Microbial Growth and Metabolism

Learning Objectives

  • Understand bacterial growth stages

  • Explain requirements for microbial growth

  • Interpret techniques for measuring microbial growth

  • Explain biofilm structure and maintenance

Bacterial Reproduction

  • Binary Fission: Main method of reproduction

  • Other Methods:

    • Multiple fission (Cyanobacteria) creates many small cells (baeocytes)

    • Unequal division (Cyanobacteria) forms basal and apical cells

    • Budding forms new cells as outgrowth (bud)

    • Spore formation (Actinomycetes) uses aerial hyphae

Cell Cycle and Growth

  • Most bacteria and archaea are haploid

  • Cell cycle involves:

    • DNA replication and segregation

    • Cytokinesis (cell division)

  • New peptidoglycan incorporated during growth

Environmental Factors Affecting Growth

  • Microbial growth depends on:

    • Temperature

    • pH

    • Osmotic pressure

    • Atmosphere

    • Radiation

Temperature Effects

  • Microorganisms have optimal, minimum, and maximum temperature ranges.

  • Different groups:

    • Psychrophiles: Optimum ≤15 °C

    • Mesophiles: 20-45 °C (most human pathogens)

    • Thermophiles: ≥55 °C

  • Adaptations for stability at high temperatures include increased hydrogen bonding in proteins and membrane saturation.

pH Requirements

  • Measuring acidity/alkalinity affects microbial growth.

  • Buffer systems stabilize pH in culture media.

  • Microbe classifications:

    • Acidophiles: pH ~0–5.5

    • Neutrophiles: pH ~5.5–7

    • Alkalophiles: pH ~8.5–11.5

Osmotic Pressure

  • Influences water movement across cell membranes.

  • Preservation involves increased osmotic pressure (e.g., salt, sugar).

  • Tolerant groups:

    • Halophiles: thrive in high salt

    • Extreme halophiles: require very high salt

Atmospheric Requirements

  • Oxygen concentration influences growth and metabolism.

  • Categories:

    • Aerobes: require oxygen

    • Anaerobes: grow without oxygen

    • Facultative anaerobes: grow better with oxygen

Radiation Effects

  • Different radiation types affect cells differently.

    • Ionizing radiation: DNA mutations

    • UV radiation: causes thymine dimers

    • Visible light: can generate singlet oxygen

Nutritional Types

  • Classification based on energy and carbon sources:

    • Phototrophs, Chemotrophs, Lithotrophs, Organotrophs

    • Autotrophs, Heterotrophs

  • Chemoorganoheterotrophs most studied in labs.

Culture Media

  • Types:

    • Defined (synthetic) media: known composition

    • Complex media: variable ingredients

  • Selective media inhibit some microbes while allowing others; differential media reveal metabolic differences.

Bacterial Colony Characteristics

  • Colony morphology aids in identification:

    • Shape, margin, elevation, size, colour, surface appearance, and texture.

Microbial Growth Dynamics

  • Growth: increase in cell number/size; population growth more significant than individual cell growth.

  • Phases of growth curve:

    • Lag, Exponential, Stationary, and Death phases.

Viable Counting Techniques

  • Include direct counts (microscopic, flow cytometry) and viable techniques (spread plate, pour plate).

Continuous Cultivation

  • Open systems support constant nutrient supply and waste removal, maintaining exponential growth phase.

Metabolism Overview

  • Catabolism: breakdown of molecules to release energy.

  • Anabolism: building complex molecules requires energy.

  • ATP as energy currency.

Biofilms

  • Structured communities of microbes attached to surfaces, providing protection and interaction with environments.

  • Formed via extracellular polymeric substance (EPS) production.

Cell Communication

  • Quorum sensing in biofilms enables coordinated behavior based on population density.

  • Impacts pathogenicity, gene transfer, and mutualistic relationships with hosts.