unit 2 - ka6: environmental control of metabolism

• Micro-organisms are archaea, bacteria and some species of eukaryotes.
• Micro-organisms use a wide variety of substrates for metabolism and produce a range of products from their metabolic pathways.
• Micro-organisms are used because of their adaptability, ease of cultivation and speed of growth.
• Variations in growth media and control of environmental factors.
• When culturing micro-organisms, their growth media require raw materials for biosynthesis as well as an energy source.
• Many micro-organisms produce all the complex molecules required for biosynthesis, for example amino acids, vitamins and fatty acids. 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.
• Culture conditions: sterility, control of temperature, oxygen levels and pH.
• Sterile conditions in fermenters reduce competition with desired micro-organisms for nutrients and reduce the risk of spoilage of the product.
• Phases of growth and changes in culture conditions.
• Phases — lag, log/exponential, stationary and death.
• The lag phase is where enzymes are induced to metabolise substrates.
• The log/exponential phase contains the most rapid growth of micro-organisms due to plentiful nutrients.
• The stationary phase occurs due to the nutrients in the culture media becoming depleted and the production of toxic metabolites. Secondary metabolites are also produced, such as antibiotics. In the wild these 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 toxic accumulation of metabolites or the lack of nutrients in the culture.
• Growth curves of micro-organisms.
• Use of semi-logarithmic scales in producing or interpreting growth curves of micro-organisms.
• Viable and total cell count.
• Viable cell counts involve counting only the living micro-organisms whereas total cell counts involve counting viable and dead cells. Only viable cell counts show a death phase where cell numbers are decreasing.