PSEUDOMONAS: CHARACTERISTICS, MORPHOLOGY, LIFE CYCLE - BIOLOGY - 2024

Pseudomonas is a genus of bacteria in the Pseudomonaceae family. The first description of these microorganisms was made by the German mycologist Walter Migula in 1894.

These bacteria are characterized by being aerobic and Gram negative. They are straight rod-shaped or have a certain curvature. They are mobile due to the presence of monotric flagella (one flagellum) or multitricus (several flagella). The flagellum tends to be in the polar position.

Image of the bacterium Pseudomonas aeruginosa. Author: Janice Haney Carr Content Providers (s): CDC / Janice Haney Carr, via Wikimedia Commons.

Most species in the genus are oxidase and catalase positive. Another characteristic of interest to recognize the group is the GC content in the DNA that ranges from 58-72%.

Pseudomonas does not develop resistance structures, such as spores. They do not present a capsule surrounding the wall or extensions of it and the cytoplasm (prosteca), which occur in other bacterial groups.

The study of Pseudomonas has been approached mainly by the Argentine microbiologist Norberto Palleroni. This researcher proposed to separate the genus into five groups based on rRNA homology.

Currently about 180 species separated into thirteen different groups are recognized. Some of these groups are recognized by the production of the fluorescent pigment known as pyoverdin.

General characteristics

Distribution

Due to its great ability to grow in diverse environments, the genus has a ubiquitous ecological and geographic distribution. They have been found in terrestrial and aquatic environments. They are chemotrophic and easily grown on nutrient agar culture media.

Temperature

Its ideal temperature range is 25-30 ° C. However, species have been found growing in temperatures below zero and others above 50 ° C.

Diseases

Among the species that make up the genus, there are some that cause diseases in animals and humans. Similarly, many species are plant pathogens causing so-called soft rot.

Applications

Other species can be very useful, since it has been proven that they stimulate plant growth and can be applied as fertilizers. They can also degrade xenobiotic compounds (which are not part of the composition of living organisms).

Among some of the xenobiotics that can degrade, aromatic hydrocarbons, chlorates and nitrates stand out. These properties make some species very useful in bioremediation programs.

Staining and respiration

Pseudomonas species are Gram negative. They are mainly aerobic, so oxygen is the final receptor for electrons in respiration.

Some species can use nitrates as alternative electron acceptors under anaerobic conditions. In this case, the bacteria reduce nitrates to molecular nitrogen.

ID

All Pseudomonas species are catalase positive. This is the enzyme that breaks down hydrogen peroxide into oxygen and water. Most aerobic bacteria produce this enzyme.

Within the group there are positive and negative oxidase species. The presence of this enzyme is considered useful in the identification of Gram negative bacteria.

Most species accumulate a glucose polysaccharide as a reserve substance. However, some groups may have polyhydroxybutyrate (PHB), which is a polymer product of carbon assimilation.

Pigments

Various species of Pseudomonas produce pigments that have been considered of taxonomic importance.

Among these are various types of phenazines. The most common of this type is the blue pigment pyoacine. This pigment is believed to contribute to increasing the ability of P. aeruginosa to colonize the lungs of patients with cystic fibrosis.

Other phenazines can give green or orange pigmentations, which are very useful in the identification of some species of the genus.

Another characteristic pigment of some groups of Pseudomonas is pyoverdin. These give yellowish green colors and are typical of the so-called fluorescent Pseudomonas.

Pyoverdin is of great physiological importance since it acts as a siderophore. This means that it can trap unavailable iron and dissolve it into chemical forms that bacteria can use.

Phylogeny and taxonomy

Pseudomonas was first described in 1894 by Walter Migula. The etymology of the name means false unity. Currently 180 species are recognized in this group.

The genus is located in the Pseudomoneacae family of the Pseudomonales order. The type species is P. aeruginosa, which is one of the best known in the group.

The characteristics used in principle to describe the genus were very general and could be shared by other groups of bacteria.

Later, more precise characters began to be used for the definition of gender. These include: the GC content in DNA, pigmentation, and the type of reserve substance, among others.

In the 70s of the 20th century, the group specialist Norberto Palleroni, together with other researchers, carried out a study of ribosomal RNA. They determined that Pseudomonas could be separated into five different groups based on rRNA homology.

By using more precise molecular techniques, it was determined that the groups II-V established by Palleroni corresponded to other groups of Proteobacteria. Currently only group I is considered to correspond to Psedomonas senso stricto.

Most of the species in this group produce pyoverdin. The way this pigment is biosynthesized and secreted can help to differentiate the species from each other.

Groups in

Based on the multilocus sequence analysis it has been proposed that Pseudomonas would be separated into five groups:

P. fluorescens group: this is very diverse and the species are saprophytic, being present in the soil, water and plant surfaces. Many species promote plant growth.

P. syringae group: it is composed mainly of species that are phytopathogenic. More than fifty pathovars (strains of bacteria with varying degrees of pathogenicity) are recognized.

P. putida group: the species of this group are found in the soil, the rhizosphere of different plants and in water. They have a high capacity to break down substances.

P stutzeri group: these bacteria are of great importance in the nutrient cycle and have high genetic diversity.

Group P aeruginosa: in this group there are species that occupy various habitats, including human pathogens.

However, in a more recent molecular study it is proposed that the genus is separated into thirteen groups consisting of two to more than sixty species.

The largest group is that of P. fluorescens, which includes the type species that is widely used in bioremediation programs. Another species of interest in this group is P. mandelii, which grows in Antarctica and has been shown to be highly resistant to antibiotics.

Morphology

Bacilli are straight to slightly curved, 0.5 - 1 µm wide x 1.5 -5 µm long. They are not able to form and accumulate polyhydroxybutyrate granules in low nitrogen culture media. This differentiates them from other aerobic bacteria.

The cell envelope is made up of the cytoplasmic membrane, the cell wall and the outer membrane that covers the latter.

The cell wall is typical of Gram negative bacteria, being thin and composed of peptidoglycan. The cytoplasmic membrane separates the cytoplasm from the other components of the cell envelope. It is formed by a lipid bilayer.

The outer membrane is made up of a lipid called lipopolysaccharide that has hydrocarbon chains. This membrane is a barrier against the passage of molecules such as antibiotics that can cause damage to the cell. On the other hand, it allows the passage of the nutrients required for the bacteria to function.

The capacity of the outer membrane to let some substances pass and others not, is given by the presence of porins. They are structural proteins of the membrane.

Flagella

The flagella in the genus are generally located in a polar position although in some cases it can be sub-polar. Lateral flagella are observed in some strains of P. stutzeri and other species.

The number of flagella has taxonomic importance. There may be one flagellum (monoteric) or several (multitrichous). In the same species the number of flagella may vary.

In some species, the presence of fimbriae (protein appendix that is thinner and shorter than a flagellum) has been observed, corresponding to evaginations of the cytoplasmic membrane.

In P. aeruginosa the fimbriae are approximately 6 nm wide, are retractable, and act as receptors for various bacteriophages (viruses that infect bacteria). The fimbriae can contribute to the adhesion of the bacterium to the epithelial cells of its host.

Lifecycle

Pseudomonas species, like all bacteria, reproduce by binary fission, a type of asexual reproduction.

In the first phase of binary fission, the bacterium enters a DNA duplication process. These have a single circular chromosome that begins to be copied by the activity of replication enzymes.

The replicated chromosomes go towards the ends of the cell, later a septum is generated and a new cell wall originates to form the two daughter cells.

Various mechanisms of genetic recombination have been observed in Pseudomonas species. This guarantees the occurrence of genetic variability in asexual reproductive organisms.

Among these mechanisms is transformation (exogenous DNA fragments can enter bacteria). Others are transduction (exchange of DNA between bacteria by a virus) and conjunction (transfer of DNA from a donor bacterium to a recipient).

Plasmids

Plasmids are small circular DNA molecules that occur in bacteria. These are separated from the chromosome and replicate and transmit independently.

In Pseudomonas, plasmids fulfill various functions as factors of fertility and resistance to various agents. In addition, some provide the ability to degrade unusual carbon sources.

Plasmids can provide resistance to various antibiotics such as gentamicin, streptomycin, and tetracycline, among others. On the other hand, some are resistant to different chemical and physical agents such as ultraviolet radiation.

They can also help prevent the action of different bacteriophages. Likewise, they give resistance against bacteriocins (toxins produced by bacteria to inhibit the growth of similar ones).

Habitat

Pseudomonas species can develop in different environments. They have been found in both terrestrial and aquatic ecosystems.

The ideal temperature for the development of the genus is 28 ° C, but species such as P. psychrophila can grow in a range of -1 ° C to 45 ° CP thermotolerans is capable of developing at a temperature of 55 ° C.

None of the species of the genus tolerates a pH lower than 4.5. They can grow in media containing nitrate ammonium ions as a nitrogen source. They require only a simple organic compound as a source of carbon and energy.

At least nine species of Pseudomonas have been found growing in Antarctica. While the species P. syringae has been associated with the water cycle, being present in rainwater, snow and clouds.

Diseases

Pseudomonas species can cause various diseases in both plants and animals and humans.

Diseases in animals and humans

Species of the genus are generally considered to have low virulence, as they tend to be saprophytic. These are opportunistic and tend to cause illness in patients with low resistance to infection. They are usually present in the urinary tract, respiratory tract, wounds, and blood.

The species that most affects humans is P. aeruginosa. It is an opportunistic species that attacks immunosuppressed patients, who have suffered severe burns or are undergoing chemotherapy.

P. aeruginosa primarily attacks the respiratory tract. In patients with bronchiectasis (dilation of the bronchi) it generates a high amount of sputum and can be fatal.

P. entomophila has been found to be a pathogen of Drosophila melanogaster (fruit fly). It is spread by ingestion and attacks the epithelial cells of the insect's intestine, which can cause death.

P. plecoglossicida has been found as a pathogen of the ayu fish (Plecoglossus altivelis). The bacteria cause hemorrhagic ascites (accumulation of fluid in the peritoneal cavity) in fish.

Plant diseases

The phytopathogenic species of Pseudomonas are the cause of a great diversity of diseases. These can generate necrotic lesions or spots on stems, leaves and fruits. They can also cause galls, putrefaction and vascular infections.

The P. syringae group attacks mainly at the foliar level. For example, in the onion they can produce spots on the leaves and rot of the bulb.

In the olive tree (Olea Europea) the species P. savastanoi is the causative agent of tuberculosis of the olive tree, which is characterized by the formation of tumors. These tumors form mainly on stems, shoots, and sometimes on leaves, fruits, and roots. They cause defoliation, a decrease in the size of the plant and later its death.

References

1. Casado MC, Urbano N, R Díaz and A Díaz (2015) Olive tree tuberculosis: in vitro study of the effect of different fungicides on six strains of Pseudomonas savastonoi. Expoliva Symposium Proceedings, Jaén, Spain, May 6 - 8.
2. Hesse C, F Schulz, C Bull, BT Shaffer, Q Yan, N Shapiro, A Hassan, N Varghese, L, Elbourne I Paulsen, N Kyrpides, T Woyke and J Loper (2018) Genome-based evolutionary history of Pseudomonas spp. Enviromental Microbiology 20: 2142-2159.
3. Higuera-Llantén S, F Vásquez-Ponce, M Núñez-Gallego, M Palov, S Marshall and J Olivares-Pacheco (2018) Phenotypic and genotypic characterization of a novel multiantibiotic-resistant, alginate hyperproducing strain of Pseudomonas mandelii isolated in Antarctica. Polar Biol. 41: 469-480.
4. Luján D (2014) Pseudomonas aeruginosa: a dangerous adversary. Acta Bioquím Clín. Latin America. 48 465-74.
5. Nishimori E, K Kita-Tsukamoto and H Wakabayashi (2000) Pseudomonas plecoglossicida sp. nov., the causative agent of bacterial haemorrhagic ascites of ayu, Plecoglossus altivelis. International Journal of Systematic and Evolutionary Microbiology. 50: 83–89.
6. Palleroni NJ and M Doudoroff (1972) Some properties and taxonomic sub-divisions of the genus Pseudomonas. Annu. Rev. Phytopathol. 10: 73-100.
7. Palleroni, N (2015) Pseudomonas. In: Whitman WB (editor) Bergey's Manual of Systematics of Archaea and Bacteria. John Wiley & Sons, Inc., in association with Bergey's Manual Trust.