Microbial Growth, Metabolism, and Genetics: Exam 2 Study Guide

Microbial Growth and Nutrition

Essential Elements for Bacterial Growth

Bacteria require several key elements for growth and survival. These elements are fundamental for cellular structure and metabolic processes.

• Carbon (C): Main component of cellular molecules.

• Hydrogen (H): Involved in energy transfer and organic molecule structure.

• Oxygen (O): Essential for aerobic respiration and water formation.

• Nitrogen (N): Required for amino acids, nucleic acids, and other cellular components.

• Phosphorus (P): Important for nucleic acids, ATP, and membrane phospholipids.

• Sulfur (S): Found in some amino acids and vitamins.

Types of Transport Across Membranes

Microorganisms use various mechanisms to transport substances across their cell membranes.

• Passive Transport: Movement of molecules without energy input.

  • Simple Diffusion: Movement from high to low concentration.

  • Facilitated Diffusion: Movement via membrane proteins, still down the concentration gradient.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Requires energy (usually ATP) to move substances against their concentration gradient.

  • Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

  • Secondary Active Transport: Uses energy from an electrochemical gradient.

  • Group Translocation: Substance is chemically modified during transport (e.g., phosphotransferase system in bacteria).

Comparison Table:

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2 diffusion
Facilitated Diffusion	No	High to Low	Glucose via transporter
Active Transport	Yes	Low to High	Na+/K+ pump
Group Translocation	Yes	Low to High	Glucose uptake in bacteria

Microorganism Classification by Temperature

Microorganisms are categorized based on their optimal temperature ranges for growth.

• Psychrophiles: Grow best at 0–15°C.

• Mesophiles: Grow best at 20–45°C (includes most human pathogens).

• Thermophiles: Grow best at 45–80°C.

• Hyperthermophiles: Grow above 80°C.

Osmotic Pressure and Microbial Growth

Osmotic pressure affects microbial cells by influencing water movement across membranes.

• Halophiles: Microorganisms that thrive in high salt concentrations.

• Osmotic Pressure: Can cause plasmolysis (cell shrinkage) or lysis (cell bursting) depending on the environment.

• Applications: High salt or sugar concentrations are used to preserve food by inhibiting microbial growth.

Oxygen Requirements

Microorganisms are classified by their oxygen requirements:

• Obligate Aerobes: Require oxygen for growth.

• Facultative Anaerobes: Can grow with or without oxygen, but grow better with oxygen.

• Obligate Anaerobes: Cannot tolerate oxygen.

• Microaerophiles: Require low levels of oxygen.

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

Example: Escherichia coli is a facultative anaerobe; Clostridium botulinum is an obligate anaerobe.

Biofilms

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

• Location: Found on medical devices, teeth (dental plaque), water pipes, etc.

• Importance: Biofilms protect microbes from antibiotics and the immune system, making infections harder to treat.

Bacterial Reproduction and Growth

Bacteria reproduce primarily by binary fission, a process of asexual reproduction.

• Steps of Binary Fission:

  1. DNA replication

  2. Cell elongation

  3. Septum formation

  4. Cell division

• Generation Time: The time required for a bacterial population to double.

• Bacterial Growth Curve: Consists of lag, log (exponential), stationary, and death phases.

Measuring Microbial Growth

Microbial growth can be measured by direct and indirect methods.

• Direct Methods: Plate counts, microscopic counts.

• Indirect Methods: Turbidity (optical density), metabolic activity, dry weight.

Metabolism

Enzymes and Coenzymes

Enzymes are biological catalysts that speed up chemical reactions in cells. Coenzymes are non-protein molecules that assist enzymes.

• Structure: Most enzymes are proteins with an active site for substrate binding.

• Importance: Enzymes lower activation energy, increasing reaction rates.

• Coenzymes: Examples include NAD+, FAD, and coenzyme A.

Enzyme Inhibition

• Competitive Inhibition: Inhibitor competes with substrate for the active site.

• Noncompetitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape and function.

REDOX Reactions

REDOX (reduction-oxidation) reactions involve the transfer of electrons between molecules, crucial in energy production.

Cellular Respiration

Cellular respiration is the process by which cells extract energy from nutrients.

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate, producing 2 ATP and 2 NADH per glucose.

• Krebs Cycle (Citric Acid Cycle): Occurs in the cytoplasm (prokaryotes) or mitochondria (eukaryotes); produces 2 ATP, 6 NADH, and 2 FADH2 per glucose.

• Electron Transport Chain (ETC): Located in the plasma membrane (prokaryotes) or inner mitochondrial membrane (eukaryotes); produces most ATP via oxidative phosphorylation.

• Total ATP Yield (Aerobic Respiration): Up to 38 ATP per glucose in prokaryotes, 36 in eukaryotes.

Key Molecules:

• NAD+: Electron carrier; reduced to NADH during glycolysis and Krebs cycle.

• ATP: Main energy currency of the cell.

Fermentation

Fermentation is an anaerobic process that allows ATP production without oxygen.

• Types: Lactic acid fermentation, alcoholic fermentation.

• Difference from Respiration: Fermentation produces less ATP and does not use the electron transport chain.

Genetics

Genes and Nucleic Acids

• Genes: Segments of DNA that code for proteins or functional RNA.

• Bases in DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

• Bases in RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).

Central Dogma: Transcription and Translation

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of protein from mRNA; occurs at the ribosome.

• Codons: Triplets of nucleotides in mRNA that specify amino acids.

• Anticodons: Triplets in tRNA complementary to mRNA codons.

Example: If DNA sequence is ATGGCTACGA, mRNA sequence is AUGGCUACGA.

Mutations

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

• Causes: Errors in DNA replication, chemical mutagens, radiation.

Genetic Transfer in Bacteria

• Transformation: Uptake of naked DNA from the environment.

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

• Conjugation: Direct transfer of DNA between bacteria via a pilus.

Summary Table: Types of Genetic Transfer

Type	Mechanism	Example
Transformation	Uptake of free DNA	Griffith's experiment with Streptococcus pneumoniae
Transduction	Virus-mediated transfer	Bacteriophage lambda in E. coli
Conjugation	Direct cell-to-cell transfer	F plasmid transfer in E. coli

Additional info: This study guide expands on the exam review questions, providing definitions, examples, and context for each topic relevant to a college-level microbiology course.