Stackebrandt et al., 1988
In a Gram stain test, a counterstain (commonly safranin) is added after the crystal violet, colouring all gram-negative bacteria with a pink colour.
The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their cell walls.
Others are free-living, and include many of the bacteria responsible for nitrogen fixation. The group is defined primarily in terms of ribosomal RNA (rRNA) sequences.
- Proteobacteria: Purple bacteria and their relatives
- alpha subdivision (purple non-sulfur bacteria, rhizobacteria, Agrobacterium, Rickettsiae, Nitrobacter)
- beta subdivision (Rhodocyclus, (some) Thiobacillus, Alcaligenes, Spirillum, Nitrosovibrio)
- gamma subdivision (enterics, fluorescent pseudomonads, purple sulfur bacteria, Legionella, (some) Beggiatoa)
- delta subdivision (Sulfur and sulfate reducers (Desulfovibrio), Myxobacteria, Bdellovibrio)