3. Assume the entire Gram procedure was carried out and the following mistakes w

Question

3. Assume the entire Gram procedure was carried out and the following mistakes were made: a) Decolourizer step was omitted for a smear made from a red isolated colony on MacConkey agar b) Inoculum from an isolated colony with a clear zone on Blood agar was removed from the center of the colony c) Human cells were stained using the Gram procedure State the colour you would expect of the cells when viewed using the oil immersion lens of the compound microscope with a corresponding explanation for your choice.

Explanation / Answer

a. Red isolated colony on MacConkey agar means it is a gram negative lactose fermenting colony. As we know MacConkey agar inhibits the growth of gram positive bacteria. During gram stinging, bacteria stained with Crystal violet- iodine complex is washed with decolorizer. If still bacteria retains the crystal violet- iodine complex then it will not be stained by safranin and will appear violet under oil immersion lens. But if we ommit decolorization step, then all bacteria will retain the color of crystal violet-iodine complex and will appear violet. So even if the bacteria on MacConkey agar is gram negative, it will be blue.

b. Question is imcomplete

c.the cells which have thick cell wall can only retain the crystal violet- iodine complex and can be gram positive. Human cell doesn't have a cell wall. So crystal violet- iodine complex will wash out in the decolorization step. The cell will then retain the color of the counter stain safranin

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