Importance of The Outer Cell Membrane in Bacterial Classification
It is important to point out that although the bacteria are traditionally divided into two main groups, Gram-positive and Gram-negative, based upon their Gram-stain retention property, 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. The Gram-positive and Gram-negative staining response is also not a reliable characteristic as these two kinds of bacteria do not form phylogenetically coherent groups. However, although 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 more reliable and fundamental characteristic of the bacterial cells. 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 which are bounded by a single membrane, but which stain Gram-negative due to either lack of the peptidoglycan layer (viz., mycoplasmas) or their inability to retain the Gram-stain due to 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. In contrast to Gram-positive bacteria, all archetypical Gram-negative bacteria, are bounded by both a cytoplasmic membrane as well as 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. The distinction between the monoderm and diderm prokaryotes is also supported by conserved signature indels in a number of important proteins (viz. DnaK, GroEL). 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 generally resistant to them, it has been proposed that the outer cell membrane in Gram negative bacteria (diderms) has as a protective mechanism against antibiotic selection pressure. 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. The diderm bacteria can also be further differentiated between simple diderms lacking lipopolysaccharide, the archetypical diderm bacteria where the outer cell membrane contains lipopolysaccharide and the diderm bacteria where outer cell membrane is made up of mycolic acid. Additionally, a number of bacterial taxa (viz. Negativicutes, Fusobacteria, Synergistetes and Elusimicrobia) that are either part of the phylum Firmicutes or branch in its proximity are also found to possess a diderm cell structure. 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, etc.) from these other atypical diderm bacteria as well as other phyla of monoderm bacteria (e.g. Actinobacteria, Firmicutes, Thermotogae, Chloroflexi, etc.). 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.
Read more about this topic: Gram-negative Bacteria
Famous quotes containing the words importance of the, importance of, importance, outer and/or cell:
“There is, I think, no point in the philosophy of progressive education which is sounder than its emphasis upon the importance of the participation of the learner in the formation of the purposes which direct his activities in the learning process, just as there is no defect in traditional education greater than its failure to secure the active cooperation of the pupil in construction of the purposes involved in his studying.”
—John Dewey (1859–1952)
“For even satire is a form of sympathy. It is the way our sympathy flows and recoils that really determines our lives. And here lies the vast importance of the novel, properly handled. It can inform and lead into new places our sympathy away in recoil from things gone dead. Therefore the novel, properly handled, can reveal the most secret places of life: for it is the passional secret places of life, above all, that the tide of sensitive awareness needs to ebb and flow, cleansing and freshening.”
—D.H. (David Herbert)
“Shall we then judge a country by the majority, or by the minority? By the minority, surely. ‘Tis pedantry to estimate nations by the census, or by square miles of land, or other than by their importance to the mind of the time.”
—Ralph Waldo Emerson (1803–1882)
“The Dada object reflected an ironic posture before the consecrated forms of art. The surrealist object differs significantly in this respect. It stands for a mysterious relationship with the outer world established by man’s sensibility in a way that involves concrete forms in projecting the artist’s inner model.”
—J.H. Matthews. “Object Lessons,” The Imagery of Surrealism, Syracuse University Press (1977)
“It is plain that there is no separate essence called courage, no cup or cell in the brain, no vessel in the heart containing drops or atoms that make or give this virtue; but it is the right or healthy state of every man, when he is free to do that which is constitutional to him to do.”
—Ralph Waldo Emerson (1803–1882)