Microbial growth

• Binary fission


  Separation of a single cell into two identical daughter cells each containing at least one copy of the parental DNA
• Steps involved in binary fission

  1. Elongation

  2. Replication of chromosomal DNA

  3. Invagination of cell envelope, formation of two new cells
• Doubling time (or Generation time)


  The period required for cells in a microbial population to enlarge, divide and produce two daughter cells for each one that existed before
• Growth of bacterial populations


  Seen by geometric progression, or exponential, 2^n, where n= number of generations
• Low doubling time


  Indicates rapid growth
• Exponential growth will continue as long as adequate nutrients are available and toxic waste products are at tolerable levels
• Phases of growth

  1. Lag phase

  2. Exponential growth phase

  3. Stationary phase

  4. Death phase
• Lag phase


  Population remains temporarily unchanged as bacteria take time to adjust to new environmental conditions
• Exponential growth phase


  Cells are dividing by binary fission and are growing by geometric progression as conditions are optimal for growth
• Stationary phase


  Limited population growth due to exhaustion of nutrients and build-up of waste products, growth rate equals death rate
• Death phase


  Number of viable cells in the population declines due to lack of nutrients and toxic metabolic by-products
• When bacteria are grown in a closed system (batch culture), the population of cells almost always exhibits these growth dynamics
• Growth requirements of microorganisms

  • Carbon
  • Oxygen
  • Nitrogen
  • Phosphates
  • Trace elements
  • Growth factors
• Autotrophs


  Obtain CO2 from atmosphere
• Heterotrophs


  Obtain carbon from organic compounds such as sugars, organic acids
• Aerobes


  Obtain oxygen from the same molecule that serves as their carbon source, or from H2O, use O2 to generate ATP
• Anaerobes


  Do not need oxygen
• Micro-aerophilic/canophilic


  Grow in the presence of carbon dioxide (CO2)
• Facultative anaerobes


  Grow in any oxygen environment
• Nitrogen


  Important constituent of protein, nucleic acids, taken into cell as nitrates, amino acids
• Phosphates


  Constituent of nucleic acids, phospholipids, taken into cell as phosphates
• Trace elements


  K, Fe, Mn, Mg, Ca, act as co-factors & as parts of co-enzymes, small quantities required for microbial growth
• Growth factors

  Amino acids, vitamins, acquired from the environment if microbe is unable to synthesize them
• Temperature

  Extrinsic factor, every species grows well over a range, optimum temperature is the one at which the organism grows most rapidly, above optimum growth rate declines due to protein inactivation
• Thermophiles

  Thrive at high temperatures because their enzymes are heat-stable
• Psychrophiles

  Can only grow at low temperatures because their enzymes are very heat-sensitive
• Hydrostatic pressure


  Pressure applied to a liquid, many prokaryotes can thrive at high pressures (barophiles), high pressure does not crush cells as H2O passes through membranes, inhibits chemical reactions that undergo an increase in molecular volume
• pH

  Intrinsic factor, prokaryotes tend to grow best at slightly alkaline pH, fungi slightly acidic pH, protozoa & algae at neutral pH
• Osmotic pressure


  Measure of how much water is available, if external solute concentration increases, bacteria react to maintain turgor pressure by pumping in potassium ions, this decreases available water and slows growth
• Halophiles


  Can withstand high salt concentrations by maintaining a high intracellular concentration of salt
• Measuring microbial growth

  1. Direct determination of dry weight of cells

  2. Turbidity measured by spectrophotometer

  3. Total cell counts

  4. Viable cell counts (plate counts)
• A number of situations make it more likely that an infection will develop, including weakened immune system, recent antibiotic use, corticosteroid use, extensive burns/trauma, and lung disease
• Infectious dose


  The number of organisms that must be consumed to cause illness, varies from one organism to another
• Alkaliphiles & acidophiles can adjust intracellular pH by pumping hydrogen ions into or out of the cell