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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". |
