Exercise 4-5 PDF

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Answers to Lab Exercise 4-5 (assigned by Dr. Amy Zimmerman)...

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Carolyn Straub Exercise 4-5 Questions 1. The dyes eosin Y and methylene blue in Eosin Methylene Blue (EMB) agar inhibit the growth of most Gram-positive bacteria and cause organisms that ferment lactose with acid end products to turn dark purple or black. Therefore, EMB agar is useful for isolating fecal coliforms. 2. a. Gelatin supplies carbon b. Gelatin supply nitrogen 3. EMB agar is an undefined, or complex, medium because the precise amounts of each ingredient are unknown. This is not preferred. Defined mediums 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. 4. The dyes eosin Y and methylene blue in EMB agar make the medium selective. 5. Lactose in EMB agar make 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 eosin Y and/or methylene blue from the plate would alter the mediums’ selectivity because they are what make EMB agar a selective medium in the first place. Without them, all organisms would be able to grow on EMB agar, so it could not be considered selective. 8. If the starting pH were 7.6 instead of 6.9, the EMB agar plates’ sensitivity would be affected. The plate would still be selective, however, because only Gram-negative bacteria 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, so there would probably be less dark purple or black color observed.

Carolyn Straub Exercise 4-5 9. I would expect the organisms to grow better on the EMB agar because it would have less

competitors. The eosin Y and methylene blue would have inhibited the growth of any other organisms that would be competing with it for nutrients on the agar plate....