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".