With regard to the cell membrane structure, bacteria can be divided in two classes:
- Gram-positive bacteria: Gram-positive bacteria like Bacilli contain only an inner membrane and a peptidoglycan layer.
- Gram-negative bacteria. Gram-negative bacteria such as E.coli have an inner and outer membrane, with an aqueous compartment between the two membranes - the periplasm. This compartment harbors the peptidoglycan layer.