Question

BIO 205 - NLV-B-TTH Fall 1 2019 Hi, kathia Sign Out Course Home <Ch 03 HW MicroCareers: Nice to See You ( 2014 Put the following descriptions in order for the staining reactions in the cells of a bacterial smear during the Gram staining procedure. View Available Hint(s) Dve molecules are rendered insoluble in presence of mordant des insed way Clear cells are stained a red or pink whereas the color of other cells Call wall of Gram-negative cells is broken down Gram-positive cells Al cells are 3 Unstained cells (start of process) cells (end of process)

Answer 1

The bacteria cam be distinguished between gram positive snd gram negative with the help of gram staining. Gram staining is used as a major diagnostic tool for the infections caused by bacteria. The mechanism used by gram staining is the difference in cell wall structure and permeability of gram positive and gram negative bacteria.

The sequence of events for gram staining are:

1. All cells are stained purple :

crystal violet dye is added to the fixed culture of bacteria. This renders all the bacterial cells purple.

2. Dye molecules are rendered insoluble in presence of mordant ; dye is rinsed away.

Iodine which acts as a mordant is added to the bacterial slide. Iodine increases the affinity of cell wall for dye and makes an insoluble complex. Thus iodine acts as a trapping agent.

After adding iodine a decoloriser is added like alcohol or acetone.

3. The cell wall of gram negative bacteria is broken down. Gram positive cells remain intact.

The addition of decoloriser results in breakage of gram negative bacterial cell wall while the gram positive bacteria remains intact. The cell wall of gram positive bacteria is made of high amount of peptidoglycan which is thick. The presence of cross linked short chains of peptides in the cell wall helps to provide it rigidity. Whereas the cell wall of gram negative bacteria contains less amount of peptidoglycan and is weak.

The breakage of cell wall of gram negative results in loss of the purple dye and the cells become colourless whereas gram positive bacteria remains purple stained.

4. Clear cells are stained red or pink whereas the color of other cells remain unchanged.

The safranine dye is then added as a counterstain to the slide. The clear gram negative bacteria turns pink or red in colour. The gram positive bacteria remains stained purple as the pink or red colour of safranine dye hides behind the purple colour.