Gram-negative bacteria

structure and -negative bacteria are differentiated chiefly by their cell wall structure Conventional Gram-negative (LPS-diderm) bacteria display : • An inner cell membrane is present (cytoplasmic) • A thin peptidoglycan layer is present (this is much thicker in Gram-positive bacteria) • Has outer membrane containing lipopolysaccharides (LPS, which consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet and phospholipids in the inner leaflet • Porins exist in the outer membrane, which act like pores for particular molecules • Between the outer membrane and the cytoplasmic membrane there is a space filled with a concentrated gel-like substance called periplasm • The S-layer is directly attached to the outer membrane rather than to the peptidoglycan • If present, flagella have four supporting rings instead of two • Teichoic acids or lipoteichoic acids are absent • Lipoproteins are attached to the polysaccharide backbone • Some contain Braun's lipoprotein, which serves as a link between the outer membrane and the peptidoglycan chain by a covalent bond • Most, with few exceptions, do not form spores However, the LPS-diderm group (corresponding to kingdom Pseudomonadati, formerly "Hydrobacteria") is not the only type of bacteria that stain negative. Mycobacterium (or rather most of Mycobacteriales), which does not belong in the group, have independently evolved an outer cell membrane, with a cell wall made of mycolic acid. This gives it very different structure and features. In many Gram-negative bacteria, the IgaA membrane protein negatively regulates the Rcs phosphorelay system, a key envelope stress response pathway that helps maintain cell envelope integrity. == Classification ==

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: Firmicutes (+), Gracillicutes (−), Mollicutes (0) and Mendocutes (var.). Since 1987, the monophyly of the Gram-negative bacteria has been disproven with molecular studies. Current knowledge divides the Gram-negatives into two large groups and some straddlers. The more "conventional" Gram-negatives with an LPS outer membrane do share a common ancestor and are grouped in kingdom Pseudomonadati. The less conventional ones are, as mentioned above, the order Mycobacteriales, have a mycolic acid cell wall and an outer membrane. The kingdom and the order are each monophyletic (or rather, not holyphyletic), but the "LPS-diderm" and "mycolic-diderm" groups are not, because some bacteria in the kingdom and the order do not, in fact, stain Gram negative. They will be discussed in the next section. == Taxonomy ==

Bacteria are traditionally classified based on their Gram-staining response into the Gram-positive and Gram-negative bacteria. Having just one membrane, the Gram-positive bacteria are also known as monoderm bacteria, while Gram-negative bacteria, having two membranes, are also known as diderm bacteria. It was traditionally thought that the groups represent lineages, i.e., the extra membrane only evolved once, such that Gram-negative bacteria are more closely related to one another than to any Gram-positive bacteria. While this is often true, the classification system breaks down in some cases, with lineage groupings not matching the staining result. Thus, Gram staining cannot be reliably used to assess familial relationships of bacteria. Nevertheless, staining often gives reliable information about the composition of the cell membrane, distinguishing between the presence or absence of an outer lipid membrane. Of these two structurally distinct groups of prokaryotic organisms, monoderm prokaryotes are thought to be ancestral. Based upon a number of different observations, including that the Gram-positive bacteria are the most sensitive to antibiotics and that the Gram-negative bacteria are, in general, resistant to antibiotics, it has been proposed that the outer cell membrane in Gram-negative bacteria (diderms) evolved 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 second group are the clinically relevant Mycobacterium, expanding to most of its encompassing order of Mycobacteriales. They do not have the CSI, and their cell wall is made of a different substance: mycolic acid. Medically relevant Gram-negative diplococci include the four types that cause a sexually transmitted disease (Neisseria gonorrhoeae), a meningitis (Neisseria meningitidis), and respiratory symptoms (Moraxella catarrhalis, A coccobacillus Haemophilus influenzae is another medically relevant coccal type. Medically relevant Gram-negative bacilli include a multitude of species. Some of them cause primarily respiratory problems (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 hospital-acquired infections include Acinetobacter baumannii, which cause bacteremia, secondary meningitis, and ventilator-associated pneumonia in hospital intensive-care units. ==Bacterial transformation==

Transformation is one of three processes for horizontal gene transfer, in which exogenous genetic material passes from one bacterium to another, the other two being conjugation (transfer of genetic material between two bacterial cells in direct contact) and transduction (injection of foreign DNA by a bacteriophage virus into the host bacterium). In transformation, the genetic material passes through the intervening medium, and uptake is completely dependent on the recipient bacterium. It has also been studied in Gram-negative species found in soil such as Pseudomonas stutzeri, Acinetobacter baylyi, and Gram-negative plant pathogens such as Ralstonia solanacearum and Xylella fastidiosa. ==Role in disease==

and/or multitarget assay. One of the several unique characteristics of Gram-negative bacteria is the structure of the bacterial outer membrane. The outer leaflet of this membrane contains lipopolysaccharide (LPS), whose lipid A portion acts as an endotoxin. == Orthography: capitalization ==

The adjectives Gram-positive and Gram-negative derive from the surname of Hans Christian Gram, a Danish bacteriologist; as eponymous adjectives, their initial letter G can be either capital or lower-case, depending on which style guide (e.g., that of the CDC), if any, governs the document being written. This is further explained at Gram staining § Orthographic note. == See also ==