Topic 14: Fermentation of Carbohydrates

• Once a bacterium has been determined to be fermentative by the OF test, further tests can determine which carbohydrates are fermented
  • In some instances, end products can also be determined.
• Monosaccharide - glucose
• Disaccharide - sucrose
• Polysaccharide - cellulose
• A fermentation tube is used to detect acid and gas production from carbohydrates
  • Fermentation medium containts peptone, phenol red, and inverted tube to trap gas
• Phenol red is red in uninoculated fermentation rube
  • Acid production turns indicator yellow
• When gas is produced during fermentation, a Durham tube traps the gas.
• Fermentation occurs with or without oxygen present
• Prolonged incubation periods (more than 24 hrs), bacteria will grow oxidatively on the peptone after exhausting carbohydrate supply - causing neutralization of indicator (red)
• MRVP test is used to distinguish between organisms that produce large amounts of acid from glucose and those that produce neutral product acetoin
  • MRVP medium is glucose supplemented nutrient broth used from the methyl red (MR) test and the Voges-Proskauer (VP) test
  • If organism produces a large amount of organic acid from glucose - medium remains red when methyl red is added in a + MR test indicating pH is below 4.4
  • If neutral products produced - will turn yellow indicating pH is above 6.0
  • Production of acetoin is detected by the addition of potassium hydroxide and alpha-napthol.
    • if present - upper part of medium will turn red
    • a neg VP test will turn medium light brown
• Production of acetoin is dependent on the length of incubation

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