MS6- environmental control of metabolism

• Micro-organisms can come from the three domains of life- archaea, bacteria and some species of eukaryotes
• Some micro-organisms are prokaryotic (archaea and bacteria) and others are eukaryotic (algae, protozoa and fungi)
• micro-organisms are able to use a wide variety of substrates for metabolism
• This allows micro-organisms to survive in a wide range of ecological niches
• In the laboratory this makes it easy for scientists to culture them
• micro-organisms are used by humans in research and industry due to their:

  • adaptability
  • ease of cultivation
  • speed of growth
• micro-organisms produce a wide range of products from their metabolic pathways which can be used for human benefit
• microorganisms can be cultured (grown) relatively easily in a laboratory
• To ensure successful growth they must be given an appropriate growth medium and the environmental conditions must be carefully controlled
• When culturing micro-organisms their growth media require;

  • raw materials for biosynthesis
  • an energy source
• many micro-organisms produce all the complex molecules required for biosynthesis;

  • amino acids
  • fatty acids
  • vitamins
• other micro-organisms require these to be supplied in the growth media
• An energy source is derived either from chemical substrates or from light in photosynthetic micro-organisms
• To allow optimal culture conditions the following environmental factors must be carefully controlled;

  • temperature
  • oxygen levels
  • pH
• Sterile conditions in fermenters reduces;

  • competition with desired micro-organism for nutrients
  • the risk of spoilage of the product
• Microorganisms are able to grow rapidly under suitable conditions
• The growth of microbes can be graphed and has four main phases
• 1. lag phase
  2. log or exponential phase
  3. stationary phase
  4. death phase
• The lag phase is when enzymes are induced to metabolise substrates
• There is little/no change in number of microbes- population remains small
• The most rapid growth of micro-organisms occurs during the log/ exponential phase due to the plentiful supply of nutrients
• The microbes multiply at a constant rate resulting in population growth
• The stationary phase occurs due to:

  nutrients in the culture becoming depleted

  the production of toxic metabolites
• The number of bacteria being produced is the same as the number dying so there is no net population growth
• Secondary metabolites, such as antibiotics, are also produced during the stationary phase
• In the wild these secondary metabolites confer an ecological advantage by allowing the micro-organisms which produce them to outcompete other micro-organisms
• The death phase occurs due to:

  • the accumulation of toxic waste/ metabolites
  • the lack of nutrients in the culture

  this results in a decline in the population
• When producing or interpreting growth curves of micro-organisms a semi-logarithmic scale is used because the data range on the y-axis is extremely large
• When carrying out a viable cell count only living cells will be counted
• When carrying out a total cell count both viable and dead cells are counted
• only viable cells will show a death phase where the cell numbers are decreasing
• A dilution of the cells to be counted is prepared and mixed with trypan blue, this stain is taken up by dead cells and excluded from live cells
• Once the cells have been stained they are counted using a haemocytometer