ENVIRONMENTAL INFLUENCES ON MICROBIAL GROWTH

• Nutrients


  Chemical substances acquired from the environment and used in cellular activities
• Essential nutrients


  • C
  • N2
  • O2
  • H2
  • P
  • S
• Nutrient categories


  • Micronutrients or trace elements
  • Macronutrients
• Micronutrients or trace elements


  Required in small amounts, involved in enzyme function and maintenance of protein structure
• Macronutrients


  Required in large amounts, play principal role in cell structure and metabolism
• Cytoplasmic (cell) membrane


  • Controls passage of nutrients into the cell
  • Semi-permeable
  • Selectively permeable

  
• Only relatively small, non-polar materials can move through the lipid bilayer (lipids, oxygen, carbon dioxide and alcohol)

  
• Water-soluble materials like glucose, amino acids and electrolytes need some assistance to cross the membrane

  
• Cell envelope


  • Comprises the cell membrane, the cell wall and outer membrane if present
• Gram-negative bacteria outer membrane


  • Plays a role in regulating the passage of solutes
  • Semi-permeable
  • No proteins are known to pump solutes across it although some activity facilitate passage
• Concentration gradient


  The difference in concentration of a substance across a space

  
• Diffusion


  The movement of particles from an area of higher concentration to an area of lower concentration

  
• Types of diffusion


  • Passive diffusion (simple diffusion)
  • Facilitated diffusion
  • Active diffusion
  • Group translocation
• Passive diffusion


  A process in which molecules from a region of higher concentration moves to one of lower concentration
• Facilitated diffusion


  Diffusion in and out of the cell of certain compounds mediated by specific membrane proteins (permeases, carrier proteins, channel proteins)
• Active transport


  Metabolic energy is utilized for the transport of substances through carrier proteins embedded in the membrane

  
• Electrical gradients


  Difference in charge across plasma membrane because atoms and molecules can form ions and carry positive or negative electrical charges
• Proton motive force (PMF)


  The transfer of hydrogen molecule (H+) through a proton pump generates an electrochemical gradient of protons, driving the conversion of ADP to ATP through ATP synthase

  
• Electron transport system (ETS) is involved in active transport

  
• Endocytosis


  A form of active transport that does not involve membrane carriers
• Exocytosis


  A form of active transport that does not involve membrane carriers
• ATP-binding cassette (ABC) transporters


  A large and diverse superfamily of proteins involved in uptake and expulsion of a variety of substrates
• Types of active transport

  • Uniport (transports one solute)
  • Symport (transports solute and co-transported solute in same direction)
  • Antiport (transports solute in and co-transported solute out)
• Secondary active transport


  Establishment of a proton motive force by proton extrusion associated with the passage of electrons through a membrane-bound transport chain or by hydrolysis of ATP by the membrane-bound ATPase

  
• Group translocation


  A process in which a molecule is chemically modified as it is brought into the cell, e.g. uptake of sugars like glucose and mannose via the phosphotransferase system (PTS)
• Comparison of transport systems

  • Passive diffusion
  • Facilitated diffusion
  • Active transport
  • Group translocation

  
• Some macromolecules and highly charged small molecules cannot pass the cell membrane
• Exoenzymes


  Enzymes that are inactive while inside the cell but become active upon release from the cell
• Osmotic pressure


  The force with which water moves through the cytoplasmic membrane from a solution containing a low concentration of dissolved substances (solute) to one containing a high solute concentration
• Bacteria based on osmotic requirement

  • Osmophiles (grow in high osmolarity solutions)
  • Halophiles (grow in high salt concentration)
• Sodium is required by some bacteria for growth on certain carbon and energy sources, especially those of marine origin, moderate halophiles and extreme halophiles
• Hydrostatic pressure


  Pressure exerted on the cells by the movement of water resting on top of them
• Barophiles


  Organisms that metabolize or function better at high pressure than at atmospheric pressure, found in deep-sea environments
• Cardinal temperatures


  Minimum, optimum, and maximum temperatures for microbial growth and metabolism
• Microbial cardinal temperatures may vary depending on the stage in the life cycle of the microorganism and the nutritional content of the medium
• High temperatures damage microbes by denaturing enzymes, transport carriers and other proteins, and disrupting microbial membranes
• As incubation temperature is lowered, enzymatic activity and growth rate decrease
• Barophiles


  • Organisms that metabolize or function better at high pressure than at atmospheric pressure
  • Organisms that need a high-pressure environment in order to grow
  • Found in deep-sea environment (ocean floors, deep lakes-pressure exceeds 380 atm)
• Temperature


  • Most important factor that determines the rate of growth, multiplication, survival and death of all microorganisms
  • Rate of enzyme reaction increases with higher temperature
• Cardinal temperatures


  Minimum, Optimum, Maximum