Gram-negative bacteria are

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

that do not retain the crystal violet stain used in the

Gram stain In microbiology and bacteriology, Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. The name comes from the Danish ...

ing method of bacterial differentiation. They are characterized by their

cell envelope The cell envelope comprises the inner cell membrane and the cell wall of a bacterium. In gram-negative bacteria an outer membrane is also included. This envelope is not present in the Mollicutes where the cell wall is absent. Bacterial cell en ...

s, which are composed of a thin

peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ...

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...

sandwiched between an inner

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. T ...

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

and a

bacterial outer membrane The bacterial outer membrane is found in gram-negative bacteria. Its composition is distinct from that of the inner cytoplasmic cell membrane - among other things, the outer leaflet of the outer membrane of many gram-negative bacteria includes ...

. Gram-negative bacteria are found in virtually all environments on

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

that support life. The gram-negative bacteria include the

model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the working ...

Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...

'', as well as many

pathogenic bacteria Pathogenic bacteria are bacteria that can cause disease. This article focuses on the bacteria that are pathogenic to humans. Most species of bacteria are harmless and are often beneficial but others can cause infectious diseases. The number of ...

, such as ''

Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common encapsulated, gram-negative, aerobic–facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. ae ...

'', ''

Chlamydia trachomatis ''Chlamydia trachomatis'' (), commonly known as chlamydia, is a bacterium that causes chlamydia, which can manifest in various ways, including: trachoma, lymphogranuloma venereum, nongonococcal urethritis, cervicitis, salpingitis, pelvic inflamma ...

'', and ''

Yersinia pestis ''Yersinia pestis'' (''Y. pestis''; formerly ''Pasteurella pestis'') is a gram-negative, non-motile, coccobacillus bacterium without spores that is related to both '' Yersinia pseudotuberculosis'' and '' Yersinia enterocolitica''. It is a fa ...

''. They are a significant medical challenge as their outer membrane protects them from many

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...

s (including

penicillin Penicillins (P, PCN or PEN) are a group of β-lactam antibiotics originally obtained from ''Penicillium'' moulds, principally '' P. chrysogenum'' and '' P. rubens''. Most penicillins in clinical use are synthesised by P. chrysogenum usin ...

detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are m ...

s that would normally damage the inner cell membrane, and

lysozyme Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycosi ...

, an

antimicrobial An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ar ...

enzyme produced by animals that forms part of the

innate immune system The innate, or nonspecific, immune system is one of the two main immunity strategies (the other being the adaptive immune system) in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is the ...

. Additionally, the outer leaflet of this membrane comprises a complex

lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the out ...

(LPS) whose

lipid A Lipid A is a lipid component of an endotoxin held responsible for the toxicity of gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide (LPS), also called endotoxin molecule, and its hydrophobic nature allows it ...

component can cause a toxic reaction when bacteria are lysed by immune cells. This toxic reaction may lead to

low blood pressure Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the dias ...

respiratory failure Respiratory failure results from inadequate gas exchange by the respiratory system, meaning that the arterial oxygen, carbon dioxide, or both cannot be kept at normal levels. A drop in the oxygen carried in the blood is known as hypoxemia; a ri ...

, reduced oxygen delivery, and

lactic acidosis Lactic acidosis is a medical condition characterized by a build-up of lactate (especially -lactate) in the body, with formation of an excessively low pH in the bloodstream. It is a form of metabolic acidosis, in which excessive acid accumulates ...

– manifestations of

septic shock Septic shock is a potentially fatal medical condition that occurs when sepsis, which is organ injury or damage in response to infection, leads to dangerously low blood pressure and abnormalities in cellular metabolism. The Third International Co ...

. Several classes of antibiotics have been designed to target gram-negative bacteria, including aminopenicillins, ureidopenicillins,

cephalosporin The cephalosporins (sg. ) are a class of β-lactam antibiotics originally derived from the fungus ''Acremonium'', which was previously known as ''Cephalosporium''. Together with cephamycins, they constitute a subgroup of β-lactam antibiotics ...

beta-lactam A beta-lactam (β-lactam) ring is a four-membered lactam. A ''lactam'' is a cyclic amide, and ''beta''-lactams are named so because the nitrogen atom is attached to the β-carbon atom relative to the carbonyl. The simplest β-lactam possible i ...

- betalactamase inhibitor combinations (e.g. piperacillin-tazobactam), Folate antagonists, quinolones, and

carbapenem Carbapenems are a class of very effective antibiotic agents most commonly used for the treatment of severe bacterial infections. This class of antibiotics is usually reserved for known or suspected multidrug-resistant (MDR) bacterial infections. ...

s. Many of these antibiotics also cover

gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bacte ...

organisms. The drugs that specifically target gram-negative organisms include

aminoglycoside Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside (sugar). The term can also refer ...

monobactam Monobactams are monocyclic and bacterially-produced β-lactam antibiotics. The β-lactam ring is not fused to another ring, in contrast to most other β-lactams. Monobactams are effective only against aerobic Gram-negative bacteria (e.g., ''Nei ...

s (

aztreonam Aztreonam, sold under the brand name Azactam among others, is an antibiotic used primarily to treat infections caused by gram-negative bacteria such as '' Pseudomonas aeruginosa''. This may include bone infections, endometritis, intra abdomina ...

) and

ciprofloxacin Ciprofloxacin is a fluoroquinolone antibiotic used to treat a number of bacterial infections. This includes bone and joint infections, intra abdominal infections, certain types of infectious diarrhea, respiratory tract infections, skin i ...

Characteristics

Conventional gram-negative (LPS-diderm) bacteria display : * An inner

is present (

ic) * A thin

layer is present (this is much thicker in

gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bacte ...

) * Has outer membrane containing

s (LPS, which consists of

, core

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...

, and

O antigen Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer me ...

) in its outer leaflet and

phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipi ...

s 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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

there is a space filled with a concentrated gel-like substance called

periplasm The periplasm is a concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane called the ''periplasmic space'' in gram-negative bacteria. Using cryo-electron microscopy it has been found tha ...

* The

S-layer An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria. The S-layers of both archaea and bacteria consists of a monomolecular layer composed of only one (or, in a few cases, two) i ...

is directly attached to the outer membrane rather than to the

* If present,

flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have fro ...

have four supporting rings instead of two *

Teichoic acid Teichoic acids (''cf.'' Greek τεῖχος, ''teīkhos'', "wall", to be specific a fortification wall, as opposed to τοῖχος, ''toīkhos'', a regular wall) are bacterial copolymers of glycerol phosphate or ribitol phosphate and carbohy ...

s or

lipoteichoic acid Lipoteichoic acid (LTA) is a major constituent of the cell wall of gram-positive bacteria. These organisms have an inner (or cytoplasmic) membrane and, external to it, a thick (up to 80 nanometer) peptidoglycan layer. The structure of LTA varies be ...

s are absent *

Lipoprotein A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, sur ...

s 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 In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae ...

Classification

Along with cell shape,

ing is a rapid diagnostic tool and once was used to group species at the subdivision of Bacteria. Historically, the kingdom

Monera Monera (/məˈnɪərə/) (Greek - μονήρης (monḗrēs), "single", "solitary") is a biological kingdom that is made up of prokaryotes. As such, it is composed of single-celled organisms that lack a nucleus. The taxon Monera was first ...

was divided into four divisions based on Gram staining: Firmacutes (+), Gracillicutes (−),

Mollicutes Mollicutes is a class of bacteria distinguished by the absence of a cell wall. The word "Mollicutes" is derived from the Latin ''mollis'' (meaning "soft" or "pliable"), and ''cutis'' (meaning "skin"). Individuals are very small, typically only 0 ...

(0) and Mendocutes (var.). Since 1987, the

monophyly In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

of the gram-negative bacteria has been disproven with molecular studies. However some authors, such as

Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford. His research has led to disco ...

still treat them as a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

taxon In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...

(though not a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

; his definition of

requires a single common ancestor but does not require

holophyly In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic g ...

, the property that all descendants be encompassed by the

) and refer to the group as a

subkingdom In biology, a kingdom is the second highest taxonomic rank, just below domain. Kingdoms are divided into smaller groups called phyla. Traditionally, some textbooks from the United States and Canada used a system of six kingdoms (Animalia, P ...

"Negibacteria".

Taxonomy

Bacteria are traditionally classified based on their

Gram-stain In microbiology and bacteriology, Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. The name comes from the Danish ...

ing response into the

and gram-negative bacteria. Having just one membrane the gram-positive bacteria are also known as monoderm bacteria, and gram-negative 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 A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Con ...

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 An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...

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 Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...

. Some bacteria such as '' Deinococcus'', which stain gram-positive due to the presence of a thick

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 (LPS); the archetypical diderm bacteria, in which the outer cell membrane contains lipopolysaccharide; and the diderm bacteria, in which the outer cell membrane is made up of

mycolic acid Mycolic acids are long fatty acids found in the cell walls of the Mycolata taxon, a group of bacteria that includes ''Mycobacterium tuberculosis'', the causative agent of the disease tuberculosis. They form the major component of the cell wal ...

(e. g. ''

Mycobacterium ''Mycobacterium'' is a genus of over 190 species in the phylum Actinomycetota, assigned its own family, Mycobacteriaceae. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis ('' M. tuberculosis'') and ...

''). The conventional LPS-''diderm'' group of gram-negative bacteria (e.g.,

Pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The ...

Aquificota The ''Aquificota'' phylum is a diverse collection of bacteria that live in harsh environmental settings. The name ''Aquificota'' was given to this phylum based on an early genus identified within this group, ''Aquifex'' (“water maker”), which ...

Chlamydiota The Chlamydiota (synonym Chlamydiae) are a bacterial phylum and class whose members are remarkably diverse, including pathogens of humans and animals, symbionts of ubiquitous protozoa, and marine sediment forms not yet well understood. All of ...

Bacteroidota The phylum Bacteroidota (synonym Bacteroidetes) is composed of three large classes of Gram-negative, nonsporeforming, anaerobic or aerobic, and rod-shaped bacteria that are widely distributed in the environment, including in soil, sediments, and ...

, Chlorobiota, "

Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blue ...

Fibrobacterota Fibrobacterota is a small bacterial phylum which includes many of the major rumen bacteria, allowing for the degradation of plant-based cellulose in ruminant animals. Members of this phylum were categorized in other phyla. The genus '' Fibrobac ...

, Verrucomicrobiota,

Planctomycetota The Planctomycetota are a phylum of widely distributed bacteria, occurring in both aquatic and terrestrial habitats. They play a considerable role in global carbon and nitrogen cycles, with many species of this phylum capable of anaerobic ammoniu ...

Spirochaetota A spirochaete () or spirochete is a member of the phylum Spirochaetota (), (synonym Spirochaetes) which contains distinctive diderm (double-membrane) gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or s ...

Acidobacteriota Acidobacteriota is a phylum of Gram-negative bacteria. Its members are physiologically diverse and ubiquitous, especially in soils, but are under-represented in culture. Description Members of this phylum are physiologically diverse, and can b ...

; " Hydrobacteria") are uniquely identified by a few conserved signature indel (CSI) in the

HSP60 HSP60, also known as chaperonins (Cpn), is a family of heat shock proteins originally sorted by their 60kDa molecular mass. They prevent misfolding of proteins during stressful situations such as high heat, by assisting protein folding. HSP60 bel ...

( GroEL) protein. In addition, a number of bacterial

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

(including Negativicutes,

Fusobacteriota Fusobacteriota are obligately anaerobic non-sporeforming Gram-negative bacilli. Since the first reports in the late nineteenth century, various names have been applied to these organisms, sometimes with the same name being applied to different ...

, Synergistota, and Elusimicrobiota) that are either part of the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclatur ...

Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the earl ...

(a monoderm group) or branches in its proximity are also found to possess a diderm cell structure. They lack the GroEL signature. The presence of this CSI in all sequenced species of conventional lipopolysaccharide-containing gram-negative bacterial phyla provides evidence that these phyla of bacteria form a

and that no loss of the outer membrane from any species from this group has occurred.

Example species

The proteobacteria are a major superphylum of gram-negative bacteria, including ''

E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Es ...

'', ''

Salmonella ''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' is the type species and is f ...

'', ''

Shigella ''Shigella'' is a genus of bacteria that is Gram-negative, facultative anaerobic, non-spore-forming, nonmotile, rod-shaped, and genetically closely related to ''E. coli''. The genus is named after Kiyoshi Shiga, who first discovered it in 18 ...

'', and other

Enterobacteriaceae Enterobacteriaceae is a large family of Gram-negative bacteria. It was first proposed by Rahn in 1936, and now includes over 30 genera and more than 100 species. Its classification above the level of family is still a subject of debate, but on ...

, ''

Pseudomonas ''Pseudomonas'' is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 described species. The members of the genus demonstrate a great deal of metabolic diversity and consequently are abl ...

'', ''

Moraxella ''Moraxella'' is a genus of gram-negative bacteria in the family Moraxellaceae. It is named after the Swiss ophthalmologist Victor Morax. The organisms are short rods, coccobacilli, or as in the case of ''Moraxella catarrhalis'', diplococci i ...

'', ''

Helicobacter ''Helicobacter'' is a genus of Gram-negative bacteria possessing a characteristic helical shape. They were initially considered to be members of the genus ''Campylobacter'', but in 1989, Goodwin ''et al.'' published sufficient reasons to justif ...

'', '' Stenotrophomonas'', ''

Bdellovibrio ''Bdellovibrio'' is a genus of Gram-negative, obligate aerobic bacteria. One of the more notable characteristics of this genus is that members can prey upon other Gram-negative bacteria and feed on the biopolymers, e.g. proteins and nucleic aci ...

'', acetic acid bacteria, '' Legionella'' etc. Other notable groups of gram-negative bacteria include the

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blue ...

spirochaete A spirochaete () or spirochete is a member of the phylum Spirochaetota (), (synonym Spirochaetes) which contains distinctive diderm (double-membrane) gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or s ...

s, green sulfur, and green non-sulfur bacteria. Medically relevant gram-negative cocci include the four types that cause a

sexually transmitted disease Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal diseases, are infections that are spread by sexual activity, especially vaginal intercourse, anal sex, and oral s ...

(''

Neisseria gonorrhoeae ''Neisseria gonorrhoeae'', also known as ''gonococcus'' (singular), or ''gonococci'' (plural), is a species of Gram-negative diplococci bacteria isolated by Albert Neisser in 1879. It causes the sexually transmitted genitourinary infection go ...

''), a

meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion ...

(''

Neisseria meningitidis ''Neisseria meningitidis'', often referred to as meningococcus, is a Gram-negative bacterium that can cause meningitis and other forms of meningococcal disease such as meningococcemia, a life-threatening sepsis. The bacterium is referred to as ...

''), and respiratory symptoms (''

Moraxella catarrhalis ''Moraxella catarrhalis'' is a fastidious, nonmotile, Gram-negative, aerobic, oxidase-positive diplococcus that can cause infections of the respiratory system, middle ear, eye, central nervous system, and joints of humans. It causes the i ...

'', ''

Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, non-motile, coccobacillary, facultatively anaerobic, capnophilic pathogenic bacterium of the family Pasteurellaceae. The bacte ...

''). Medically relevant gram-negative

bacilli Bacilli is a taxonomic class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as ''Bacillus anthracis'' (the cause of anthrax). ''Bacilli'' are almost exclusively gram-posit ...

include a multitude of species. Some of them cause primarily respiratory problems (''

Klebsiella pneumoniae ''Klebsiella pneumoniae'' is a Gram-negative, non-motile, encapsulated, lactose- fermenting, facultative anaerobic, rod-shaped bacterium. It appears as a mucoid lactose fermenter on MacConkey agar. Although found in the normal flora of the m ...

'', ''

Legionella pneumophila ''Legionella pneumophila'' is a thin, aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative bacterium of the genus '' Legionella''. ''L. pneumophila'' is the primary human pathogenic bacterium in this group and is the causative ...

'', ''

''), primarily urinary problems (''

'', ''

Proteus mirabilis ''Proteus mirabilis'' is a Gram-negative, facultatively anaerobic, rod-shaped bacterium. It shows swarming motility and urease activity. ''P. mirabilis'' causes 90% of all ''Proteus'' infections in humans. It is widely distributed in soil a ...

'', ''

Enterobacter cloacae ''Enterobacter cloacae'' is a clinically significant Gram-negative, facultatively-anaerobic, rod-shaped bacterium. Microbiology In microbiology labs, ''E. cloacae'' is frequently grown at 30 °C on nutrient agar or at 35 °C in try ...

'', ''

Serratia marcescens ''Serratia marcescens'' () is a species of rod-shaped, Gram-negative bacteria in the family Yersiniaceae. It is a facultative anaerobe and an opportunistic pathogen in humans. It was discovered in 1819 by Bartolomeo Bizio in Padua, Italy.Serr ...

''), and primarily gastrointestinal problems (''

Helicobacter pylori ''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, microaerophilic, spiral (helical) bacterium usually found in the stomach. Its helical shape (from which the genus name, helicobacter, derives) is thoug ...

'', '' Salmonella enteritidis'', ''

Salmonella typhi ''Salmonella enterica'' subsp. ''enterica'' is a subspecies of '' Salmonella enterica'', the rod-shaped, flagellated, aerobic, Gram-negative bacterium. Many of the pathogenic serovars of the ''S. enterica'' species are in this subspecies, inc ...

''). Gram-negative bacteria associated with

hospital-acquired infection A hospital-acquired infection, also known as a nosocomial infection (from the Greek , meaning "hospital"), is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is so ...

s include ''

Acinetobacter baumannii ''Acinetobacter baumannii'' is a typically short, almost round, rod-shaped ( coccobacillus) Gram-negative bacterium. It is named after the bacteriologist Paul Baumann. It can be an opportunistic pathogen in humans, affecting people with compromi ...

'', which cause

bacteremia Bloodstream infections (BSIs), which include bacteremias when the infections are bacterial and fungemias when the infections are fungal, are infections present in the blood. Blood is normally a sterile environment, so the detection of micro ...

, secondary

, and

ventilator-associated pneumonia Ventilator-associated pneumonia (VAP) is a type of lung infection that occurs in people who are on mechanical ventilation breathing machines in hospitals. As such, VAP typically affects critically ill persons that are in an intensive care unit (I ...

in hospital intensive-care units.

Bacterial transformation

Transformation is one of three processes for

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HG ...

, in which exogenous genetic material passes from

bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were amon ...

to another, the other two being

conjugation Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics *Complex conjugation, the change ...

(transfer of

genetic material Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cla ...

between two bacterial cells in direct contact) and transduction (injection of foreign DNA by a

bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacter ...

virus into the host bacterium). In transformation, the

passes through the intervening medium, and uptake is completely dependent on the recipient bacterium. As of 2014 about 80 species of bacteria were known to be capable of transformation, about evenly divided between

and gram-negative bacteria; the number might be an overestimate since several of the reports are supported by single papers. Transformation has been studied in medically important gram-negative bacteria species such as ''

'', ''

'' and ''

Vibrio cholerae ''Vibrio cholerae'' is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and o ...

''. It has also been studied in gram-negative species found in soil such as ''

Pseudomonas stutzeri ''Pseudomonas stutzeri'' is a Gram-negative soil bacterium that is motile, has a single polar flagellum, and is classified as bacillus, or rod-shaped. While this bacterium was first isolated from human spinal fluid, it has since been found in man ...

'', '' Acinetobacter baylyi'', and gram-negative plant pathogens such as ''

Ralstonia solanacearum ''Ralstonia solanacearum'' is an aerobic non-spore-forming, Gram-negative, plant pathogenic bacterium. ''R. solanacearum'' is soil-borne and motile with a polar flagellar tuft. It colonises the xylem, causing bacterial wilt in a very wide ran ...

'' and '' Xylella fastidiosa''.

Role in disease

One of the several unique characteristics of gram-negative bacteria is the structure of the

. The outer leaflet of this membrane contains

(LPS), whose

portion acts as an

endotoxin Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O- antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the out ...

. If gram-negative bacteria enter the

circulatory system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

, LPS can trigger an innate immune response, activating the

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...

and producing

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autoc ...

s (hormonal regulators). This leads to

inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...

and can cause a toxic reaction, resulting in fever, an increased respiratory rate, and

. That is why some infections with gram-negative bacteria can lead to life-threatening

. The outer membrane protects the bacteria from several

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution a ...

s, and

s that would normally damage either the inner membrane or the cell wall (made of

). The outer membrane provides these bacteria with resistance to

and

. The periplasmic space (space between the two cell membranes) also contains

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...

s which break down or modify antibiotics. Drugs commonly used to treat gram negative infections include amino, carboxy and ureido penicillins (

ampicillin Ampicillin is an antibiotic used to prevent and treat a number of bacterial infections, such as respiratory tract infections, urinary tract infections, meningitis, salmonellosis, and endocarditis. It may also be used to prevent group B strepto ...

amoxicillin Amoxicillin is an antibiotic medication used to treat a number of bacterial infections. These include middle ear infection, strep throat, pneumonia, skin infections, and urinary tract infections among others. It is taken by mouth, or less ...

, pipercillin,

ticarcillin Ticarcillin is a carboxypenicillin. It can be sold and used in combination with clavulanate as ticarcillin/clavulanic acid. Because it is a penicillin, it also falls within the larger class of beta-lactam antibiotics. Its main clinical use is as a ...

) these drugs may be combined with beta-lactamase inhibitors to combat the presence of enzymes that can digest these drugs (known as

beta-lactamase Beta-lactamases, (β-lactamases) are enzymes () produced by bacteria that provide multi-resistance to beta-lactam antibiotics such as penicillins, cephalosporins, cephamycins, monobactams and carbapenems (ertapenem), although carbapenems ...

s) in the peri-plasmic space. Other classes of drugs that have gram negative spectrum include

s (

), aminoglycosides, quinolones,

macrolide The Macrolides are a class of natural products that consist of a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered. M ...

chloramphenicol Chloramphenicol is an antibiotic useful for the treatment of a number of bacterial infections. This includes use as an eye ointment to treat conjunctivitis. By mouth or by injection into a vein, it is used to treat meningitis, plague, cho ...

, folate antagonists, and

Orthographic note

The adjectives ''Gram-positive'' and ''Gram-negative'' derive from the surname of

Hans Christian Gram Hans Christian Joachim Gram (13 September 1853 – 14 November 1938) was a Danish bacteriologist noted for his development of the Gram stain, still a standard technique to classify bacteria and make them more visible under a microscope. Early ...

, a Danish bacteriologist; as eponymous adjectives, their initial letter can be either capital ''G'' or lower-case ''g'', depending on which

style guide A style guide or manual of style is a set of standards for the writing, formatting, and design of documents. It is often called a style sheet, although that term also has multiple other meanings. The standards can be applied either for gener ...

(e.g., that of the CDC), if any, governs the document being written. This is further explained at '' Gram staining § Orthographic note''.

References

Notes

External links

3D structures of proteins from inner membranes of Ellie Wyithe's gram-negative bacteria
{{Authority control Staining Bacteriology

Characteristics

Classification

Taxonomy

Example species

Bacterial transformation

Role in disease

Orthographic note

See also

References

Notes

External links