Microbial Growth

• Requirements for microbial growth

  • Physical
  • Chemical
• Physical requirements


  • Temperature
  • pH
  • Osmotic pressure
• Psychrophiles


  Cold-loving microbes, grow at 0°C
• Mesophiles


  Moderate-temperature loving microbes, grow at 25-40°C
• Thermophiles


  Heat-loving microbes, grow at 50-60°C
• Hyperthermophiles


  Optimum growth temperature of 80°C or higher
• Extreme thermophiles


  Grow at 121°C and above
• Acidophiles


  Bacteria that love acidic environments
• Hypertonic


  Concentration of solutes is higher than in the cell
• Plasmolysis


  Shrinkage of cell cytoplasm
• Types of Halophiles


  • Extreme Halophiles
  • Obligate Halophiles
  • Facultative Halophiles
• Chemical requirements


  • Carbon
  • Sulfur, Nitrogen and Phosphorus
  • Trace elements
  • Organic Growth factors
  • Oxygen
• Types of Oxygen Requirement

  • Obligate aerobe
  • Facultative anaerobe
  • Obligate anaerobe
  • Aerotolerant anaerobes
  • Microaerophile
• Cell Division and Population Growth

  1. Binary fission

  2. Budding division

  3. Generation time
• During one generation, all cellular constituents increase proportionally
• Each daughter cell receives a copy of the chromosome(s) and sufficient copies of ribosomes and all other macromolecular complexes, monomers, and inorganic ions to begin life as an independent entity
• Biofilms


  An attached polysaccharide matrix containing embedded bacterial cells
• Quantification of Microbial Growth

  • Growth is an increase in the number of cells
  • Exponential growth is a repetitive pattern where the number of cells doubles in a constant time interval
  • Bacteria grow quickly in batch culture (enclosed vessel), and cell numbers increase dramatically in a short period of time
  • By measuring the rate of cell population increase over time, the growth depicts a certain 'growth curve'
• Phases of microbial growth cycle

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


  An open system used to control both specific growth rate and cell density independently
• Chemostat


  The most common type of continuous culture
• Two Broad Classes of Culture Media

  • Defined media
  • Complex media
• Total Cell Count


  • Microscopic counting is a quick and easy way of estimating microbial cell numbers
  • Stained samples to increase contrast between cells and their background
  • Liquid samples, counting chambers consisting of a grid with squares of known area etched on the surface of a glass slide are used
  • A viable cell is one that is able to divide and form offspring
• Methods of Viable Plate Count

  • Spread plate method
  • Pour plate method
• Direct Measurement Methods of Microbial Growth

  • Plate count
  • Serial Dilution
  • Pour plates and spread plates
  • Filtration
  • Most Probable Number (MPN) method
  • Direct Microscopic Count
• Petroff-Hausser cell counter


  Used for direct microscopic counting
• Coulter counter


  Used for electronic cell counters
• Estimating Bacterial Numbers by Indirect Methods

  • Turbidity
  • Spectrophotometer (or colorimeter)
  • Metabolic Activity
  • Dry Weight