出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/03/07 16:22:32」(JST)
Gram-negative bacteria are so called because they do not retain crystal violet dye, used in the Gram staining protocol.[1] Gram staining is a method used to differentiate the two large groups of bacterial species into gram-positive and gram-negative groupings. Gram-positive bacteria do retain the crystal violet dye, which shows up in their different cell wall structure. In a Gram stain test, a counterstain (commonly safranin) is added after the crystal violet, giving all gram-negative bacteria a red or pink colouring. Without the counterstain, the Gram-negative bacteria would be colourless, which indicates that they have the much thinner cell wall associated with belonging to the gram-negative class. Their thinner peptidoglycan cell wall layer does not retain crystal violet dye. Gram-positive bacteria have a thicker cell wall, which can retain the crystal violet dye when washed in a decolorizing solution. Compared with Gram-positive bacteria, Gram-negative bacteria are more resistant to antibiotics, despite their thinner peptidoglycan layer. This is seen to be due to their having an additional relatively impermeable lipid membrane known as a bacterial outer membrane.
The pathogenic capability of Gram-negative bacteria is often associated with certain components of their membrane, in particular, the lipopolysaccharide layer (also known as the LPS or endotoxin layer).[1] In humans, the presence of LPS triggers an innate immune response, activating the immune system and producing cytokines (hormonal regulators). Inflammation is a common reaction to cytokine production, which can also produce host toxicity. The innate immune response to LPS, however, is not synonymous with pathogenicity, or the ability to cause disease. In fact, the innate immune response is triggered purely by LPS.
The following characteristics are displayed for gram-negative bacteria:
Along with cell shape, Gram staining is a rapid diagnostic tool and once was used to group species at the subdivision of Bacteria.
Historically, the kingdom Monera was divided into four divisions based on Gram staining: Firmacutes (+), Gracillicutes (−), Mollicutes (0) and Mendocutes (var.).[2]
Since 1987, the monophyly of the Gram-negative bacteria has been disproven with molecular studies.[3] However some authors, such as Cavalier-Smith still treat them as a monophyletic clade and refer to the group as subkingdom "Negibacteria".[4]
It is important to point out that, though the bacteria are traditionally divided into two main groups, Gram-positive and Gram-negative, based on their Gram-stain retention, this classification system is ambiguous, as it can refer to three distinct aspects (staining result, cell-envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species.[5][6][7][8] The gram-positive and gram-negative staining response is also not a reliable phylogenetic character as these two kinds of bacteria do not form phylogenetically coherent groups.[5] However, Gram-staining response of bacteria is an empirical criterion; its basis lies in the marked differences in the ultrastructure and chemical composition of the two main kinds of prokaryotic cells that are found in nature. These two kinds of cells are distinguished from each other based upon the presence or absence of an outer lipid membrane, which is a reliable and fundamental characteristic of bacterial cells.[5][9] All gram-positive bacteria are bounded by only a single unit lipid membrane and they generally 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 (viz., 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 (prokaryotic) cells that are bounded by a single cell membrane, the term Monoderm Bacteria or Monoderm Prokaryotes has been proposed.[5][5][9] In contrast to gram-positive bacteria, all archetypical gram-negative bacteria are bounded by both a cytoplasmic membrane and an outer cell membrane, and they contain only a thin layer of peptidoglycan (2–3 nm) in between these two membranes. The presence of both inner and outer cell membranes defines a new compartment in these cells, the periplasmic space or the periplasmic compartment. These bacteria/prokaryotes have been designated as Diderm Bacteria.[5][5][9] The distinction between the monoderm and diderm prokaryotes is also supported by conserved signature indels in a number of important proteins (viz. DnaK, GroEL).[5][6][9][10] Of these two structurally distinct groups of prokaryotic organisms, monoderm prokaryotes are indicated to be ancestral. Based upon a number of different observations including that the gram-positive bacteria are the major producers of antibiotics and that gram-negative bacteria are, in general, resistant to them, it has been proposed that the outer cell membrane in gram-negative bacteria (diderms) evolved as a protective mechanism against antibiotic selection pressure.[5][6][9][10] Some bacteria such as Deinococcus, which stain gram-positive due to the presence of a thick peptidoglycan layer, but also possess an outer cell membrane are suggested as intermediates in the transition between monoderm (gram-positive) and diderm (gram-negative) bacteria.[5][10] The diderm bacteria can also be further differentiated between simple diderms lacking lipopolysaccharide, the archetypical diderm bacteria, in which the outer cell membrane contains lipopolysaccharide, and the diderm bacteria, in which outer cell membrane is made up of mycolic acid.[7][8][10][11]
In addition, a number of bacterial taxa (viz. Negativicutes, Fusobacteria, Synergistetes, and Elusimicrobia) that are either part of the phylum Firmicutes or branches in its proximity are also found to possess a diderm cell structure.[8][10][11] However, a conserved signature indel (CSI) in the HSP60 (GroEL) protein distinguishes all traditional phyla of gram-negative bacteria (e.g., Proteobacteria, Aquificae, Chlamydiae, Bacteroidetes, Chlorobi, Cyanobacteria, Fibrobacteres, Verrucomicrobia, Planctomycetes, Spirochetes, Acidobacteria) from these other atypical diderm bacteria as well as other phyla of monoderm bacteria (e.g., Actinobacteria, Firmicutes, Thermotogae, Chloroflexi).[10] The presence of this CSI in all sequenced species of conventional LPS-containing gram-negative bacterial phyla provides evidence that these phyla of bacteria form a monophyletic clade and that no loss of the outer membrane from any species from this group has occurred.[10]
The proteobacteria are a major group of gram-negative bacteria, including Escherichia coli (E. coli), Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella, and numerous others. Other notable groups of gram-negative bacteria include the cyanobacteria, spirochaetes, green sulfur, and green non-sulfur bacteria.
Medically relevant gram-negative cocci include the three organisms that cause a sexually transmitted disease (Neisseria gonorrhoeae), a meningitis (Neisseria meningitidis), and respiratory symptoms (Moraxella catarrhalis).
Medically relevant gram-negative bacilli include a multitude of species. Some of them cause primarily respiratory problems (Hemophilus influenzae, Klebsiella pneumoniae, Legionella pneumophila, Pseudomonas aeruginosa), primarily urinary problems (Escherichia coli, Proteus mirabilis, Enterobacter cloacae, Serratia marcescens), and primarily gastrointestinal problems (Helicobacter pylori, Salmonella enteritidis, Salmonella typhi).
Gram-negative bacteria associated with nosocomial infections include Acinetobacter baumannii, which cause bacteremia, secondary meningitis, and ventilator-associated pneumonia in intensive-care units of hospital establishments.
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One of the several unique characteristics of gram-negative bacteria is the structure of the outer membrane. The outer leaflet of the membrane comprises a complex lipopolysaccharide (LPS) whose lipid portion acts as an endotoxin. If LPS enters the circulatory system, it causes a toxic reaction, with the sufferer developing a high temperature, high respiration rate, and low blood pressure. This may lead to endotoxic shock, which can be fatal.
This outer membrane protects the bacteria from several antibiotics, dyes, and detergents that would normally damage either the inner membrane or the cell wall's (peptidoglycan). The outer membrane provides these bacteria with resistance to lysozyme and penicillin. However, alternative medicinal treatments such as lysozyme with EDTA and the antibiotic ampicillin have been developed to combat the protective outer membrane of some pathogenic gram-negative organisms. Other drugs can also be used, significant ones being chloramphenicol, streptomycin, and nalidixic acid.
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| リンク元 | 「Gram陰性」「グラム陰性」 |
| 拡張検索 | 「Gram-negative bacteria」「Gram-negative bacterial infection」「Gram-negative aerobic bacteria」「Gram-negative bacterium」「Gram-negative facultatively anaerobic rod」 |
| 関連記事 | 「negative」 |