Question

1. Obligate anaerobes are often grown in an anaerobe jar, which completely excludes oxygen from the environment. How is the environment within a tube of fluid thioglycollate media different from that found within the anaerobe jar?

2. Phenol red is another indicator commonly used to detect the presence of acid. This indicator is red above pH7 and yellow below pH of 7. What would be the appearance of E. coli (gram negative rod, lactose fermenter) and S. aureus (gram positive cocci, lactose fermenter) if neutral red in MacConkey agar was replaced with phenol red?
3. There are 4 classes of biological molecule, which class of biological molecule is responsible for the ability of a bacterium to ferment a sugar? Hint: think enzyme
4.What is the enzyme that is being detected in a positive starch test?
5. Hydrogen peroxide is used as an antiseptic to kill bacteria in cuts and abrasions. Many of these bacteria are catalase positive, yet they are still killed by the actions of hydrogen peroxide. Why does this occur?

Answer 1

Q1. Anaerobe jar is used to culture anaerobic bacteria, while fluid thioglycollate media is used to determine the oxygen requirements of the bacteria. Anaerobe jar contains sodium borohydride and sodium bicarbonate which break down to CO2 and hydrogen, the later reacts with oxygen resulting in anaerobic condition. Fluid thioglycollate media is differential medium, there is a gradual increase in oxygen concetration from bottom of the tube to the top.

Q2. MacConkey agar is used to selectively isolate Gram-negative bacteria. It contains some salts and dye which inhibit most Gram-positive bacteria. Both bacteria ferment the lactose, producing acid, and it was expected for both cultures to be colored red. But, since S. aureus is Gram-positive bacteria, its growth will be inhibited, and, consequently, there will be no produced acid.

Q3: Proteins are responsible for the ability of a bacterium to ferment a sugar. Proteins include enzymes, such as beta-galactosidase or sucrase, which ferment lactose or sucrose, respectively.

Q4: In a positive starch test, alpha-amylase is detected. Alpha-amylase breaks down starch into di- and trisaccharide. If iodine solution reacts with starch, it will change color into black. But, a presence of clear halos in the medium indicates a presence of alpha-amylase because it will break down starch.

Q5: Catalase is able to break down hydrogen peroxide into water and oxygen. Still, if there is a great amount of hydrogen peroxide, catalase won't be able to break down all molecules. Its capacity is limited. So, if there is a lot of hydrogen peroxide it will eventually kill the bacteria.