microbial growth

• Microbial Growth


  Increase in number of cells (not the size of cells)
• Growing microbes accumulate into colonies
• In nutrient poor environments, they form biofilms
• Significance of microbial growth topic

  • By understanding the conditions necessary for microbial growth, we can determine how to control the growth of microbes that cause diseases and food spoilage
  • We can also learn how to encourage the growth of helpful microbes and those we wish to study
• Requirements for growth


  • Physical requirements
  • Chemical requirements
• Physical requirements


  • Temperature
  • pH
  • Osmotic pressure
• Chemical requirements


  • Carbon
  • Nitrogen
  • Sulfur
  • Phosphorus
  • Trace Elements
  • Oxygen
  • Organic Growth Factors
• Temperature


  • Most microorganisms grow well at temperatures that humans favor, while some thrive at extremes of temperatures
  • Most bacteria grow only within a limited range of temperatures with their maximum and minimum growth temperatures only approx. 30qC apart
• Microbial classification by preferred range of temperature

  • Psychrophiles (cold loving)
  • Mesophiles (moderate temperature)
  • Thermophiles (heat loving)
• Minimum growth temperature


  The lowest temperature at which a microorganism can grow
• Optimum growth temperature


  The temperature at which a microorganism grows best
• Maximum growth temperature


  The highest temperature at which a microorganism can grow
• Refrigeration is the most common method of food preservation as low temperatures decrease microbial reproduction rates
• Refrigerating rice in a pot half-full


  Versus transferring to and flattening it in a wider plate or pan
• pH


  Refers to the acidity or alkalinity of a solution
• Most bacteria grow best in a narrow pH range near neutrality, between pH 6.5 and 7.5
• Pickled foods


  Foods preserved from spoilage by acids produced by bacterial fermentation
• Acidophiles


  Microorganisms that tolerate acidity and thrive in lower pH levels
• Molds & yeasts grow over a greater pH range than bacteria, but their optimum pH is about 5 to 6
• Chemical buffers


  Chemicals added to the growth medium used to neutralize the acids and maintain proper pH
• Chemical buffers help maintain proper pH despite changes in pH
• Osmotic pressure


  The pressure that causes water to move across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration
• Microorganisms require water for growth and even obtain almost all their nutrients in solution from the surrounding water
• Hypertonic solution


  A solution with a higher concentration of solutes than the cell, causing water to leak out of the cell (plasmolysis)
• Plasmolysis


  The shrinkage of a cell's cytoplasm due to osmotic loss of water, inhibiting cell growth
• The high salt or sugar concentrations in salted or sweetened foods draw water out of any microbial cells that are present and thus prevent their growth
• Adaptations to different salt concentrations

  • Obligate/extreme halophiles (thrive in high salt, require it for growth)
  • Facultative halophiles (able to grow at salt concentrations up to 2%)
• Hypotonic environment


  An environment with very low osmotic pressure, causing water to enter the cell and potentially lyse it (cytolysis)
• Carbon is the structural backbone of living matter, needed for all the organic compounds that make up a living cell, or provide energy source
• Chemoheterotrophs


  Organisms that get most of their carbon from organic materials such as proteins, carbohydrates, and lipids
• Autotrophs


  Organisms that derive their carbon from CO2
• Nitrogen, Sulfur, Phosphorus


  Important for protein synthesis, DNA/RNA synthesis, and ATP synthesis
• Nitrogen acquisition


  • Decomposition of proteins & reincorporation
  • Acquisition of ammonium ions (NH4+) from organic material
  • Nitrogen fixation (directly use gaseous nitrogen (N2) from the atmosphere)
• Sulfur is used to synthesize sulfur-containing amino acids & vitamins such as thiamine & biotin
• Phosphorus is essential for the synthesis of nucleic acids and the phospholipids of cell membranes, and is also found in the energy bonds of ATP
• Trace elements such as iron, copper, molybdenum, and zinc are required in very small amounts, usually as cofactors for enzymes
• Aerobic respiration


  Metabolic system that requires oxygen as the final electron acceptor, extracting more energy from nutrients than anaerobic respiration
• Classification based on need for oxygen


  • Obligate aerobes
  • Microaerophiles
  • Facultative anaerobes
  • Aerotolerant anaerobes
  • Obligate anaerobes
• Oxygen is actually toxic in its reactive forms (singlet oxygen, superoxide radicals, peroxide anion, hydroxyl radical)
• Organic growth factors


  Essential organic compounds that an organism is unable to synthesize and must directly obtain from the environment, including vitamins, essential amino acids, and purines & pyrimidines