Which step in the gram stain is the critical step in differentiating gram-positive cells from gram-negative cells? a) safranin b) alcohol-acetone c) iodine d) crystal violet

The correct answer is: B) alcohol-acetone

Gram staining includes next steps:

•Fixation- usually  by heating or by using methanol

•Primary stain (crystal violet)- it stains all cells blue/purple  

•Mordant- added to form a crystal violet iodine

•Decolorization- distinguishes gram-positive from gram-negative cells. Acetone or ethanol are usually used. Acetone/ethanol extracts blue dye from Gram - bacteria (the lipid-rich, thin wall) more than from the, Gram + bacteria (lipid poor, thick walled).  The Gram -bacteria appear colorless and Gram + bacteria remain blue.

•Safranin-the red dye stains the decolorized Gram- bacteria.

the dark variety, as they are not visible on the dark tree barks

explanation:

1. is geographic distribution

2 is increasing complexity

3 is transitional forms