Pseudomonas is a genus of Gram-negative, aerobic gammaproteobacteria, belonging to the family
Pseudomonadaceae containing 191 validly described species.
The members of the genus demonstrate a great deal of metabolic diversity, and consequently
are able to colonise a wide range of niches.
Their ease of culture in vitro and availability of an increasing number of Pseudomonas strain
genome sequences has made the genus an excellent focus for scientific research; the best studied
species include P. aeruginosa in its role as an opportunistic human pathogen, the plant
pathogen P. syringae, the soil bacterium P. putida, and the plant growth-promoting P.
fluorescens.
Because of their widespread occurrence in water and plant seeds such as dicots, the
pseudomonads were observed early in the history of microbiology.
The generic name Pseudomonas created for these organisms was defined in rather vague terms
by Walter Migula in 1894 and 1900 as a genus of Gram-negative, rod-shaped and polar-flagellated
bacteria with some sporulating species, the latter statement was later proved incorrect
and was due to refractive granules of reserve materials.
Despite the vague description, the type species, Pseudomonas pyocyanea, proved the best descriptor.
Classification history Like most bacterial genera, the pseudomonad
last common ancestor lived hundreds of millions of years ago.
They were initially classified at the end of the 19th century when first identified
by Walter Migula.
The etymology of the name was not specified at the time and first appeared in the seventh
edition of Bergey's Manual of Systematic Bacteriology as Greek pseudes "false" and -monas "a single
unit", which can mean false unit; however, Migula possibly intended it as false Monas,
a nanoflagellated protist.
Soon, other species matching Migula's somewhat vague original description were isolated from
many natural niches and, at the time, many were assigned to the genus.
However, many strains have since been reclassified, based on more recent methodology and use of
approaches involving studies of conservative macromolecules.
Recently, 16S rRNA sequence analysis has redefined the taxonomy of many bacterial species.
As a result, the genus Pseudomonas includes strains formerly classified in the genera
Chryseomonas and Flavimonas.
Other strains previously classified in the genus Pseudomonas are now classified in the
genera Burkholderia and Ralstonia.
In 2000, the complete genome sequence of a Pseudomonas species was determined; more recently,
the sequence of other strains has been determined, including P. aeruginosa strains PAO1, P. putida
KT2440, P. protegens Pf-5, P. syringae pathovar tomato DC3000, P. syringae pathovar syringae
B728a, P. syringae pathovar phaseolica 1448A, P. fluorescens PfO-1, and P. entomophila L48.
Pseudomonas may be the most common nucleator of ice crystals in clouds, thereby being of
utmost importance to the formation of snow and rain around the world.
Characteristics Members of the genus display these defining
characteristics: Rod-shaped
Gram-negative One or more polar flagella, providing motility
Aerobic Nonspore-forming
Positive catalase test Positive oxidase test
Other characteristics that tend to be associated with Pseudomonas species include secretion
of pyoverdine, a fluorescent yellow-green siderophore under iron-limiting conditions.
Certain Pseudomonas species may also produce additional types of siderophore, such as pyocyanin
by Pseudomonas aeruginosa and thioquinolobactin by Pseudomonas fluorescens,.
Pseudomonas species also typically give a positive result to the oxidase test, the absence
of gas formation from glucose, glucose is oxidised in oxidation/fermentation test using
Hugh and Leifson O/F test, beta hemolytic, indole negative, methyl red negative, Voges–Proskauer
test negative, and citrate positive.
Biofilm formation All species and strains of Pseudomonas have
historically been classified as strict aerobes.
Exceptions to this classification have recently been discovered in Pseudomonas biofilms.
A significant number of cells can produce exopolysaccharides associated with biofilm
formation.
Secretion of exopolysaccharides such as alginate makes it difficult for pseudomonads to be
phagocytosed by mammalian white blood cells.
Exopolysaccharide production also contributes to surface-colonising biofilms that are difficult
to remove from food preparation surfaces.
Growth of pseudomonads on spoiling foods can generate a "fruity" odor.
Pseudomonas species have the ability to metabolize a variety of nutrients.
Combined with the ability to form biofilms, they are, thus, able to survive in a variety
of unexpected places.
For example, they have been found in areas where pharmaceuticals are prepared.
A simple carbon source, such as soap residue or cap liner-adhesives is a suitable place
for them to thrive.
Other unlikely places where they have been found include antiseptics, such as quaternary
ammonium compounds, and bottled mineral water.
Antibiotic resistance Being Gram-negative bacteria, most Pseudomonas
spp.
are naturally resistant to penicillin and the majority of related beta-lactam antibiotics,
but a number are sensitive to piperacillin, imipenem, ticarcillin, or ciprofloxacin.
Aminoglycosides such as tobramycin, gentamicin, and amikacin are other choices for therapy.
This ability to thrive in harsh conditions is a result of their hardy cell walls that
contain porins.
Their resistance to most antibiotics is attributed to efflux pumps, which pump out some antibiotics
before they are able to act.
Pseudomonas aeruginosa is increasingly recognized as an emerging opportunistic pathogen of clinical
relevance.
One of its most worrying characteristics is its low antibiotic susceptibility.
This low susceptibility is attributable to a concerted action of multidrug efflux pumps
with chromosomally encoded antibiotic resistance genes and the low permeability of the bacterial
cellular envelopes.
Besides intrinsic resistance, P. aeruginosa easily develops acquired resistance either
by mutation in chromosomally encoded genes or by the horizontal gene transfer of antibiotic
resistance determinants.
Development of multidrug resistance by P. aeruginosa isolates requires several different
genetic events that include acquisition of different mutations and/or horizontal transfer
of antibiotic resistance genes.
Hypermutation favours the selection of mutation-driven antibiotic resistance in P. aeruginosa strains
producing chronic infections, whereas the clustering of several different antibiotic
resistance genes in integrons favours the concerted acquisition of antibiotic resistance
determinants.
Some recent studies have shown phenotypic resistance associated to biofilm formation
or to the emergence of small-colony-variants may be important in the response of P. aeruginosa
populations to antibiotic treatment.
Taxonomy The studies on the taxonomy of this complicated
genus groped their way in the dark while following the classical procedures developed for the
description and identification of the organisms involved in sanitary bacteriology during the
first decades of the 20th century.
This situation sharply changed with the proposal to introduce as the central criterion the
similarities in the composition and sequences of macromolecular components of the ribosomal
RNA.
The new methodology clearly showed the genus Pseudomonas, as classically defined, consists
of a conglomerate of genera that could clearly be separated into five so-called rRNA homology
groups.
Moreover, the taxonomic studies suggested an approach that might prove useful in taxonomic
studies of all other prokaryotic groups.
A few decades after the proposal of the new genus Pseudomonas by Migula in 1894, the accumulation
of species names assigned to the genus reached alarming proportions.
The number of species in the current list has contracted more than 90%.
In fact, this approximated reduction may be even more dramatic if one considers the present
list contains many new names; i.e., relatively few names of the original list survived in
the process.
The new methodology and the inclusion of approaches based on the studies of conservative macromolecules
other than rRNA components constitutes an effective prescription that helped to reduce
Pseudomonas nomenclatural hypertrophy to a manageable size.
Pathogenicity Animal pathogens
Infectious species include P. aeruginosa, P. oryzihabitans, and P. plecoglossicida.
P. aeruginosa flourishes in hospital environments, and is a particular problem in this environment,
since it is the second-most common infection in hospitalized patients.
This pathogenesis may in part be due to the proteins secreted by P. aeruginosa.
The bacterium possesses a wide range of secretion systems, which export numerous proteins relevant
to the pathogenesis of clinical strains.
Plant pathogens P. syringae is a prolific plant pathogen.
It exists as over 50 different pathovars, many of which demonstrate a high degree of
host-plant specificity.
Numerous other Pseudomonas species can act as plant pathogens, notably all of the other
members of the P. syringae subgroup, but P. syringae is the most widespread and best-studied.
Although not strictly a plant pathogen, P. tolaasii can be a major agricultural problem,
as it can cause bacterial blotch of cultivated mushrooms.
Similarly, P. agarici can cause drippy gill in cultivated mushrooms.
Use as biocontrol agents Since the mid-1980s, certain members of the
Pseudomonas genus have been applied to cereal seeds or applied directly to soils as a way
of preventing the growth or establishment of crop pathogens.
This practice is generically referred to as biocontrol.
The biocontrol properties of P. fluorescens and P. protegens strains are currently best-understood,
although it is not clear exactly how the plant growth-promoting properties of P. fluorescens
are achieved.
Theories include: the bacteria might induce systemic resistance in the host plant, so
it can better resist attack by a true pathogen; the bacteria might outcompete other soil microbes,
e.g. by siderophores giving a competitive advantage at scavenging for iron; the bacteria
might produce compounds antagonistic to other soil microbes, such as phenazine-type antibiotics
or hydrogen cyanide.
Experimental evidence supports all of these theories.
Other notable Pseudomonas species with biocontrol properties include P. chlororaphis, which
produces a phenazine-type antibiotic active agent against certain fungal plant pathogens,
and the closely related species P. aurantiaca, which produces di-2,4-diacetylfluoroglucylmethane,
a compound antibiotically active against Gram-positive organisms.
Use as bioremediation agents Some members of the genus are able to metabolise
chemical pollutants in the environment, and as a result, can be used for bioremediation.
Notable species demonstrated as suitable for use as bioremediation agents include:
P. alcaligenes, which can degrade polycyclic aromatic hydrocarbons.
P. mendocina, which is able to degrade toluene.
P. pseudoalcaligenes, which is able to use cyanide as a nitrogen source.
P. resinovorans, which can degrade carbazole.
P. veronii, which has been shown to degrade a variety of simple aromatic organic compounds.
P. putida, which has the ability to degrade organic solvents such as toluene.
At least one strain of this bacterium is able to convert morphine in aqueous solution into
the stronger and somewhat expensive to manufacture drug hydromorphone.
Strain KC of P. stutzeri, which is able to degrade carbon tetrachloride.
Food spoilage agents As a result of their metabolic diversity,
ability to grow at low temperatures, and ubiquitous nature, many Pseudomonas species can cause
food spoilage.
Notable examples include dairy spoilage by P. fragi, mustiness in eggs caused by P. taetrolens
and P. mudicolens, and P. lundensis, which causes spoilage of milk, cheese, meat, and
fish.
Species previously classified in the genus Recently, 16S rRNA sequence analysis redefined
the taxonomy of many bacterial species previously classified as being in the Pseudomonas genus.
Species that moved from the Pseudomonas genus are listed below; clicking on a species will
show its new classification.
The term 'pseudomonad' does not apply strictly to just the Pseudomonas genus, and can be
used to also include previous members such as the genera Burkholderia and Ralstonia.
α proteobacteria: P. abikonensis, P. aminovorans, P. azotocolligans, P. carboxydohydrogena,
P. carboxidovorans, P. compransoris, P. diminuta, P. echinoides, P. extorquens, P. lindneri,
P. mesophilica, P. paucimobilis, P. radiora, P. rhodos, P. riboflavina, P. rosea, P. vesicularis.
β proteobacteria: P. acidovorans, P. alliicola, P. antimicrobica, P. avenae, P. butanovorae,
P. caryophylli, P. cattleyae, P. cepacia, P. cocovenenans, P. delafieldii, P. facilis,
P. flava, P. gladioli, P. glathei, P. glumae, P. graminis, P. huttiensis, P. indigofera,
P. lanceolata, P. lemoignei, P. mallei, P. mephitica, P. mixta, P. palleronii, P. phenazinium,
P. pickettii, P. plantarii, P. pseudoflava, P. pseudomallei, P. pyrrocinia, P. rubrilineans,
P. rubrisubalbicans, P. saccharophila, P. solanacearum, P. spinosa, P. syzygii, P. taeniospiralis,
P. terrigena, P. testosteroni.
γ-β proteobacteria: P. beteli, P. boreopolis, P. cissicola, P. geniculata, P. hibiscicola,
P. maltophilia, P. pictorum.
γ proteobacteria: P. beijerinckii, P. diminuta, P. doudoroffii, P. elongata, P. flectens,
P. halodurans, P. halophila, P. iners, P. marina, P. nautica, P. nigrifaciens, P. pavonacea,
P. piscicida, P. stanieri.
δ proteobacteria: P. formicans.
Bacteriophage There are a number of bacteriophage that infect
Pseudomonas, e.g. Pseudomonas phage Φ6
Pseudomonas aeruginosa phage EL Pseudomonas aeruginosa phage ΦKMV
Pseudomonas aeruginosa phage LKD16 Pseudomonas aeruginosa phage LKA1
Pseudomonas aeruginosa phage LUZ19 Pseudomonas aeruginosa phage ΦKZ
See also culture collection for a list of culture collections
Footnotes
References
External links General
Pseudomonas at origin of world's rain and snow
Pseudomonas survive in nuclear reactor Pseudomonas genome database
Pseudomonas Fluorescent Pseudomonas video