Microbiology Study Guide: Bacterial Growth, Microbial Control, Genetics, and Biotechnology

Chapter 6: Microbial Nutrition and Growth

Physical and Chemical Requirements for Bacterial Growth

Bacterial growth depends on several physical and chemical factors that influence their ability to survive and reproduce.

• Physical Requirements: Include temperature, pH, and osmotic pressure.

• Chemical Requirements: Include sources of carbon, nitrogen, sulfur, phosphorus, trace elements, and oxygen.

• Example: Escherichia coli requires a neutral pH and moderate temperature for optimal growth.

Biofilms

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

• Biofilms protect microbes from environmental stress and antimicrobial agents.

• They are important in medical and industrial contexts due to their resistance and persistence.

• Example: Dental plaque is a biofilm formed by oral bacteria.

Chemically Defined vs. Complex Media

Microbial culture media can be classified based on their composition.

• Chemically Defined Media: Exact chemical composition is known. Used for specific research applications.

• Complex Media: Contains extracts (e.g., peptone, beef extract) with unknown exact composition. Used for routine cultivation.

• Example: Nutrient broth is a complex medium; minimal salts medium is chemically defined.

Selective and Differential Media

Media can be designed to select for or differentiate between microbial species.

• Selective Media: Inhibits growth of some organisms while allowing others to grow.

• Differential Media: Allows differentiation of organisms based on biochemical reactions.

• Example: MacConkey agar is both selective (for Gram-negative bacteria) and differential (lactose fermenters turn pink).

Categories of Microbes with Respect to Oxygen Utilization

Microbes are classified based on their oxygen requirements.

• Obligate Aerobes: Require oxygen for growth.

• Obligate Anaerobes: Cannot tolerate oxygen.

• Facultative Anaerobes: Can grow with or without oxygen.

• Microaerophiles: Require low levels of oxygen.

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

Phases of the Bacterial Growth Curve

Bacterial populations grow in a predictable pattern when cultured.

• Lag Phase: Adaptation period; no increase in cell number.

• Log (Exponential) Phase: Rapid cell division; population doubles at regular intervals.

• Stationary Phase: Growth rate slows; nutrients deplete; cell death equals cell division.

• Death Phase: Cell death exceeds cell division; population declines.

Chapter 7: Microbial Control

Types of Microbial Control

Microbial control methods are used to reduce or eliminate microorganisms in various environments.

• Sterilization: Complete destruction of all microbial life, including spores.

• Commercial Sterilization: Kills Clostridium botulinum spores in canned goods.

• Disinfection: Removal of pathogens from inanimate objects.

• Antisepsis: Removal of pathogens from living tissue.

• Degerming: Mechanical removal of microbes (e.g., hand washing).

• Sanitization: Lowering microbial counts to safe levels.

Mechanisms of Action for Microbial Control Agents

Agents control microbes by targeting essential cellular structures or functions.

• Disrupt cell membranes or walls.

• Denature proteins and enzymes.

• Damage nucleic acids.

Microbial Resistance to Disinfectants and Antiseptics

Some microbes possess characteristics that make them more resistant to control agents.

• Endospores: Highly resistant to heat and chemicals.

• Mycobacteria: Waxy cell wall resists disinfectants.

• Gram-negative bacteria: Outer membrane limits penetration.

Chapter 8: Microbial Genetics

Genotype vs. Phenotype

Genotype refers to the genetic makeup of an organism, while phenotype is the observable characteristics resulting from gene expression.

• Example: A bacterium may have a gene for antibiotic resistance (genotype), which is expressed as resistance to antibiotics (phenotype).

DNA Replication and Protein Synthesis

Genetic information is copied and expressed through two fundamental processes.

• DNA Replication: The process by which DNA is copied before cell division.

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

• Sequence of Events:

  • DNA unwinds and replicates.

  • Transcription produces mRNA.

  • Translation synthesizes proteins from mRNA.

• Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Pre-Transcriptional and Post-Transcriptional Control Mechanisms

Gene expression is regulated at multiple levels in both prokaryotes and eukaryotes.

• Pre-Transcriptional: Regulation of gene expression before mRNA is made (e.g., operons in prokaryotes).

• Post-Transcriptional: Regulation after mRNA is produced (e.g., mRNA splicing in eukaryotes).

Mutations

Mutations are changes in the DNA sequence that can affect an organism's phenotype.

• Can be beneficial, neutral, or harmful.

• Important for evolution and adaptation.

• Types: Point mutations, insertions, deletions, frameshift mutations.

Horizontal vs. Vertical Gene Transfer

Microbes can acquire genetic material through different mechanisms.

• Vertical Transfer: Transmission of genes from parent to offspring.

• Horizontal Transfer: Transfer of genes between organisms of the same generation.

Genetic Recombination

Genetic recombination is the exchange of genetic material between different DNA molecules, increasing genetic diversity.

• Important for adaptation and evolution.

Transformation, Conjugation, and Transduction

These are mechanisms of horizontal gene transfer in bacteria.

• Transformation: Uptake of naked DNA from the environment.

• Conjugation: Transfer of DNA via direct cell-to-cell contact (usually plasmids).

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

Plasmids and Transposons

Plasmids are small, circular DNA molecules that replicate independently and often carry genes for antibiotic resistance. Transposons are DNA sequences that can move from one location to another within a genome.

• Both contribute to genetic variation and evolution in microbes.

Chapter 9: Recombinant DNA Technology

Recombinant DNA Technology

Recombinant DNA technology involves combining DNA from different sources to create new genetic combinations.

• Used in research, medicine, and industry.

• Example: Production of insulin using genetically modified Escherichia coli.

Biotechnology Tools and Techniques

Several tools are used in genetic engineering and biotechnology.

• Restriction Enzymes: Cut DNA at specific sequences.

• Artificial Selection: Choosing organisms with desirable traits.

• Directed Mutation: Introducing specific mutations.

• Vectors: DNA molecules used to transfer genetic material (e.g., plasmids).

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

• Transformation: Introduction of foreign DNA into cells.

• Cloning: Producing identical copies of DNA or organisms.

Methods of Inserting Foreign DNA into Cells

• Transformation

• Electroporation

• Microinjection

• Gene gun

Applications of Biotechnology

Biotechnology has both therapeutic and scientific applications.

• Production of pharmaceuticals (e.g., insulin, growth hormone).

• Gene therapy for treating genetic disorders.

• Genetically modified organisms for agriculture.

Safety Issues and Ethical Concerns

Biotechnology raises important safety and ethical questions.

• Potential for unintended consequences (e.g., gene transfer to non-target species).

• Ethical concerns about genetic modification and cloning.

• Regulation and oversight are necessary to ensure safety.