Exam 2 Lab Questions

Would mannitol salt agar still work (be selective for S. aureus) if it did not have salt in it?

Eliminating the high concentration of salt would make the medium no longer selective. Any thing could grow, not just Staphs and Vibrios. The media would still be differential for mannitol fermentation, however

Do all bacteria that grow on blood agar break down the blood? Explain why just about all bacteria will grown on blood agar. How is it a differential media?

No, all bacteria that grow on blood agar do not break down blood. Most bacteria will grow on blood agar because it is an enriched medium. only some bacteria can make toxins which break down blood and cause a visible change on the plate this makes the medium differential.

An alternate method of performing the catalase test exists where hydrogen peroxide is dropped directly onto a colony growing on an agar plate. The test can be done if the bacterium is growing on nutrient agar but not blood agar. Why?

Red blood cells produce catalase and would give a false positive catalase test result if the test was performed on colonies growing on blood agar

What is the agent or chemical in mannitol salt agar that makes the medium differential? How does the agent work?

The addition of phenol red, which is a pH indication, makes the medium differential for the detection of acid. Since Mannitol is the only carbohydrate added, metabolism of mannitol is demonstrated when the agar around the colonies changes from a red color to a yellow color

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?

The small amount of catalase present in these cells is overwhelmed by the volume of hydrogen peroxide used. Even when all enzyme molecules are converting hydrogen peroxide without interruption, many other molecules are free to cause damage to the cell