Question

Read Exercise 3-7 in your lab manual, then complete the following item 3. Why is the order of the reagents in a Gram stain so important? (2 pts) 4. Rarely is a Gram stain ever perfect, so proper interpretation of a Gram stain result requires you to formulate a "consensus" decision about the general Gram reactivity of the cells stained. Study the three microscopic fields of view pictured below and for each develop a consensus decision about the overall Gram reaction of the bacterial cells shown. Record your answers in the blanks provided as either Gram positive, Gram negative, or Gram variable, as appropriate. (3 pts) CELL LEGEND: • • o purple pink unstained Gram reaction:

Answer 1

Answer 3. Order of reagents

Primary stain (crystal violet),

Mordant (iodine),

Decolorizer (ethanol or acid-alcohol)

Counter stain (safranin or dilute carbol-fuchsin).

Bacteria Stained with + crystal violet All bactenia will be stained bluish or bluish purple Kl solution Btain will be fixed due to formation of a complex of crystal violet and Ki ( Mordant) I Destaining is done t with water & alcohol If Stain comes Stain does not come out out gram +ve Bacteria (violet/purple) qram -ve Bacteria ( Pink)

The order of reagents are important because when stained with a primary stain and fixed by a mordant, some bacteria are able to retain the primary stain by resisting declorization while others get decolorized by a decolorizer.

Answer 4.

4. Rarely is a Gram stain ever perfect, so proper interpretation of a Gram stain result requires you to formulate a "consensus" decision about the general Gram reactivity of the cells stained. Study the three microscopic fields of view pictured below and for each develop a consensus decision about the overall Gram reaction of the bacterial cells shown. Record your answers in the blanks provided as either Gram positive, Gram negative, or Gram variable, as appropriate. (3 pts) CELL LEGEND: purple pink unstained o Gram reaction: purple - pink unstrained

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