Eosin methylene blue (emb) agar contains the dyes | Homework Help

1)     
Eosin Methylene Blue (EMB) agar contains the dyes eosin and methylene blue, which inhibit the growth of gram-positive bacteria and therefore select for gram-negative bacteria. These bacteria take up the EMB dye, giving large colonies a dark blue appearance and smaller colonies a green , shiny appearance.

2)     
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3)     
If 75% ethanol is poured to a single celled organism, the diluted alcohol also coagulates the protein, but at a slower rate, so that it penetrates all the way through the cell before coagulation can block it. Then the entire cell is coagulated and the organism dies.Whereas, 100% ethanol coagulates protein in contact. Suppose the pure alcohol is poured over a single celled organism. The alcohol will go through the cell wall of the organism in all direction, coagulating the protein just inside the cell wall. The ring of the coagulated protein would then stop the ethanol from penetrating farther from the cell, and no more coagulation would take place. At this time the cell would become inactive but not dead. Under the favorable conditions, the cell would then begin to function.

Take home assignment (5pt each) 1. Why does not EMB plates allow the growth of gram-positive bacteria? 

2. Why should you use tetracycline but not penicillin as a positive control in the experiment 3 of the microbiology lab? 

3. 100% ethanol can not kill bacteria as effectively as 75% ethanol. Please try to explain the reason.