8. Microbial Growth

• Microbial growth


  Increasing in number, accumulating into colonies
• Biofilms


  Attached polysaccharide matrix containing embedded bacterial cells
• Growth factors


  Small amounts of certain organic compounds required for growth because they are essential substances that the organism is unable to synthesize from available nutrients
• Requirements for microbial growth

  • Physical
  • Chemical

  
• Psychrophiles

  • Cold-loving microbes living at 0°C

  
• Psychrotrophs


  • Particular group of psychrophiles, prefer refrigerator temperature (4°C)
• Psychroduric microbes


  • Prefer warm temperatures but can endure very cold or even freezing temperatures
• Mesophiles


  • Moderate temperature-loving microbes withstanding 25 to 40°C; the most common type of microbe

  
• Thermophiles


  • Heat-loving microbes withstanding 50 to 60°C

  
• Hyperthermophiles


  • Have an optimum growth temperature of 80°C or higher
• Extreme thermophiles


  • 121°C and above
• pH


  Acidity or alkalinity of a solution
• Acidophiles


  • Bacteria that loves acidic environment (pH of 2 to 5)
• Alkaliphiles


  • Bacteria that loves basic or alkaline environment (pH of >8.5)
• Osmotic pressure

  Pressure exerted on a cell membrane by solutions both inside and outside the cell
• Osmosis

  Movement of a solvent, through a permeable membrane, from a lower concentration of solutes to a higher concentration of solutes
• Hypertonic

  When the concentration of solutes in the external environment of a cell is greater than that of solutes inside the cell
• Plasmolysis

  Condition in which the cell membrane and cytoplasm of a cell shrink away from the cell wall; occurs when bacteria with rigid cell walls are placed in a hypertonic solution
• Hypotonic

  When the concentration of solutes outside a cell is less than that of solutes inside a cell
• Plasmoptysis

  Process where the cytoplasm escapes when a bacterial cell placed in a hypotonic solution bursts
• Isotonic

  When the concentration of solutes outside a cell equals the concentration of solutes inside the cell
• Extreme Halophiles

  • Require high salt concentration
• Obligate Halophiles

  • Require 30% of salt for growth
• Facultative Halophiles

  • Requires 15% of salt for growth
• Haloduric organisms

  Microbes that do not prefer to live in salty environments but are capable of surviving there
• Piezophiles

  • Microbes that can survive in high atmospheric pressure (>14.7 psi)
• Major chemical elements in living protoplasm

  • Carbon
  • Hydrogen
  • Nitrogen
  • Phosphorus
  • Sulfur
• Trace elements


  Elements such as iron, copper, molybdenum, and zinc that serve as micronutrients
• Organic growth factors


  Required for microbes to grow and survive in their environment
• Types of oxygen requirement

  • Obligate aerobes
  • Facultative anaerobes
  • Obligate anaerobes
  • Aerotolerant anaerobes
  • Microaerophiles
  • Capnophiles

  
• Binary fission


  Forms a totally new daughter cell, with the mother cell retaining its original identity
• Budding division


  Forms a totally new daughter cell, with the mother cell retaining its original identity
• Generation time


  When one cell eventually separates to form two cells, one generation has occurred
• Culture medium or growth medium

  Liquid or gel designed to support the growth of microorganisms

  
• Types of culture media

  • Defined media
  • Complex media
• Phases of microbial growth cycle

  • Lag phase
  • Exponential or log phase
  • Stationary phase
  • Death phase

  
• Exponential growth


  Repetitive pattern where the number of cells doubles in a constant time interval
• Chemostat


  Most common type of continuous culture, where a known volume of sterile medium is added at a constant rate while an equal volume of spent culture medium is removed at the same rate
• Microscopic counting


  Quick and easy way of estimating microbial cell numbers

  
• Plate count


  Most frequently used method of measuring bacterial populations, often reported as colony-forming units (CFU)