Exercise 4-3 PDF

Preview

Summary

Answers to Lab Exercise 4-3 (assigned by Dr. Amy Zimmerman)...

Description

BIOL 360

Exercises 4-3 – 4-6 Carolyn Straub

Lab Partner: Patricia Takam

Carolyn Straub Exercise 4-3 Questions 1. The high salt content of Mannitol salt agar (MSA) kills most bacteria, but Staphylococcus thrive on it. While most Staphylococcus bacteria do not ferment mannitol and therefore appear pink on the medium, Staphylococcus aureus does. When mannitol ferments, it lowers the pH of the medium and turns it yellow. Therefore, MSA is useful for distinguishing S. aureus from other Staphylococcus bacteria. 2. a. Mannitol supplies carbon b. The enzymatic digests of casein and animal tissue, as well as beef extract, supply nitrogen 3. MSA is a defined medium because the precise amounts of each ingredient are known. This is preferred because it can provide trace elements and vitamins to the organism as well as very defined carbon and nitrogen sources. Undefined media are often used out of necessity because they are less expensive and the organism being used may have never been grown on a defined media before. A sample ingredient list for an MSA plate is shown below: a. 5.0 g/L enzymatic digest of casein b. 5.0 g/L enzymatic digest of animal tissue c. 1.0 g/L beef extract d. 10.0 g/L D-mannitol e. 75.0 g/L sodium chloride f.

0.025 g/L phenol red

g. 15.0 g/L agar h. pH 7.4 ± 0.2 at 25°C 4. The sodium chloride in MSA makes the medium selective. Because the high concentration of NaCl makes the environment inhabitable to most bacteria, it ensures that only Staphylococcus, which are comfortable in saline settings, will be able to survive on the plate.

Carolyn Straub Exercise 4-3 5. The carbohydrate mannitol serves as the carbon source for the bacteria. It is also what S. aureus ferment, lowering the pH of the medium to change the color from pink to yellow. Mannitol therefore makes the medium differential. 6. a. I looked at how many individual colonies were present and how thick the confluent growth was at the beginning of the streak. b. Even though the organisms may be the same, they were grown in very different conditions, so their presentations after incubations cannot be compared. 7. The removal of NaCl from the plate would alter the mediums’ selectivity because sodium chloride is what makes MSA a selective medium in the first place. Without NaCl, all organisms would be able to grow on MSA, so it could not be considered selective. 8. If the starting pH were 7.8 instead of 7.2, the MSA plates’ sensitivity would be affected. The plate would still be selective, however, because only Staphylococcus would be able to survive. If the pH started higher, a greater difference in pH would be required for a change in color to be observed (the plate turns yellow below a pH of 6.8), so there would probably be less yellow color observed. 9. Because of the selectivity of MSA, even though there is massive diversity of microorganisms in the world, there can only be Staphylococcus bacteria growing on an MSA plate. Therefore, we can narrow our search and we only have to compare S. aureus to Staphylococcus.

Results Summary All three organisms (Staphylococcus aureus, Esherichic coli, and Staphylococcus epidermis) grew well in white, translucent, confluent colonies on the nutrient agar plate. In contrast, E. coli did not grow at all on the MSA plate. S. epidermis and S. aureus grew in lighter colonies with some confluent growth and some individual colonies. No yellow colonies were observed.

Carolyn Straub Exercise 4-3 Conclusion As was expected, E. coli did not grow on the MSA plate. What was a surprise was the fact that S. aureus did not produce yellow colonies....