🦠 Gram positive and gram negative bacteria

Amongst bacteria, the cell wall composition is a key determinant of what type they belong to. This is important in terms of predicting their response to various antibiotics. Based on different bacteria species’ response to crystal violet stain, Gram positive bacteria are able to take up the stain and appear violet under a microscope, while Gram negative bacteria do not take the stain up and will appear pink if a counterstain is added after washing off the crystal violet stain (this will persist in the Gram positive bacteria).

The difference arises because different bacteria have different cell walls. The bacterial
cell wall is one of the main targets of antibiotics.



Notice the difference in thickness of the murein layer in gram positive versus gram negative cells. This layer is what absorbs the violet stain. Hence gram positive bacteria turn violet, while gram negative bacteria lose the stain upon washing.

Penicillin is an antibiotic used against gram positive bacteria. It doesn’t work on gram negative bacteria because their outer membrane (cell envelope) protects against it. Penicillin works by interfering with the production of the cell wall component murein, and as gram positive bacteria have so much of…