Atypical bacteria

Atypical bacteria are bacteria that do not color with gram-staining but rather remain colorless: they are neither Gram-positive nor Gram-negative. These include the Chlamydiaceae, Legionella and the Mycoplasmataceae (including mycoplasma and ureaplasma); the Rickettsiaceae are also often considered atypical. Atypical bacteria are bacteria that do not color with gram-staining but rather remain colorless: they are neither Gram-positive nor Gram-negative. These include the Chlamydiaceae, Legionella and the Mycoplasmataceae (including mycoplasma and ureaplasma); the Rickettsiaceae are also often considered atypical. Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet during Gram staining, resulting in a purple color. Gram-negative bacteria have a thin peptidoglycan layer which does not retain the crystal violet, so when safranin is added during the process, they stain red.