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.
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