It is important to recognize that this class is defined morphologically (by the presence of a bacterial outer membrane), and not histologically (by a pink appearance when stained), though the two usually coincide.
One reason for this division is that the outer membrane is of major clinical significance: it can play a role in the reduced effectiveness of certain antibiotics, and it is the source of endotoxin.
The gram status of some organisms is complex or disputed:
Mycoplasma are sometimes considered gram negative, but because of its lack of a cell wall and unusual membrane composition, it is sometimes considered separately from other gram-negative bacteria.
Gardnerella is often considered gram negative, but it is classified in MeSH as both gram positive and gram negative. It has some traits of gram positive bacteria, but has a gram negative appearance. It has been described as a "gram-variable rod".