BM

Microbial Growth Lecture Notes

Rationale for Growing Microbes

  • Laboratory Experimentation: Essential for studies in microbiology to understand microbial behavior.
  • Industrial Applications: Used in several industries including pharmaceuticals to produce drugs and other compounds.
  • Environmental Purposes: Important for bioremediation and studying ecosystems.

Common Approaches to Growing Microbes

  • Based on the purpose, different approaches are employed:
    • Isolation & Enrichment: Selective growth of specific microorganisms.
    • Identification/Testing: Used for characterizing and evaluating microbes.
    • Experimentation: Conducting controlled experiments to study microbial properties.
    • Bioprocessing/Bioengineering: Application of microbial processes to manufacture products.

Types of Culture Systems

Closed System (Batch Culture)

  • Definition: All nutrients provided at the beginning; no further additions during the culture process.
  • Growth Stages:
    • Lag Phase: Initial adaptation period with no increase in cell number.
    • Exponential Phase: Rapid cell division occurs; maximum growth rate is achieved.
    • Stationary Phase: Cell division equals cell death; population stabilizes.
    • Death Phase: Cellular lysis occurs; viable cell count decreases.

Open System (Continuous Culture)

  • Definition: Continuous addition of fresh medium and removal of cells to maintain constant growth conditions.
  • Characteristics:
    • Maintains stable environmental conditions over time.
    • Chemostat is a common type; allows control of growth rates and population density.

Population Growth Curve Phases

  1. Lag Phase:

    • Delay in growth due to acclimatization to new media conditions.
    • Duration varies based on medium conditions (e.g., nutrient richness).

  2. Exponential Phase:

    • Cells grow at maximum rate; e.g., E. coli has a generation time of ~20 min.
    • Most microorganisms grow exponentially, but depends on culture conditions.

  3. Stationary Phase:

    • No net increase or decrease in cell numbers; birth rate = death rate.
    • Cells may undergo stress adaptations such as endospore formation.

  4. Death Phase:

    • Viable cell count decreases post-stationary phase.

Characteristics of Continuous Cultures

  • Chemostat:
    • Maintains a steady state with constant nutrient supply and growth rate control.
    • Optimum for industrial processes due to stable cell populations and metabolic products.

Growth Dynamics in Continuous Culture

  • Dilution Rate (D):

    • Defined as: D = \frac{f}{v}, where $f$ is flow rate and $v$ is culture volume.

  • Nutrient Concentration:

    • Affects both growth rate and biomass yield.
    • Nutrient uptake must meet metabolic demand to prevent slow growth.

Measuring Growth

  • Population size determination for experimental validation uses:
    • Titre: Number of bacterial cells per specific volume (e.g., cells/ml).
    • Turbidity: Light scattering measurement of cell suspension.
    • Optical Density (OD600): Measurement for growth stage assessment, typically at 600 nm due to minimal absorption by media.

Additional Methods of Measuring Growth

  1. Wet and Dry Weight:

    • Wet weight provides mass measurement of cell suspension.
    • Dry weight reflects mass accurately (wet weight is about 10-20% of dry weight).

  2. Chemical Probes:

    • O2/CO2 Probes to evaluate metabolic processes (e.g., fermentation in yeast).
    • pH Changes, due to acid production over time, can also indicate microbial activity.
    • Catabolite Concentrations: Measuring specific nutrients or metabolic products (e.g., sugar or DNA levels) provides insights into growth dynamics.