Question

what is the influence of pH on the gram stain reaction?

Answer 1

pH affects the uptake of crystal violet by the bacteria

Crystal violet at low pH and safranin do not stain the walls of yeast and bacteria. When organisms of a gram positive species are stained with crystal violet at a high pH and then are washed at a low pH, the dye disappears from the walls. Organisms with stained walls resist both decolorizer and counterstain after treatment with iodine and thus appear gram positive.

Source: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC357432/pdf/jbacter00549-0019.pdf