<P> Along with cell shape, Gram staining is a rapid method used to differentiate bacterial species . Such staining, together with growth requirement and antibiotic susceptibility testing, and other macroscopic and physiologic tests, forms the full basis for classification and subdivision of the bacteria (e.g., see figure and pre-1990 versions of Bergey's Manual). </P> <P> Historically, the kingdom Monera was divided into four divisions based primarily on Gram staining: Firmicutes (positive in staining), Gracilicutes (negative in staining), Mollicutes (neutral in staining) and Mendocutes (variable in staining). Based on 16S ribosomal RNA phylogenetic studies of the late microbiologist Carl Woese and collaborators and colleagues at the University of Illinois, the monophyly of the Gram - positive bacteria was challenged, with major implications for the therapeutic and general study of these organisms . Based on molecular studies of the 16S sequences, Woese recognised twelve bacterial phyla . Two of these were both Gram - positive and were divided on the proportion of the guanine and cytosine content in their DNA . The high G + C phylum was made up of the Actinobacteria and the low G + C phylum contained the Firmicutes . The Actinobacteria include the Corynebacterium, Mycobacterium, Nocardia and Streptomyces genera . The (low G + C) Firmicutes, have a 45--60% GC content, but this is lower than that of the Actinobacteria . </P> <P> Although bacteria are traditionally divided into two main groups, Gram - positive and Gram - negative, based on their Gram stain retention property, this classification system is ambiguous as it refers to three distinct aspects (staining result, envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species . The Gram - positive and Gram - negative staining response is also not a reliable characteristic as these two kinds of bacteria do not form phylogenetic coherent groups . However, although Gram staining response is an empirical criterion, its basis lies in the marked differences in the ultrastructure and chemical composition of the bacterial cell wall, marked by the absence or presence of an outer lipid membrane . </P> <P> All Gram - positive bacteria are bounded by a single - unit lipid membrane, and, in general, they contain a thick layer (20--80 nm) of peptidoglycan responsible for retaining the Gram stain . A number of other bacteria--that are bounded by a single membrane, but stain Gram - negative due to either lack of the peptidoglycan layer, as in the Mycoplasmas, or their inability to retain the Gram stain because of their cell wall composition--also show close relationship to the Gram - positive bacteria . For the bacterial cells bounded by a single cell membrane, the term "monoderm bacteria" or "monoderm prokaryotes" has been proposed . </P>

Can bacteria be gram positive and gram negative