<P> After decolorization, the gram - positive cell remains purple and the gram - negative cell loses its purple color . Counterstain, which is usually positively charged safranin or basic fuchsine, is applied last to give decolorized gram - negative bacteria a pink or red color . </P> <P> Gram - positive bacteria generally have a single membrane (monoderm) surrounded by a thick peptidoglycan . This rule is followed by two phyla: Firmicutes (except for the classes Mollicutes and Negativicutes) and the Actinobacteria . In contrast, members of the Chloroflexi (green non-sulfur bacteria) are monoderms but possess a thin or absent (class Dehalococcoidetes) peptidoglycan and can stain negative, positive or indeterminate; members of the Deinococcus - Thermus group, stain positive but are diderms with a thick peptidoglycan . </P> <P> Historically, the gram - positive forms made up the phylum Firmicutes, a name now used for the largest group . It includes many well - known genera such as Bacillus, Listeria, Staphylococcus, Streptococcus, Enterococcus, and Clostridium . It has also been expanded to include the Mollicutes, bacteria like Mycoplasma that lack cell walls and so cannot be stained by Gram, but are derived from such forms . </P> <P> Some bacteria have cell walls which are particularly adept at retaining stains . These will appear positive by Gram stain even though they are not closely related to other gram - positive bacteria . These are called acid fast bacteria, and can only be differentiated from other gram - positive bacteria by special staining procedures . </P>

Could bacteria that do not contain cell walls be stained using gram stain