Microbial Growth Requirements

Physical Requirements:
- Temperature:
  - Minimum growth temperature: Lowest temperature at which a species can grow.
  - Optimum growth temperature: Temperature at which a species grows best.
  - Maximum growth temperature: Highest temperature at which a species can grow.
  - Temperature classifications:
    - Psychrophilic: Grow at low temperatures (0-20°C).
    - Psychrotroph: Prefers moderate temperatures (20-30°C) but can grow at low temperatures (0°C).
    - Mesophilic: Grow at moderate temperatures (20-45°C).
    - Thermophilic: Grow at high temperatures (45-70°C).
    - Hyperthermophilic: Grow at very high temperatures (70°C and above).
- pH:
  - Acidophile: Optimum growth at low pH (below 5.5).
  - Neutrophile: Optimum growth at neutral pH (around 7).
  - Alkaliphile: Optimum growth at high pH (above 8.5).
- Osmotic pressure:
  - Halophile: Requires high salt concentration for growth.
  - Extreme halophile: High salt concentration is essential for growth.
  - Haltolerant: Can tolerate high salt concentrations but do not require it for growth.
Chemical Requirements:
- Carbon:
  - Chemoheterotroph: Obtain carbon from organic compounds.
  - Chemoautotroph: Obtain carbon from inorganic compounds.
- Nitrogen, Phosphorus, Sulfur, and Trace Elements: Essential for microbial growth.
- Oxygen:
  - Obligate aerobes: Require oxygen for growth.
  - Obligate anaerobes: Cannot grow in the presence of oxygen.
  - Facultative anaerobes: Can grow with or without oxygen, but grow better in the presence of oxygen.
  - Aerotolerant anaerobes: Can tolerate oxygen but do not utilize it for growth.
  - Microaerophiles: Require oxygen for growth but only at low concentration.

Biofilm

A complex polymer containing communities of bacteria.
Formation:
- Planktonic bacteria attach to a surface.
- Monolayer formation and slime production.
- Multilayering of cells.
- Development of pillar-like structures with channels.
- Cells detach and revert to planktonic form.
Communication:
- Quorum sensing: Bacteria communicate within a biofilm using chemical signals to coordinate their behavior.

Culture Media

Nutrient materials used for microbial growth in the laboratory.
Chemically defined media: Exact chemical composition is known.
Complex media: Composed of undefined complex ingredients, such as yeast extract or nutrient broth.
Special culture techniques:
- Cultivation of anaerobic microbes:
  - Reducing media: Contains chemicals that absorb oxygen.
  - Anaerobic chamber: Removes oxygen from the environment.
- Cultivation of CO2-required microbes:
  - Candle jars: Creates a CO2-enriched environment.
  - CO2 incubator: Provides a controlled CO2-rich atmosphere.
- Cultivation of pathogenic microbes:
  - Biosafety cabinets (BSCs):
    - Level 2 microbes: Standard BSC for containment.
    - Level 3 microbes: BSC with directional airflow for additional protection.
  - Bioseal personal protective equipment (PPE): Required for Level 4 microbes.
- Selective and differential media:
  - Selective: Inhibit the growth of unwanted species.
  - Differential: Distinguish desired species from others.
  - Combined: Mannitol salt agar is both selective and differential for Staphylococcus aureus.
- Enrichment culture: A selective medium for favoring the growth of desired species.

Bacterial Cultures

Pure culture: Contains only one species of bacteria.
- Streak plate method: Used to isolate individual colonies of bacteria.
Preserving Bacterial Cultures:
- Deep-freezing: Storing pure cultures suspended in liquid at -80°C.
- Lyophilisation (freeze-drying): Freezing the culture at -80°C, removing water by vacuum, and sealing in glass containers.
Bacterial division:
- Binary fission: Bacteria divide by splitting into two identical daughter cells.
Bacterial Growth:
- Generation time: Time taken for bacteria to double their population size.
- Phases of growth:
  - Lag phase: Intense metabolic activity in preparation for growth, but no increase in population.
  - Log phase: Exponential increase in population.
  - Stationary phase: Growth rate slows and population reaches equilibrium.
  - Death phase: Population decreases due to cell death.
- Direct measurement of microbial growth:
  - Serial dilutions + plate count:
    - Colony forming units (CFU): Colonies counted on plates are used to determine the original number of bacteria in the sample.
    - Pour plate method: Suitable for most species, but not heat-sensitive microbes, as some may grow within the agar.
    - Spread plate method: Suitable when microbial density is high.
  - Membrane filtration: For low microbial densities, bacteria are filtered onto a membrane and counted.
- Direct microscopic count:
  - Haemocytometry: Uses a special counting chamber for direct cell counting.
  - Turbidity: Measures the optical density (OD) of a culture using a spectrophotometer.
  - Metabolic activity: Indirectly measures growth rate by measuring the production of CO2 or acid.
  - Dry weight: Measures the dry mass of a culture as a proxy for cell number.