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Regrade_Form - REQUEST TO REVIEW AN EXAM ITEM/REGRADE DATE...

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Unformatted text preview: REQUEST TO REVIEW AN EXAM ITEM/REGRADE DATE NAME !"#$%&'()*+(,--.(/01223(0144$5&&67 )*+%#+,-.*+, QUIZ/SGD/CASE STUDY/Mid-term QUESTION # "#$%&'( I believe that answer ___ should also be considered correct because I found supporting evidence ! from the lecture notes/lecture outline (briefly describe below): From our knowlege about h ow Gram stains work, we are only able to conclude that Archea should stain GRAM NEGATIVE. Gram Stains work by Birst d ying cells with C rystal ­ Violet then with Iodine to form Iodine ­Violet complexes within cells. Then ethanol is u sed to d ecolorize cells without a thick p eptidoglycan because ethanol extracts lipids from membranes and collapses it. Any cell without a thick p eptidoglycan cannot h old the color in, therefore at this stage, only cells with thick peptidoglycan (Gram + cells) are able to retain color and remain p urple. All other cells are colorless. We then restain with safrarin. C ells that were colorless (Gram  ­) are n ow red/pink. Archea d o n ot h ave a thick peptidoglycan layer (only p seudopeptioglycan layer) therefore should n ot be able to retain p urple color and should look like a Gram Negative C ell. According to Todlar's Textbook of Bacteriology (http://www.textbookoBbacteriology.net/procaryotes.html) "Although the Gram reaction d epends on both the structural format and the chemical composition of the cell envelope in bacteria, most archaea stain Gram ­n egative, independent of their basic cell envelope structure or chemical composition. An interesting exception is Methanobacterium formicicum that stains Gram ­p ositive, since its cell wall contains p seudomurein, a type of p eptidoglycan that lacks muramic acid. In one study, all other archaea stained Gram ­n egative because their cell walls were so d isrupted d uring staining, that the crystal violet ­mordant complex could n ot be retained by the cells. Methanococcus jannaschii was grown at both 5 0 d egrees C and 7 0 d egrees C , which increases the tetraether lipids in its plasma membrane from 2 0% (50 d egrees C ) to 4 5% (70 d egrees C ) of the total lipids; in both cases the cells stained Gram ­n egative. O ne conclusion from these observations is that in a Gram stain p reparation of a mixed microbial p opulation containing archaea and bacteria, the archaea are among the Gram ­n egative cells, and the Gram ­p ositive cells are p robably bacteria." ...
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