Why are gram positive bacteria susceptible to penicillin?

Penicillin is an antibody of the fungus Penicillium that prevents gram positive from forming cell walls. Without a cell wall the bacteria dies because it cannot maintain homeostasis.
Gram positive bacteria have a thick peptidoglycan layer in their cell wall which differentiates them from gram negative bacteria that do not that thick peptidoglycan layer as gram positive have.
Gram Stain shows difference in cell walls which is present in a thick layer in gram-positive bacteria. Gram – bacteria walls are thinner and the blue stain is lost in the staining process.
In a Gram stain test gram-positive bacteria retain the (crystal violet) dye while a counterstain (pink) added after the crystal violet gives all gram-negative bacteria a red or pink coloring.
All bacteria which are stained are given the same process as you do not know until the end which will retain the violet die or will be red.
The violet color is not washed out in Gram+ bacteria and it will cover over the red counter stain.
Gram staining is almost always used first to identify organisms.