unit 2 :1

• Microbial Growth


  All life need basic physical and chemical requirements to be met to survive. For most microbes, survival is limited to certain environmental niches where these physical and chemical requirements may be met.
• Physical requirements (environmental)


  • Temperature
  • pH
  • Osmotic pressure
• Chemical requirements


  • Carbon
  • Nitrogen, sulfur, and phosphorous
  • Trace elements
  • Oxygen
  • Organic growth factors
• Minimum growth temperature


  The lowest temperature at which a species will grow

  
• Optimum growth temperature


  The temperature at which it grows best

  
• Maximum growth temperature


  The highest temperature at which growth is possible
• Classification by optimal growth temp

  • Psychrophiles (~-10 to 20°C)
  • Mesophiles (~10 to 45°C)
  • Thermophiles (~40 to 70 °C)
  • Hyperthermophiles (~65 to 110 °C)

  
• Hyperthermophiles


  • Thermophiles
  • Mesophiles
• Most bacteria are mesophiles and thrive between 20 and 40°C. Exposure of foods to extended durations around these temperatures is likely to result in rapid spoilage and potential expansion of bacteria harboring toxins.

  
• Some bacteria are capable of growth and activity extending into lower temperatures and are often responsible for the slow but steady decay of food. These bacteria are referred to as psychrotrophs due to their ability to eat and grow at lower temperatures, even though they may prefer and optimally grow at warmer temperatures.
• Classification by Optimal pH

  • Acidophiles: pH 1 – 5.5
  • Neutrophile: pH 5.5 – 8.5
  • Alkaliphile: pH 7.5 – 11+
• Acidic (pH ~ 0) Hot Spring at Yellowstone Nat'l Park


  • Alkaline (pH 9.4) Lake Turkana in Kenya

  
• Organisms that require certain conditions

  • Halophiles: organisms which thrive in high salt concentrations [high osmolarity]
  • Osmophiles: organisms which thrive in solute concentrations such as high sugar concentrations [high osmolarity]
  • Xerophiles: organisms which tolerate desiccation or dry conditions [low osmolarity]
  • Piezophiles (Barophiles): organisms that tolerate intense atmospheric pressure
• The elements required for all life are carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Elements needed in small amounts are called trace elements (e.g. iron, copper, zinc).
• Organisms classified based on their ability to use or tolerate oxygen

  • Obligate aerobes: organisms that require oxygen to live
  • Facultative anaerobes: organisms that can tolerate the absence of oxygen, but prefer to use it (generate more energy in its presence – recall ETC)
  • Obligate anaerobes: organisms that require the absence of oxygen for growth
  • Aerotolerant anaerobes: organisms that cannot use oxygen, but can tolerate it
  • Microaerophiles: aerobic organisms that require only low doses of oxygen
• Chemotrophs


  Organisms that acquire energy through chemical metabolism
• Phototrophs


  Organisms that acquire energy through light reactions
• Chemotrophs further classified based on carbon source

  • Chemoheterotrophs: use organic compounds
  • Chemoautotrophs: use CO2
• Phototrophs further classified based on carbon source

  • Photoheterotrophs: use organic compounds
  • Photoautotrophs: use CO2
• Binary fission


  A form of asexual reproduction whereby a bacteria divides producing two genetically identical daughter cells
• Events of Binary Fission

  1. The circular DNA chromosome of the bacterium is replicated from the origin of replication (Ori)

  2. The replicated DNA chromosomes migrate to the poles of the cell

  3. The cell elongates

  4. The cell wall constricts forming a septum with a ring of proteins (FtsZ and possibly MreB) leading to a new cell wall and membrane between daughter cells
• Generation Time


  The time it takes a cell to divide (or the population to double)
• Bacterial growth is logarithmic (grows exponentially rather than linearly).

  
• Phases of a bacterial growth curve

  • Lag Phase
  • Log Phase
  • Stationary Phase
  • Death Phase
• Lag Phase


  Very little or no cell division, the bacteria are generating enzymes and adjusting to their environment in preparation of growth
• Log Phase


  Bacteria divide rapidly (logarithmically) at the generation time rate, continues as long as resources are available and the environment is stable
• Stationary Phase


  Bacterial growth is equal to bacterial death, the population has reached its top capacity based on the resources available
• Death Phase


  Bacterial death exceeds bacterial growth, as resources are consumed, the population capacity continues to diminish. Dying cells may provide some nutrients resulting in potentially a long and persistent surviving population.
• Direct Methods for quantifying prokaryotic culture sizes

  • Direct Plate Count
  • Most Probable Number (MPN) Method
  • Filtration
• Indirect Methods for quantifying prokaryotic culture sizes

  • Turbidity
  • Metabolic Activity
  • Dry Weight
• Turbidity


  Growth of bacteria in liquid culture results in the solution becoming cloudy (turbidity). Measurement of light transmission (optical density or OD) may be used as an indirect measure of the extent of microbial growth