Microbial Growth and Nutrition

Microbial Growth and Nutrition

• Microbial growth is an increase in the number of microbial cells. It can also be described as an increase in total cell mass.
• Prokaryotic organisms generally grow through binary fission.
• Some prokaryotes can also grow through budding.
• Microbial growth depends on chemical requirements and physical (environmental) factors.

Chemical Factors (Nutrient Requirements)

• Oxygen
• Carbon source
• Nitrogen source
• Sulfur source
• Phosphorus source
• Trace elements (e.g., iron, copper, zinc)

Microbial Growth Based on Oxygen Requirements

• Obligate aerobes (e.g., Neisseria)
• Obligate anaerobes (e.g., Clostridium)
• Facultative anaerobes (e.g., Escherichia)
• Aerotolerant anaerobes (e.g., Lactobacillus)
• Microaerophiles (e.g., Campylobacter)

Microbial Growth Based on Carbon Source

• Heterotrophs: utilize organic compounds (e.g., carbohydrates, proteins, vitamins) as carbon source.
• Autotrophs: obtain carbon from inorganic sources (e.g., CO2).

Physical (Environmental) Requirements

• pH:
  • Acidophiles (e.g., Sulfolobus, Ferroplasma): optimal growth at pH 0–5.5
  • Neutrophiles (e.g., Escherichia): optimal growth at pH 5.5–8.0
  • Alkaliphiles (e.g., B. alcalophilus): optimal growth at pH 8.0–11.5
• Temperature:
  • Psychrophiles (e.g., B. psychrophilus): optimal growth 0°C–20°C
  • Mesophiles (e.g., Escherichia): optimal growth 20°C–45°C
  • Thermophiles (e.g., Thermus aquaticus): optimal growth 50°C–60°C
  • Hyperthermophiles (e.g., Pyrococcus): optimal growth above 60°C
• Osmotic pressure
• Water activity
• Hydrostatic pressure (Barophiles/Piezophiles)

The Microbial Growth Curve

• Four phases:
  • Lag phase: initial adjustment to the new environment (no significant increase in cell population)
  • Log/Exponential phase: rapid cell division (constant growth rate)
  • Stationary phase: cell division rate equals death rate (population remains constant)
  • Death/Senescence phase: exponential decrease in viable cells

Growth Curve Additional Details

• Lag phase: cells are adjusting and synthesizing new components/repairing from transfer damage.
• Exponential/Log phase: cells actively divide, metabolic activity is high, growth rate is constant, and cells are uniform for biochemical studies.
• Stationary phase: nutrients are depleted, metabolic waste accumulates, balance is established between cell division and cell death.
• Death/senescence: nutrients are depleted further, and toxic waste continues to accumulate; death rate exceeds division rate; viable cells decrease exponentially.

Generation Time and Population Expression:

• Generation time = time for a population to double. Cell population at any time N_n = N₀2ⁿ, where N₀ is the initial cell number and 'n' is the number of generations.

Laboratory Culture of Microorganisms

• Culturing = cultivating microorganisms in labs.
• This is possible with the use of culture media.
• Culture media: solid or liquid media with crucial nutrients for microbial growth.
• Culture media can be classified by parameters (e.g. chemical composition, physical nature, function).

Parameters for Classifying Culture Media

• Based on Chemical Composition: Defined media (all components known) and Complex media (some unknown components).

• Example components for defined media: sodium carbonate as a carbon source, ammonia as a nitrogen source.

• Example components for complex media: meat extract, yeast extract.

• Based on Physical Nature: Liquid (broths), Semi-solid, and Solid (agar).

• Based on Function: General purpose, enriched, selective, differential, and enrichment media.

Additional Terms

• Inoculum: microbes introduced to initiate growth
• Culture: microbes growing on/in media
• Colony: visible cluster of microbes on solid media
• Batch culture (closed culture): No fresh medium added during incubation.
• Continuous culture (open culture): Fresh medium is added during incubation.