Gram-negative bacteria

thumb|Microscopic image of gram-negative [[Pseudomonas aeruginosa bacteria (pink-red rods)]]

Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane. These bacteria are found in all environments that support life on Earth.

Within this category, notable species include the model organism Escherichia coli, along with various pathogenic bacteria, such as Pseudomonas aeruginosa, Chlamydia trachomatis, and Yersinia pestis. They pose significant challenges in the medical field due to their outer membrane, which acts as a protective barrier against numerous antibiotics (including penicillin), detergents that would normally damage the inner cell membrane, and the antimicrobial enzyme lysozyme produced by animals as part of their innate immune system. Furthermore, the outer leaflet of this membrane contains a complex lipopolysaccharide (LPS) whose lipid A component can trigger a toxic reaction when the bacteria are lysed by immune cells. This reaction may lead to septic shock, resulting in low blood pressure, respiratory failure, reduced oxygen delivery, and lactic acidosis.

Several classes of antibiotics have been developed to target gram-negative bacteria, including aminopenicillins, ureidopenicillins, cephalosporins, beta-lactam-betalactamase inhibitor combinations (such as piperacillin-tazobactam), folate antagonists, quinolones, and carbapenems. Many of these antibiotics also cover gram-positive bacteria. The antibiotics that specifically target gram-negative bacteria include aminoglycosides, monobactams (such as aztreonam), and ciprofloxacin.

Characteristics

thumb|500px|right|Gram-negative (LPS-diderm) [[cell wall structure]]

thumb|right|[[Gram-positive and gram-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-negative bacteri into two large groups and some straddlers. The more "conventional" with an LPS outer membrane do share a common ancestor and are grouped in kingdom Pseudomonadati. 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.

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 is being adopted. For instance, the style guide of the American CDC recommends writing: Gram stain, this species is gram negative, and a gram-negative species.

References

Notes

External links

3D structures of proteins from inner membranes of Ellie Wyithe's Gram-negative bacteria