BI108_bacterial-growth_notes_cn

Bacterial Growth

Overview

  • Closed Batch Systems: Nutrients added, no waste removal after inoculation.
  • Recap of binary fission and importance of closed batch systems.

Phases of Bacterial Growth

Lag Phase
  • Characteristics:
    • Bacteria adjust to new environments.
    • Little to no binary fission.
    • Preparation for binary fission (synthesis of ribosomes, cell wall, and membrane components).
    • Significant anabolic and catabolic processes occur.
Exponential Phase (Log Phase)
  • Characteristics:
    • Significant binary fission occurs.
    • Optimal conditions lead to maximum growth rate.
    • Cells are healthier and morphologically uniform.
    • Growth rate is much higher than death rate.
Stationary Phase
  • Characteristics:
    • Limited food availability.
    • Rate of binary fission decreases.
    • Cells reduce in size due to nutrient scarcity.
    • Competition for space leads to crowding.
    • Some bacteria produce antibiotics or form endospores for survival.
    • Growth rate equals death rate.
Death Phase
  • Characteristics:
    • Growth rate is significantly lower than the death rate.
    • Majority of bacterial cells die.

Exceptions

  • Endospores: Alive but non-reproductive structure of some Gram-positive bacteria, protecting cells from harsh conditions.
  • VBNC State (Viable But Non-Culturable):
    • Bacteria can enter this state in hosts causing latent infections (e.g., tuberculosis).
    • Remain undetectable and non-reproductive until conditions improve.

Summary of Phases

  • Lag Phase: Preparation; little fission.
  • Log Phase: Active growth; rapid fission.
  • Stationary Phase: Balance; limited resources.
  • Death Phase: Decline; majority die.

Environmental Factors Affecting Bacterial Growth

Human Body and Ecosystem

  • Oversimplified model to represent the immune response.
  • Temperature variations (cooler skin than body temp) affect microbial survival.
  • Microorganisms exhibit growth in diverse temperature ranges from freezing (psychrophiles) to boiling (thermophiles).

Growth Conditions

  • Temperature:

    • Optimal ranges for growth.
    • Mesophiles: Preferred by human pathogens (~37°C).
    • Thermophiles: Prefer high temperatures.
    • Psychrophiles: Adapted to cold environments.

  • pH Levels:

    • Each microorganism thrives in specific pH levels.
    • Acidic (

  • Osmotic Pressure/Salt Concentration:

    • Halophiles: Thrive in high-salt environments.

Chemical Requirements for Microbial Growth

  • Essential Elements Needed:
    • Large quantities: Carbon, Nitrogen, Hydrogen, Sulfur, Phosphorus.
    • Small quantities: Iron, Zinc, Sodium, Chloride, Magnesium.

Harmful Forms of Oxygen

  • Oxygen gas is toxic to obligate anaerobes.
  • Reactive Oxygen Species (ROS) including peroxides and radicals can damage cellular components, leading to mutations.
  • Antioxidants are crucial for neutralizing ROS and preventing damage.