Exam 2: Microbial Growth, Control, Genetics, and Biotechnology: Study Guide Notes

Microbial Growth

Physical and Chemical Requirements for Bacterial Growth

Bacteria require specific physical and chemical conditions to grow and reproduce. Understanding these requirements is essential for culturing bacteria and controlling their growth.

• Physical Requirements:

  • Temperature: Bacteria are classified by their optimal temperature ranges: psychrophiles (cold-loving), mesophiles (moderate temperature), and thermophiles (heat-loving).

  • pH: Most bacteria grow best near neutral pH (6.5–7.5), but acidophiles and alkaliphiles can tolerate extreme pH conditions.

  • Osmotic Pressure: High salt or sugar concentrations can inhibit growth; halophiles thrive in high-salt environments.

• Chemical Requirements:

  • Carbon: Main structural component; obtained from organic or inorganic sources.

  • Nitrogen, Sulfur, Phosphorus: Needed for synthesis of proteins, nucleic acids, and other cell components.

  • Trace Elements: Inorganic elements (e.g., iron, copper, zinc) required in small amounts.

  • Oxygen: Requirement varies among bacteria (see below).

  • Organic Growth Factors: Vitamins, amino acids, purines, and pyrimidines that some bacteria cannot synthesize.

Biofilms

Biofilms are complex communities of microorganisms attached to surfaces and embedded in a self-produced extracellular matrix.

• Importance:

  • Protect bacteria from environmental stress and antimicrobial agents.

  • Facilitate nutrient exchange and genetic material transfer.

  • Play a role in chronic infections and industrial biofouling.

Culturing Media: Chemically Defined vs. Complex Media

• Chemically Defined Media: Exact chemical composition is known. Used for fastidious organisms and research.

• Complex Media: Contains extracts (e.g., peptones, beef extract); composition varies. Commonly used for routine cultivation.

• Examples:

  • Chemically defined: Glucose salts medium.

  • Complex: Nutrient broth, tryptic soy agar.

Selective vs. Differential Media

• Selective Media: Suppress unwanted microbes and encourage desired microbes (e.g., MacConkey agar selects for Gram-negative bacteria).

• Differential Media: Distinguish between different microbes based on metabolic reactions (e.g., blood agar shows hemolysis patterns).

Oxygen Utilization Categories

Bacteria are classified by their oxygen requirements:

• Obligate aerobes: Require oxygen.

• Facultative anaerobes: Grow with or without oxygen, but better with oxygen.

• Obligate anaerobes: Cannot tolerate oxygen.

• Aerotolerant anaerobes: Do not use oxygen but tolerate its presence.

• Microaerophiles: Require low oxygen concentrations.

Bacterial Growth Curve Phases

Bacterial populations grow in a predictable pattern when cultured in a closed system:

1. Lag Phase: Adaptation, no increase in cell number.

2. Log (Exponential) Phase: Rapid cell division and population growth.

3. Stationary Phase: Growth rate slows; nutrients deplete, waste accumulates.

4. Death Phase: Cells die faster than new cells are produced.

Control of Microbial Growth

Types of Microbial Control

• Sterilization: Destruction of all microbial life, including endospores.

• Commercial Sterilization: Sufficient heat treatment to kill Clostridium botulinum endospores in canned food.

• Disinfection: Destruction of vegetative pathogens on inanimate objects.

• Antisepsis: Destruction of pathogens on living tissue.

• Degerming: Mechanical removal of microbes from a limited area (e.g., skin before injection).

• Sanitization: Lowering microbial counts to safe public health levels.

Mechanisms of Action for Microbial Control Agents

• Alteration of membrane permeability (e.g., detergents, alcohols).

• Damage to proteins (e.g., heat, heavy metals, alcohols denature enzymes).

• Damage to nucleic acids (e.g., radiation, some chemicals).

Microbial Resistance to Disinfectants and Antiseptics

• Endospores: Highly resistant structures (e.g., Bacillus, Clostridium).

• Mycobacteria: Waxy cell wall resists chemicals.

• Gram-negative bacteria: Outer membrane limits entry of chemicals.

• Prions: Extremely resistant to conventional methods.

Microbial Genetics

Genotype vs. Phenotype

• Genotype: The genetic makeup of an organism (its DNA sequence).

• Phenotype: Observable characteristics resulting from gene expression.

DNA Replication and Protein Synthesis

• DNA Replication: Semi-conservative process where DNA is copied before cell division.

  • Key enzymes: DNA polymerase, helicase, ligase.

  • Direction: 5' to 3'.

• Protein Synthesis: Involves transcription (DNA to mRNA) and translation (mRNA to protein).

  • Transcription: RNA polymerase synthesizes mRNA from DNA template.

  • Translation: Ribosomes read mRNA and assemble amino acids into proteins.

Gene Expression Control Mechanisms

• Pre-transcriptional Control: Regulation before mRNA is made (e.g., operons in prokaryotes, DNA methylation in eukaryotes).

• Post-transcriptional Control: Regulation after mRNA is made (e.g., mRNA splicing, RNA interference in eukaryotes).

Mutations

• Definition: Permanent changes in DNA sequence.

• Importance: Source of genetic diversity; can be beneficial, neutral, or harmful.

• Types:

  • Point mutations: Single base changes (silent, missense, nonsense).

  • Insertions/Deletions: Addition or loss of bases; may cause frameshifts.

Genetic Transfer in Microbes

• Vertical Transfer: Genes passed from parent to offspring during reproduction.

• Horizontal Transfer: Genes transferred between cells of the same generation.

Genetic Recombination

• Definition: Exchange of genetic material between different DNA molecules.

• Importance: Increases genetic diversity and adaptation.

Mechanisms of Horizontal Gene Transfer

• Transformation: Uptake of naked DNA from the environment.

• Conjugation: Direct transfer of DNA via cell-to-cell contact (often involves plasmids).

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

Plasmids and Transposons

• Plasmids: Small, circular DNA molecules independent of the chromosome; often carry antibiotic resistance genes.

• Transposons: "Jumping genes" that can move within and between DNA molecules, causing mutations and gene rearrangements.

Biotechnology and DNA Technology

Recombinant DNA Technology

• Definition: Techniques for combining DNA from different sources into a single molecule.

• Importance: Enables genetic engineering, production of medicines, and research advances.

Biotechnology Tools and Techniques

• Restriction Enzymes: Cut DNA at specific sequences.

• Vectors: DNA molecules (e.g., plasmids, viruses) used to carry foreign DNA into host cells.

• PCR (Polymerase Chain Reaction): Amplifies specific DNA sequences.

  • Equation: $DNA_{n} = DNA_{0} \times 2^{n}$ (where n = number of cycles)

• Transformation: Introduction of foreign DNA into a cell.

• Cloning: Making identical copies of DNA or organisms.

• Artificial Selection & Directed Mutation: Selecting or inducing desired traits in organisms.

Methods for Inserting Foreign DNA

• Transformation (chemical or electrical methods)

• Microinjection

• Gene gun (biolistics)

• Viral vectors

Applications of Biotechnology

• Therapeutic: Production of insulin, growth hormones, vaccines.

• Scientific: Gene function studies, genetic mapping, transgenic organisms.

Safety and Ethical Issues in Biotechnology

• Potential for creating harmful organisms (biosafety).

• Ethical concerns about genetic modification, cloning, and gene therapy.

• Regulation of genetically modified organisms (GMOs).

• Example: Debate over labeling GMO foods.