L2-Microbial Growth and Control

• Microbial Nutrition


  • All microbes require a core set of nutrients
  • Macronutrients are required in large amounts
  • Micronutrients are required in minute amounts
  • Nutrients are important to the cell's chemical makeup
• Basic elements required by all living organisms

  • Hydrogen
  • Carbon
  • Nitrogen
  • Phosphorus
  • Sulfur
  • Selenium
• Macromolecules


  • Proteins
  • Nucleic acids
  • Lipids
  • Polysaccharides
• • Proteins and RNA are the most abundant macromolecules in a cell
  • DNA contributes a tiny percentage of a cell's dry weight
• Carbon


  • The main element in all classes of macromolecules
  • Organic compounds e.g., amino acids, fatty acids, organic acids, sugars, and nitrogen bases assimilated by bacteria
• Nitrogen


  • A typical bacterial cell is ~13% nitrogen (by dry weight)
  • Key elements in proteins, nucleic acids
• Other macronutrients


  • Phosphorus: nucleic acids and phospholipid synthesis
  • Sulphur: amino acids (cysteine, methionine), vitamins (thiamine, biotin and co-enzyme-A)
  • Potassium: Required for enzyme activity
  • Magnesium: Stabilizes membranes, ribosomes and nucleic acids. Also required for enzyme activity
  • Calcium: Stabilizes cell walls and provides heat stability to endospores
  • Sodium: Required for growth of some microbes
• Metals


  • Iron: cellular respiration. Siderophores bind iron from minerals and transport it into the cell
  • Trace metals: (Cr, Co, Cu, Mn, Mo, Ni, Se, W, V, Zn): components of enzymes
• Growth factors


  • Required by some microbes in small quantities
  • Vitamins, amino acids, purines and pyrimidines
• Microbial growth


  • An increase in the number of cells in a population
  • A result of cell division
• Binary fission


  The cell elongates twice the original size and then separates into two cells

  
• Budding


  • The cell divides because of unequal cell growth
  • Simple budding, budding from hyphae, division of stalked organisms, polar growth without differentiation of cell size
• Generation


  When a cell divides into two cells
• Generation time


  The time it takes for a cell to divide into two cells
• The total cells numbers and mass doubles per generation time
• Cell numbers in a bacterial culture can quickly become very large
• Exponential (logarithmic) growth


  Cells in a bacterial population double at a constant time interval
• • The generation time of bacteria can vary depending on the bacterial species and the environmental conditions
  • Most bacteria have shorter generation times than eukaryotic microbes
• Exponential growth phase


  The growth phase of a culture where cells double at a constant rate, visualized as a straight line on a semilogarithmic graph

  
• Exponential growth can have both beneficial and detrimental consequences
• Microbial growth cycle


  Lag, exponential, stationary, and death phases
• Lag phase


  Cells prepare themselves for growing in a new medium/environment
• Exponential/log phase


  Cells in the healthiest state, growing at the fastest rate
• Stationary phase


  The growth rate is zero (no net increase or decrease in cell number), energy metabolism and biosynthetic processes at a reduced rate
• Death phase


  Cells eventually die
• Continuous culture


  An open system where fresh medium is continuously added and used medium and cells are harvested at the same time, allowing cells to be kept in an exponential growth phase for long periods

  
• Biofilm


  A population of cells enmeshed in a polysaccharide matrix that is attached to a surface, going through stages of attachment, colonization, development, and dispersal

  
• Microscopic count


  Microbial cells are enumerated by microscopic observations using counting chambers
• Microscopic count has limitations such as not distinguishing between living and dead cells, overlooking small cells, difficulty counting low density suspensions, and mistaking debris for cells
• Viable/plate count


  Counting viable cells that can divide and form colonies, using serial dilutions and spread or pour plate methods

  
• The "Great Plate Count Anomaly" refers to the fact that counts of microbial cells obtained via cultivation are orders of magnitude lower than those directly observed via microscope

  
• Turbidimetric method


  Using a spectrophotometer to estimate cell numbers based on the density/turbidity of a liquid culture, requiring a standard curve
• Cardinal temperatures


  The range that a particular bacterium (microbe) grows in, including the minimum, optimum, and maximum temperatures
• Series Dilution


  A method for diluting a sample to count viable cells
• Counting Viable Cells


  Spread plate and pour plate methods

  
• Great Plate Count Anomaly
• Spectrophotometer


  Used to estimate cell numbers based on the density of a liquid culture
• Turbidity


  Suspension of cells scatters light passing through the suspension
• Turbidimetric Method


  1. Require preparation of a standard curve relating cell numbers or mass dry weight to turbidity

  2. Growth curve can be generated over time
• Drawbacks of turbidimetric method