Pseudomonas

In 1786, the physician Otto Friedrich Müller drew up a bacterial system distinguishing between motile microbes (tremors or vibrions) and immobile "primordial corpuscles" or monads (monas puctum = cocci). Later it was recognized that there were also motile rod-shaped bacteria that did not fit into this scheme, i.e. false (because motile) primordial corpuscles, i.e. pseudomonads.

Pseudomonas are Gram-negative, non-spore-forming rod-shaped bacteria. In some cases, Pseudomonas species use nitrate as a terminal electron acceptor. Then growth occurs anaerobically (nitrate respiration). Pseudomonas are motile by one or more polar flagella and consist of straight to curved rods that are 1.5-5.0 μm long and 0.5-1.0 μm in diameter. They are chemoorganotrophic and do not require organic growth factors. Pseudomonas are catalase and usually oxidase positive. They are not very acid tolerantand do not grow below pH 4.5. Pseudomonas species degrade a variety of low molecular weight organic compounds and hydrolytic products produced by actinomycete and fungal growth. All species are so-called nonfermenters (non-fermenters: fermentation is not used as energy metabolism by this genus).

The human-pathologically immensely important Pseudomonas aeruginosa forms a blue-green dye (the phenacetin derivative pyocyanin) and develops a peculiar sweetish odor through the formation of aminoacetophenone. This odour can be used diagnostically at the bedside.

Ecology: Pseudomonads are ubiquitous in the environment worldwide, especially in moist habitats and soil. These "wet germs" often live on dead organic material from plants. They are also commonly found in the rhizosphere.

The genus Pseudomonas belongs to the Pseudomonadaceae, a family of Gram-negative Gammaproteobacteria that includes the genera

• Pseudomonas
• Cellvibrio
• Mesophilobacter
• Rhizobacter
• Rugamonas and
• Serpens

includes. The genus Pseudomonas now includes over 200 species; some of which have major human pathological significance. However, through the use of genetic and molecular tools such as 16S and 23S rRNA gene sequences, rRNA-DNA and DNA-DNA hybridization, and cellular lipid and fatty acid profiling, some species have been defined as new genera. These include:

• Burkholderia
• Burkholderia cepacia (complex of 20 species, infection-related exacerbation of cystic fibrosis).
• Burkholderia mallei (glanders - in solipeds, rarely in humans)
• Burkholderia pseudomallei (melioidosis - most severe or fatal pneumonia, especially in tropical countries)
• Stenotropomonas
• Stenotropomonas maltophilia (nosocomial infection /water)

Other medically important pseudomonads that occur in water are: Shewanella, Sphingomonas, Comamonas, Delftia, Ralstonia, Chryobacterium.

The genus Pseudomonas in the narrower sense includes the following species with different pathogenic activity:

• P. aeruginosa (pathogenic, green pus -aeruginosa =green pus)
• P. cannabina
• P. denitrificans
• P. fluorescens (little pathogenic)
• P. marginalis
• P. putida (little pathogenic)
• P. stutzeri (low pathogenicity)
• P. syringae

Environmental germs/opportunists: Most pseudomonads are environmental germs, especially so-called puddle germs from aqueous sites (e.g. Syphon). They are physiologically highly flexible and can occur as opportunistic pathogens in already weakened plants and animals. Species such as Pseudomonas aeruginosa can be pathogenic to immunocompromised humans. Virulence genes are localized to plasmids and can be easily transferred from one Pseudomonas species to another.

R-plasmid: Pseudomonads often possess a so-called R-plasmid, which serves to transfer the resistance genes.

Tetrodotoxin (TTX): Tetrodotoxin (also the variant anhydro-TTX) a highly toxic neurotoxin is produced by some Pseudomonas species.

Pseudomonas as pathovars: Plant-associated species may be plant pathogens(phytopathogens -so-called pathovars) or act as plant growth promoters. Pseudomonas syringae is a major plant pathogen with more than 50 pathogenic variants defined by the plant species it infects; these include a number of economically important species such as tomato, beans, rice, tobacco and a number of tree hosts such as European horse chestnut, olive and cherry.

Pseudomonas as phytoprotectants: In contrast, a number of species are important for colonizing the rhizosphere and promoting plant health by antagonizing plant pathogens. Some, such as Pseudomonas fluorescens, produce insecticides and can therefore be used as biocontrol agents. Nitrogen-fixing species, such as Pseudomonas stutzeri, colonize plant roots and their nitrogen-fixing abilities and are a positive stimulator of plant growth.

The extraordinary nutritional versatility of the genus means that they can use a wide range of compounds as carbon sources, including hazardous environmental contaminants;

Pseudomonas as bioremediators: Pseudomonas putida is a typical example with a wide range of biodegradative capabilities that can degrade unusual carbon sources such as toxic organic wastes (e.g., petroleum and aromatic hydrocarbons) and is therefore important for bioremediation.

Animal kingdom: Pseudomonas are thought to be responsible for the toxicity of various puffer fish, among others. The fish presumably ingest the bacteria through food. It is assumed that there is a symbiosis between the puffer fish and the bacteria.

Pseudomonas aeruginosa is a dreaded cause of nosocomial infections that can be very severe, often fatal. In addition, the pathogen is frequently found in persons with burns, on large ulcers of the lower legs and in the lungs of cystic fibrosis patients.

Antibiotic resistance: Clinically important, numerous Pseudomonas species exhibit resistance to antibiotics. They are also capable of biofilm formation (from special sugars alginates).

Nosocomial infections: While bacteria of the genus Pseudomonas rarely cause disease in people with intact immune systems, they can cause infections of wounds, respiratory and urinary tracts, pneumonia as well as sepsis and heart disease, especially in immunocompromised patients. Wound infections caused by P. aeruginosa are characterized by their blue-green coloration and sweet-smelling odor. Patients with cystic fibrosis(cystic fibrosis) are particularly at risk, and Pseudomonas-induced pneumonia is the most common cause of death in these patients.

Other diseases caused by Pseudomonas species are:

• Nosocomial pneumonia especially in ventilated patients. Pneumonia or sinus infections due to Pseudomonas pathogens are common in HIV-infected patients.

• Soft tissue infections include infections of muscles, tendons, ligaments, and skin. These infections can occur in deep puncture wounds

• Panniculitis, infectious

• Urinary tract infections

Dermatological infections

• Otitis externa (swimmer's ear, bathing otitis) is a mild external infection that can occur in otherwise healthy individuals.

• Malignant external otitis is a more serious external ear infection. It is particularly common in people with diabetes.

• Pseudomonas folliculitis (whirlpool dermatitis) is another mild external infection. Follicular pyoderma occurs in hot tub users, especially when hot tubs are inadequately maintained.

• Ecthyma gangraenosum which occur in neutropenic patients.
• Infectious keratitis: usually the result of injury, but sometimes arising from contaminated contact lenses or contact lens fluid.
• Folliculitis, gram-negative
• Foot infection gram-negative

• Green nail syndrome(see below nail pigmentation)

• Pseudomonas hot-foot syndrome

• Wound infections, gram-negative (see below Wound, chronic)

Acylaminopenicillins such as piperacillin, cephalosporins from the third generation onwards, especially ceftazidime and cefepime. Ceftriaxone and cefotaxime, on the other hand, are not effective. Effective are fluoroquinolones, aminoglycosides and carbapenems.

Remark: It is particularly noticeable that the number of antibiotic-resistant as well as multi-resistant strains is increasing. For Europe it can be assumed that up to 25 % of the Pseudomonas aeruginosa strains are resistant to cephalosporins of the 3rd generation, for the carbapenems and quinolones it is even up to 50 %. In the meantime, there are so-called 4MRGN strains against which none of the currently available antibiotics are effective.

It remains to be seen to what extent Pseudomonas vaccinations will be an option for people at risk in the future.

1. Chegini Z et al. (2020) Bacteriophage therapy against Pseudomonas aeruginosa biofilms: a review. Ann Clin Microbiol Antimicrob 19:45.
2. Lee K et al. (2017) Pseudomonas aeruginosa biofilm, a Programmed Bacterial Life for Fitness. J Microbiol Biotechnol 27:1053-1064.
3. Mielko KA et al. (2019) , Jabłoński SJ, Milczewska J, Sands D, Łukaszewicz M, Młynarz P. Metabolomic studies of Pseudomonas aeruginosa. World J Microbiol Biotechnol 35:178.
4. Moradali MF et al.(2017) Pseudomonas aeruginosa, Lifestyle: A Paradigm for Adaptation, Survival, and Persistence. Front Cell Infect Microbiol.7:39.
5. Mulcahy LR et al. (2014) Pseudomonas aeruginosa biofilms in disease. Microb Ecol 68:1-12.
6. Silby MW et al. (2011) Pseudomonas genomes: diverse and adaptable. FEMS Microbiol Rev 35:652-680.