Download .Bacterial metabolism--carbohydrate fermentation

.Bacterial metabolism--carbohydrate fermentation
Introduction
Most bacteria produce energy (ATP) by one or more of three mechanisms:
1. Aerobic respiration.
2. Anaerobic respiration.
3. Fermentation.
Aerobic respiration is an oxidative process which uses oxygen as a final electron acceptor.
Anaerobic respiration is similar to aerobic respiration, but it uses an inorganic molecule other
than oxygen as the final electron acceptor.
Fermentation uses an organic molecule as a final electron acceptor.
Aerobic respiration produces 36-38 ATP per glucose molecule, and is the most efficient form of
energy production. Fermentation is the least efficient means of energy production; it produces
only two ATP per glucose molecule. Anaerobic respiration is more efficient than fermentation,
but less efficient than aerobic respiration. The ATP yield per glucose molecule varies,
depending on the final electron acceptor used.
.Figure : Carbohydrate fermentation tube
1
Fermentation is an anaerobic process. However, many aerobic bacteria are capable of
fermentation, and may do so even when oxygen is available. In this experiment, we will
examine the capability of bacteria to ferment a variety of carbohydrates. We will also determine
the end products of bacterial fermentation.
1. A cid end product.
2. A cid and gas end products.
Most bacteria produce organic acids as by-products of fermentation. Incorporation of a pH
indicator into a medium allows detection of acidic fermentation products. Phenol red is one of
the most commonly used indicators. At acid pH, phenol red changes from red to yellow. Other
indicators may be used as well. Many bacteria also produce gases in addition to acids. Gases
may be detected by placing an inverted glass tube, called a Durham tube, into broth tubes. If the
bacteria produce gas, the Durham tube traps the bubbles (Figure 9.1). Acid and gas production
from different carbohydrates are important bacterial identification characteristics.
Materials
Each student/team:
3 (each) Phenol red broths (with Durham tubes) of: glucose (dextrose), lactose, sucrose.
Lab supplies:
Nutrient broth cultures of Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa
(all 24- to 48-hour).
Procedure
1. Inoculate each of the above bacteria into separate tubes of glucose, lactose and
sucrose broth.
2. Incubate all of the tubes for 24-48 hours at 37°C.
3. Examine the tubes for acid and gas production.
2