chap 8-9

• Factors that Affect Microbial Growth

  • Availability of Nutrients
  • Moisture
  • Temperature
  • pH
  • Osmotic Pressure and Salinity
  • Barometric Pressure
• Nutrients


  Energy sources. Organisms obtain energy by breaking chemical bonds.
• Water


  Essential for life. Needed to carry out normal metabolic processes.
• Temperature


  • Every organism has an optimum growth temperature
  • The temperature (and pH) ranges over which an organism grows best are largely determined by its enzymes
  • Thermophiles are microorganisms that grow best at high temperatures
  • Mesophiles are microbes that grow best at moderate temperatures (e.g., 37° C)
  • Psychrophiles prefer cold temperatures (like deep ocean water)
  • Psychrotrophs, a particular group of psychrophiles, prefer refrigerator temperature (4°C)
  • Psychroduric organisms prefer warm temperatures, but can endure very cold or even freezing temperatures
• pH


  • Most microorganisms prefer a neutral or slightly alkaline growth medium (pH 7.0 - 7.4)
  • Acidophiles prefer a pH of 2 to 5
  • Alkaliphiles prefer a pH > 8.5
• Osmotic pressure


  The pressure that is exerted on a cell membrane by solutions both inside and outside the cell
• Osmosis


  The movement of a solvent, through a permeable membrane, from a lower concentration of solutes (dissolved substances) 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


  A 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 into a hypertonic solution
• Hypotonic


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


  The process where a bacterial cell bursts when placed into a hypotonic solution and the cytoplasm escapes
• Isotonic


  When the concentration of solutes outside a cell equals the concentration of solutes inside the cell
• Halophilic organisms


  Organisms that prefer to live in salty environments
• Haloduric organisms


  Organisms that do not prefer to live in salty environments, but which are capable of surviving there (e.g., Staphylococcus aureus)
• Piezophiles


  Microbes that can survive in high atmospheric pressure (> 14.7 psi)
• Isotonic solution: Concentration of solutes outside a cell equals concentration inside
• Hypotonic solution: Concentration of solutes outside a cell is less than inside
• Hypertonic solution: Concentration of solutes outside a cell is greater than inside
• Plasmoptysis/Hemolysis: Bursting of bacterial/red blood cell in hypotonic solution
• Plasmolysis: Shrinking of bacterial cell membrane and cytoplasm in hypertonic solution
• Crenation: Shrinking of red blood cell in hypertonic solution
• Bacterial growth


  Increase in the number of organisms rather than an increase in their size
• Binary fission


  One cell divides to become two cells
• Generation time


  The time it takes for one cell to become two cells (e.g., E. coli = 20 minutes)
• Culture media


  Used in microbiology labs to culture (i.e., grow) bacteria; media prepared in the lab are referred to as artificial media or synthetic media
• Chemically defined medium


  One in which all ingredients are known
• Enriched medium


  A broth or solid containing a rich supply of special nutrients that promote the growth of fastidious organisms; example = chocolate agar
• Selective medium


  Has added inhibitors that discourage growth of certain organisms while allowing the growth of a desired organism; example == PEA agar
• Obligate aerobes will grow where there is 20-21% oxygen
• Microaerophiles will grow where there is about 5% oxygen
• Obligate anaerobes will grow where there is 0% oxygen
• Selective & differential media


  • MacConkey agar
  • Mannitol-salt agar
• Blood agar


  • Enriched and differential medium
• Inoculation


  Adding a portion of a specimen to the medium
• Aseptic technique


  Practiced when it is necessary to exclude microbes from a particular area (e.g., when inoculating culture media)
• Contaminants


  Unwanted organisms; the growth medium or plate is said to be contaminated
• Viable plate count


  Used to determine the number of viable bacteria in a liquid sample by making serial dilutions of the liquid and inoculating onto nutrient agar; after overnight incubation, the number of colonies is counted
• Bacterial population growth curve


  A graph prepared by plotting the logarithmic number of viable organisms (on the vertical or Y-axis) against the incubation time (on the horizontal or X-axis)
• A population growth curve for any particular species of bacterium may be determined by growing a pure culture of the organism in a liquid medium at a constant temperature
• A Chemostat is used for continuous cultures