Lecture 8

• Bacterial growth


  grown on agar plates or in broth
• Logarithmic growth


  bacterial growth in nutrient rich media
• Doubling time


  growth rate of bacteria for logarithmic growth
• E coli doubling time

  20 minutes
• Plate counts


  count number of colony forming units (CFU) on plate
• Optical density (turbidity)


  directly measure ability to absorb light at specific wavelength
• Microscopic visualization


  count bacteria in a given amount of culture directly
• Plate count

  • dilute culture in medium
  • plate onto agar plate
  • count CFU after 24 hours
  • advantage: only counts CFU
  • disadvantage: time consuming, experimental error in dilution
• Optical density (turbidity)

  • use spectrophotometer
  • shines light at specific wavelength
  • measures amount that passes through
  • advantage: quick and accurate
  • disadvantage: counts dead cells, narrow range of effectiveness
• Microscopy

  • direct sample and count cells
  • uses hemocytometer or Petroff-Hauser cell counter
  • advantage: quick and precise
  • disadvantages: vary greatly (need to repeat many times), counts dead cells, intensive
• Lag phase


  when inoculating culture, cells take time to adjust to new environment
• Log phase


  maximal growth rate
• Stationary phase


  number of cells is steady
• Death phase


  rate of cell death exceeds division rate
• Batch cultures


  the microbes are exposed to the same media, even as they utilize the nutrients and release various products
• Continuous culture

  • done in chemostat
  • the spent media is removed and fresh media is added at a constant flow rate
  • used in industrial fermentation
• Maintenance energy 


  energy required for bacteria to stay at homeostasis
• Washout

  continuous culture: high dilution rates
• Growth rates affected by

  temperature, pH, water activity, oxygen
• Optimal growth rate


  reflect niches
• Psychrophilic


  cold loving

  example: flavobacterium
• Mesophile


  room temperature

  example: escherichia
• Thermophile


  heat loving

  example: thermus
• Hyperthermophile


  extreme heat loving

  example: thermococcus
• Acidophile


  acidic: pH 1-4.5
• Neutrophile


  neutral: pH 5.5-8.5
• Alkalophile


  basic: pH 7.5-11.5
• Hypotonic solution

  • membrane stretches
  • swelling
  • turgor pressure
  • generally normal for many organism
• Hypertonic solution

  • membrane shrinks
  • plasmolysis: contraction of protoplasm
• Water activity


  available water for organisms
• Low water activity


  life is difficult for cell
• Osmotic pressure 


  pressure caused by water at different [ ] due to dilution of water by solutes
• Matrix pressure

  adsorption to solids

  adhesion of water to solids --> dessication
• Cell wall


  protects against modest changes in water activity
• Leibig's law of the minimum 


  total biomass of organisms is determined by nutrient present at lowest concentration
• Shelford's law of tolerance


  above or below certain environmental limits, a microorganism will not grow, regardless of nutrient supply
• VBNC

  • viable but non-culturable
  • identified by staining
  • response to stress/starvation
• Oligotrophy


  organism that can survive with nutrient stress, low levels of nutrients
• Carbon sources

  • Autotrophy is fixation of carbon dioxide
  • Heterotrophy is use of organic carbon
• Energy sources

  • Phototrophy is energy from light
  • Chemotrophy is energy from chemical reactions