Gram-positive Bacteria - Classification

Classification

Along with cell shape, Gram staining is a rapid diagnostic tool of use to group species of Bacteria. In traditional and even some areas of contemporary microbiological practice, such staining, alongside 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).

As such, historically, the kingdom Monera was divided into four divisions based primarily on Gram staining: Firmicutes (positive in staining), Gracillicutes (negative in staining), Mollicutes (neutral in staining) and Mendocutes (variable in staining).

Since 1987 and the seminal 16S ribosomal RNA phylogenetic studies of Carl Woese (Department of Microbiology, University of Illinois) and collaborators and colleagues, the monophyly of the Gram-positive bacteria has been challenged, with striking productive implications for the therapeutic and general study of these organisms. Based on molecular studies of 16S sequences, Woese recognised twelve bacterial phyla, two being Gram-positive: high-GC Gram-positives and low-GC Gram-positives (where G and C refer to the guanine and cytosine content in their genomes), which are now referred to by these names, or as Actinobacteria and Firmicutes. The former, the Actinobacteria, are the high GC content Gram-positive bacteria and contains genera such as Corynebacterium, Mycobacterium, Nocardia and Streptomyces. The latter, the Firmicutes are the "low-GC" Gram-positive bacteria, which actually have 45%–60% GC content but lower than that of the Actinobacteria,. The Firmicutes contain the well-known genera that are majority of Gram-positives of medical interest: Staphylococcus, Streptococcus, Enterococcus (cocci), Bacillus, Clostridium and Listeria (bacilli/rods). This group also been expanded to include the Mycoplasma, or Mollicutes, bacteria-like organisms that lack cell walls and cannot be Gram-stained, but appear to have derived evolutionarily from such forms.

Despite the wide acceptance and practical record of utility of the new molecular phylogeny, a small group, including Cavalier-Smith, still treat the Monera as a monophyletic clade and refer to the group as division "Posibacteria".