Topic 13: Carbohydrate Catabolism

• Catabolism: chemical reactions that release energy from the decomposition of complex organic molecules
• Most bacteria catabolize carbohydrates for carbon and energy.
• Carbohydrates: organic molecules that contain carbon, hydrogen, and oxygen in the ratio : (CH2O)n
  • Carboyhydrates can be classified based on size: monosaccharides, oligosaccharides, and polysaccharides.
• Monosaccharides: simple sugars containing three to seven carbon atoms.
  • Oligosaccharides: composed of two to about twenty monosaccharide molecules.
  • Disaccharides most common oligosaccharide
  • Polysaccharides: consist of eight or more monosaccharide molecules
• Exoenzymes are mainly hydrolytic enzymes that leave the cell and break down, by the addition of water, large substrates into smaller components, which can then be transported into the cell.
  • Amylase hydrolyzes the polysaccharide starch into smaller carbohydrates
  • Glucose, a monosaccharide, can be released by hydrolysis
• In lab, presence of an exoenzyme is determined by looking for a change in the substrate outside of a bacterial colony.
• Glucose can enter a cell and be catabolized
  • some bateria using endoenzymes, catabolize glucose oxidatively producing carbon dioxide and water
• Oxidative catabolism requires the presence of molecular oxygen (O2)
• Most bacteria can ferment glucose without using oxygen.
  • Fermentative catabolism - do not require oxygen but may occur in its presence
  • end products of fermentation are small organic molecules, usually organic acids; some bacteria may produce gas
• Determination of oxidative or fermentative bacterium can be done by using OF medium.
  • OF medium: nutrient semisolid agar deep containing a high concentration of carbohydrate and a low concentration of peptone.
  • peptone will support growth of bacteria that don’t use the carbohydrate
  • two tubes used: one open to air and one closed to keep air out
  • contains bromthymol blue - turns yellow in presence of acids (indicates catabolism); turns dark blue in presence of alkaline conditions (indicates fermentation)
  • an organism that can only use the carbohydrate under aerobic conditions will produce acid in open tube only
    • acids are produced as intermediates in respiration and indicator will turn yellow in the top of the open test tube - oxidative organism

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