Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain (safranin or fuchsine) and appearing red or pink. Gram-positive organisms are able to retain the crystal violet stain because of the high amount of peptidoglycan in the cell wall. Gram-positive cell walls typically lack the outer membrane found in Gram-negative bacteria.
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The following characteristics are generally present in a Gram-positive bacterium:[1]
Both Gram-positive and Gram-negative bacteria may have a membrane called an S-layer. In Gram-negative bacteria, the S-layer is attached directly to the outer membrane. In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Unique to Gram-positive bacteria is the presence of teichoic acids in the cell wall. Some particular teichoic acids, lipoteichoic acids, have a lipid component and can assist in anchoring peptidoglycan, as the lipid component is embedded in the membrane.
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).[2]
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,[3] with striking productive implications for the therapeutic and general study of these organisms. Based on molecular studies of 16S sequences, Woese recognised 12 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),[3] 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,[1]. 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".[4]
In general, Gram-positive bacteria have a single lipid bilayer (monoderms), whereas Gram-negative have two (diderms). Some taxa lack peptidoglycan (such as the domain Archaea, the class Mollicutes, some members of the Rhickettsiales, and the insect-endosymbionts of the Enterobacteriales) and are Gram-variable. This, however, does not always hold true.
The Deinococcus-Thermus bacteria have Gram-positive stains, although they are structurally similar to Gram-negative bacteria with two layers (diderms).
The Chloroflexi have a single layer, yet (with some exceptions[5]) stain negative.[6] Two related phyla to the Chloroflexi, the TM7 clade and the Ktedonobacteria, are also monoderms.[7][8]
Some Firmicute species are not Gram-positive; these belong to the class Mollicutes (alternatively considered a class of the phylum Tenericutes), which lack peptidoglycan (Gram-indeterminate), and the class Negativicutes, which includes Selenomonas and which stain Gram-negative.[9]
Most pathogens in humans are Gram-positive organisms. In the classical sense, six Gram-positive genera are typically pathogenic in humans. Two of these, Streptococcus and Staphylococcus, are cocci (sphere-shaped bacteria). The remaining organisms are bacilli (rod-shaped bacteria) and can be subdivided based on their ability to form spores. The non-spore formers are Corynebacterium and Listeria (a coccobacillus), whereas Bacillus and Clostridium produce spores.[10] The spore-forming bacteria can again be divided based on their respiration: Bacillus is a facultative anaerobe, while Clostridium is an obligate anaerobe.
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