Finals

• Microbial growth requirements

  • Physical
  • Chemical
• Physical requirements


  • Temperature
  • pH
  • Osmotic pressure
• Microorganisms classified by temperature range

  • Psychrophile
  • Psychrotroph
  • Mesophile
  • Thermophile

  
• Minimum growth temperature


  Lowest temperature at which the species will grow
• Optimum growth temperature


  Temperature at which the species grows best
• Maximum growth temperature


  Highest temperature at which growth is possible
• Most bacteria grow best in a narrow pH range near neutrality, between pH 6.5 and 7.5
• The optimum pH of molds and yeasts is generally below that of bacteria, usually about pH 5 to 6
• Osmotic pressure


  High osmotic pressure has the effect of removing necessary water from a cell
• Plasmolysis


  The osmotic loss of water, or the shrinkage of the cell's cytoplasm
• Microbes adapted to high salt concentrations

  • Extreme halophiles (obligate halophiles)
  • Facultative halophiles
• Chemical requirements

  • Carbon
  • Nitrogen, Sulfur, and Phosphorus
  • Trace elements
  • Oxygen
• Carbon


  The structural backbone of living matter, needed for all organic compounds in a cell
• Nitrogen


  About 14% of the dry weight of a bacterial cell, obtained from decomposing protein-containing material, ammonium ions, or nitrates
• Phosphorus


  Synthesis of nucleic acids, phospholipids, and ATP
• Sulfur


  Synthesis of sulfur-containing amino acids and vitamins
• Trace elements

  Fe, Cu, Mo, Zn; cofactors for enzymes
• Microbes classified by oxygen requirements

  • Obligate aerobe
  • Facultative anaerobe
  • Anaerobe

  
• Singlet oxygen


  Normal molecular oxygen boosted into a higher-energy state, extremely reactive
• Superoxide radicals


  Formed in small amounts during normal respiration, toxic to cellular components
• Superoxide dismutase (SOD)


  Enzyme that neutralizes superoxide radicals
• Hydrogen peroxide


  Toxic byproduct of aerobic respiration, neutralized by catalase and peroxidase enzymes
• Hydroxyl radical


  Highly reactive intermediate form of oxygen, formed by ionizing radiation
• Other oxygen-related microbes


  • Aerotolerant anaerobes
  • Microaerophiles
• Organic growth factors


  Essential organic compounds an organism is unable to synthesize, must be obtained from the environment (e.g. vitamins, amino acids, purines, pyrimidines)
• Bacterial growth


  An increase in bacterial numbers
• Bacterial division


  • Binary fission (single cell divides into two identical cells)
  • Budding
  • Aerial spore formation
  • Fragmentation
• Generation time


  The time required for a cell to divide (and its population to double)
• Most bacteria have a generation time of 1 to 3 hours; others require more than 24 hours per generation
• Logarithmic representation of bacterial populations

  Bacterial division occurs according to a logarithmic progression (two cells, four cells, eight cells, and so on)

  
• Phases of growth


  1. Lag phase

  2. Log phase (or exponential growth phase)

  3. Stationary phase

  4. Death phase (or logarithmic decline phase)
• Lag phase


  Little or no change in the number of cells, but metabolic activity is high
• Log phase (or exponential growth phase)


  Bacteria multiply at the fastest rate possible under the conditions provided

  
• Stationary phase


  Equilibrium between cell division and death
• Carrying capacity


  The number of organisms that an environment can support
• Death phase (or logarithmic decline phase)


  The number of deaths exceeds the number of new cells formed
• Direct measurement of microbial growth

  1. Plate counts

  2. Filtration

  3. Most probable number (MPN) method

  4. Direct microscopic count

  
• Plate counts


  Determining the number of bacteria in a sample by counting the number of colony-forming units (CFU) on a solid culture medium

  
• Filtration


  Bacteria are retained on the surface of a membrane filter and then transferred to a culture medium to grow and subsequently be counted

  
• Most probable number (MPN) method


  A statistical estimation used for microbes that will grow in a liquid medium