BIOS5480 Microbial World - Flashcards

Week 32 – Microbial Culturing and Growth of Microorganisms

• Understanding the requirements for microbial growth.

• Importance of culture medium in cultivation.

Overview

• Nutritional classification of microorganisms.

• Requirements for microbial growth.

• Types and preparation of culture media.

• Microbial growth and liquid culture.

• Summary, outcomes, and possible questions.

Nutritional Classification and Modes of Growth

Importance of Culturing Microorganisms

• Cultivating microorganisms is crucial to establish their function, phenotype, and ecological roles.

• Typically, it is suggested that only around 1% of bacteria/archaea are culturable, but there's debate surrounding this statistic (Steen et al. 2019).

• A diverse representation in culture collections is beneficial for scientific research.

• Understanding growth conditions is essential for manipulation and engineering microorganisms.

Nutritional Classification

Nutritional Classification Based on Carbon, Energy, and Electron Sources

• Carbon Sources:

  • Autotrophs: Use CO2 as the sole carbon source.

  • Heterotrophs: Obtain organic compounds from other organisms.

• Energy Sources:

  • Phototrophs: Use light energy.

  • Chemotrophs: Use chemical energy sources (organic/inorganic).

• Electron Sources:

  • Lithotrophs: Use reduced inorganic substances.

  • Organotrophs: Use organic compounds.

Examples of Nutritional Classifications

• Photoautotrophs: Use light energy and CO2 (e.g., Cyanobacteria such as Spirulina).

• Photoheterotrophs: Light energy with organic compounds as sources (e.g., Purple non-sulfur bacteria).

• Chemoautotrophs: Oxidize inorganic compounds for energy (e.g., Nitrosomonas spp. in nitrogen cycle).

• Chemoheterotrophs: Use organic compounds for all energy and carbon needs (e.g., Escherichia coli).

Requirements for Microbial Growth

Macronutrients

• Carbon

  • Essential for all cells; found in organic forms for heterotrophs and CO2 for autotrophs.

  • Typical bacterial cell composition: 50% Carbon.

• Nitrogen (N)

  • Critical for cellular constituents (12% of cell mass).

  • Available as inorganic forms (NH3, NO3-, N2) with some bacteria capable of nitrogen fixation (e.g., Azotobacter spp.).

• Other Macronutrients

  • Phosphorous (P): vital for nucleic acids and phospholipids.

  • Sulfur (S): used in amino acids and vitamin synthesis.

  • Potassium (K), Magnesium (Mg), Calcium (Ca): important for enzyme function and cell stabilization.

Iron (Fe)

• A key redox-active metal in proteins.

• Exists in soluble ferrous (Fe2+) under anoxic conditions and insoluble ferric (Fe3+) under oxic conditions.

• Many organisms employ siderophores to scavenge iron from the environment.

• Some bacteria can substitute manganese (Mn) for iron (e.g., Lactobacillus plantarum).

Micronutrients

• Essential trace elements include Boron, Nickel, Selenium, and Zinc, among others, needed for cellular functions or specific enzymes.

• Specific elements like Cobalt (B12) and Copper are important in various metabolic pathways.

Growth Factors

• Organisms may require specific growth factors such as vitamins, which play roles in numerous metabolic processes.

• Example vitamins: Folic acid (precursor), Biotin (fatty acid biosynthesis), Cobalamin (B12, involved in one-carbon transfer).

Culture Media - Types and Preparation

Types of Media

1. Non-Selective (Complex) Media: Contains undefined nutrient-rich components to support a wide range of microorganisms.

   • Examples: Nutrient Agar, Tryptone Soya Agar, Brain Heart Infusion agar.

2. Selective Media: Contains ingredients to inhibit certain organisms while allowing others to grow.

   • Example: Mannitol salt agar for Staphylococci.

3. Differential Media: Distinguishes between groups of microorganisms based on visual differences in the colonies due to biochemical characteristics.

   • Example: Blood agar for hemolysis types, MacConkey agar for lactose fermentation.

Preparation of Media

• Culture media can be liquid or solid and must be sterilized via autoclaving (121-132°C for 30-40 mins).

• Heat-sensitive components should be filter-sterilized.

• Gelling agents like agar must be added at concentrations above 1.5% to solidify.

Microbial Growth in Liquid Culture

Growth Cycle/Curve

• Lag Phase: Period of adjustment with low cell numbers.

• Exponential Growth Phase: Cells adapt and grow exponentially without limitations.

• Stationary Phase: Nutrient depletion leads to growth slowing.

• Death Phase: Cells die due to lack of nutrients and buildup of toxic products.

Measurement of Growth

• Total cells and viable counts give insights into growth characteristics under controlled environments.

• Growth media impacts both growth rates and yields in batch versus continuous (chemostat) cultures.

Continuous Cultures

• Continuous cultures maintain steady-state growth conditions allowing for detailed physiological studies, similar to ecosystems in nature.

• Adjusting dilution rates and nutrient concentrations can significantly affect population densities and growth rates.

Summary of Learning Outcomes

• Comprehend basic microbial composition, differences between complex and minimal media, and the importance of selective enrichment.

• Understand growth in different cultivation methods (batch vs continuous) and how growth parameters can be manipulated.

Possible Exam Questions

• What do microorganisms need to grow in liquid and solid media?

• Describe the microbial growth curve and differences between batch and continuous cultures.

• Compare and measure the growth characteristics of different bacteria, including benefits of using a chemostat.