• Submitted By: natted
  • Date Submitted: 02/10/2009 10:12 AM
  • Category: Science
  • Words: 2116
  • Page: 9
  • Views: 1


The purpose of this experiment was to determine the identity of three unknown bacteria (#5, CC, and DD) using several differentials tests and cell structural characteristics. The structural differences examined included rod or coccus cell type and gram staining. The differential tests used included catalase test, oxidase test, phenol red broths, triple sugar iron agar, gelatinase test, mannitol salt agar, starch hydrolysis, nitrate reduction test, tryptophanase test, urease test, citrate test, methyl red test, Voges-Proskauer test, and coagulase test.

Gram staining was used to determine wither the cells were gram positive or gram negative and identification of the cells as rods or cocci was made at the same time. Gram positive cells have a thick peptidoglycan layer that is external and has s higher degree of cross-thinking that traps the crystal violet-iodine (CVI) complex better than gram negative cells. Therefore, they are less susceptible to decolorization by alcohol making them appear purple in color. Gram negative cells have a thinner peptidoglycan layer and a lipid membrane external to the cell wall which makes them susceptible to decolorization thus allowing them to be counterstained with safranin making them appear red in color. Rod shaped cells will appear oblong or cylindrical shaped whereas coccus shaped cells will appear as spheres.
Since my unknown was a mixture of gram negative and gram positive, I have to separate the two bacteria by inoculating them into a selective media. To separate the gram negative from the gram positive, I used the MacConkey agar, which is selective to gram negative bacteria. I used phenylethanol agar, PEA, which is selective to gram positive bacteria. As a result, I found that the gram positive grew on the PEA agar; it also changed the color of the media from red to yellow. The gram negative grew well on the MacConkey agar, which also changes the agar color to brown showing that the bacteria ferment...

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