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
