and protein synthesis.

• Transcription: DNA is transcribed to mRNA by replacing thymine (T) with uracil (U).

• Translation: mRNA codons are read by ribosomes to assemble amino acids into proteins using the genetic code.

• Example: Given DNA: ATG CCG TAA → mRNA: AUG CCG UAA → Amino acids: Methionine-Proline-Stop.

Comparison Table: Physical vs. Chemical Methods for Controlling Microbial Growth

Method Type	Examples	Main Purpose
Physical	Autoclaving, Filtration, UV Radiation	Sterilization, Disinfection
Chemical	Alcohols, Chlorine, Antibiotics	Disinfection, Antisepsis

Summary Table: Types of Media

Type of Media	Description	Purpose
Defined	Exact chemical composition known	Research, precise experiments
Complex	Contains natural extracts, composition not exact	Routine cultivation
Selective	Inhibits some microbes, allows others	Isolation of specific organisms
Differential	Shows differences in metabolic reactions	Distinguish between species

Additional info: These tables summarize key comparisons and classifications relevant to microbial growth and control.