Pseudomonas

Pseudomonas is a gram-negative bacteria. It is a genus belonging to the Pseudomonadaceae family. There are about 144 species in this genus. Most of the Pseudomonas species are saprotrophic, meaning they release enzymes that break down big substances into small ones.

## History

Pseudomonas is one of the earliest found bacteria. It was first called 'Bacterium aeruginosm' by Schroter in 1872. However, in 1894, German botanist Walter Migula classified it as _Pseudomonas aeruginosa_. In Greek, pseudo means false and monas means “unit”. This bacteria is associated with many chronic infections and is considered dangerous due to its resistance to many antibiotics. There has been an increase in the number of species discovered year by year. In 2013, 10 species have been found and 6 more in the subsequent year. Pseudomonas was well-classified 100 years later by a Canadian scientist, Roger Stanier.  The classification was based on the physiological and biochemical properties.

## Phylogeny

The basic tool which is used to classify Pseudomonas is the 16S rRNA gene.  However, it is hard to differentiate closely related Pseudomonas species, thus genes such as atpD, gyrB, rpoB, recA, and rpoD are also used in the taxonomy of Pseudomonas.Using gyrB and rpoD in phylogenetic analysis, Pseudomonas came from two predominant clusters; intrageneric cluster I (IGC I) and intrageneric cluster II (IGC II).

IGC I consists of two sub clusters, the ‘_Pseudomonas aeruginosa_ complex’  comprises of _Pseudomonas aeruginosa, Pseudomonas alcaligenes, Pseudomonas citronellolis, Pseudomonas mendocina, Pseudomonas oleovorans_ and _Pseudomonas pseudoalcaligenes,_ and the ‘_Pseudomonas stutzeri_ complex’, comprises of _Pseudomonas balearica_ and _Pseudomonas stutzeri_.

IGC II consists of three subclusters, the ‘_Pseudomonas syringae_ complex’, the ‘_Pseudomonas fluorescens_ complex’ and the ‘_Pseudomonas putida_ complex’.  The ‘_Pseudomonas syringae_ complex’ are phytopathogens and pathovars. The former encompasses _Pseudomonas amygdali, Pseudomonas caricapapayae, Pseudomonas cichorii, Pseudomonas ficuserectae_ and _Pseudomonas viridiflava_  and the latter consists of _Pseudomonas savastanoi_ and _Pseudomonas syringae_. The ‘_Pseudomonas fluorescens_ complex’ is divided into two subpopulations, the ‘_Pseudomonas fluorescens_ lineage’ and the ‘_Pseudomonas chlororaphis_ lineage’. The ‘_Pseudomonas fluorescens_ lineage’ contains _Pseudomonas fluorescens_ biotypes A, B and C, _Pseudomonas azotoformans, Pseudomonas marginalis_ pathovars, _Pseudomonas mucidolens_, _Pseudomonas synxantha_ and _Pseudomonas tolaasii_, while the ‘_Pseudomonas chlororaphis_ lineage’ includes _Pseudomonas chlororaphis, Pseudomonas agarici, Pseudomonas asplenii_ and _Pseudomonas corrugata_. The ‘_Pseudomonas putida_ complex’ comprises of _Pseudomonas putida_ and _Pseudomonas fulva_.

## Clinical manifestations

More than 25 species of Pseudomonas cause diseases in humans. Species such as _Pseudomonas aeruginosa, Pseudomonas fluorescens, Pseudomonas putida, Pseudomonas cepacia, Pseudomonas stutzeri, Pseudomonas maltophilia,_ and _Pseudomonas putrefaciens_ are said to cause opportunistic infections in humans.  Opportunistic infections more commonly affect people with weak immune systems compared to the ones with stronger immune system. Only two of the species are known to cause specific disease. These are _Pseudomonas mallei_ and _Pseudomonas pseudomallei_ which cause glanders and melioidosis respectively. However, the most common species involved in clinical problems is _Pseudomonas aeruginosa_. The common clinical problems caused by _Pseudomonas aeruginosa_ is shown in the following bullet points.

- Bone and joint infections (BJI): It has been noted that _Pseudomonas aeruginosa_ infections are the most difficult to treat compared to other BJIs.
    - Osteomyelitis
    - Implant-associated bone infections (IABI)
- Cardiovascular (CV) infections
    - Pseudomonal infectious endocarditis (IE)
    - Constrictive Pericarditis
- Central Nervous System (CNS) infections
    - Brain Abscess
    - Community-acquired _Pseudomonas aeruginosa_ meningitis: It commonly occurs after surgery.
- Ear infections
    - Otitis media and Otitis externa
    - Chronic suppurative otitis media
- Eye infections
    - Endophthalmitis
    - Keratitis
    - Ophthalmia neonatorum: It is rare and the complications are eyelid edema, erythema, and purulent discharge causing corneal perforation, endophthalmitis, blindness, and also death if not treated properly.
- Gastro-intestinal(GI) infections
    - Epidemic diarrhoea: _Pseudomonas aeruginosa_ causes enterocolitis in children. One of the common symptoms of enterocolitis is diarrhoea.
    - Necrotising enterocolitis
    - Neutropenic Enterocolitis (Typhlitis): It commonly affects immunocompromised patients.
    - Shanghai fever
- Genitourinary(GU) infections
    - Epididymitis
    - Urinary tract infections (UTIs): _Pseudomonas aeruginosa_ is the common cause of nosocomial UTIs. It is difficult to distinguish from other bacterial UTIs.
- Respiratory infections
    - Cystic fibrosis (CF)
    - Ventilator-associated pneumonia (VAP)
- Sepsis
- Skin and soft tissue infections
    - Ecthyma gangrenosum (EG): It is rare but has very high mortality rate ranging from 38%-77%.
    - Dermatitis: It is a form of mild cutaneous infection.
    - Necrotizing fasciitis: It is rare with only 12 cases recorded so far.

## Biotechnology

Pseudomonas strains have been useful in the biotechnology field. The table below shows the Pseudomonas strains and their respective uses.
|Scientific name of the strains|Uses|
| --- | --- |
|_Pseudomonas aeruginosa_|Treatment of rice blast disease|
|_Pseudomonas syringae_|Production of frozen foods|
|_Pseudomonas putida_|Control black root rot disease of tobacco  and the production of biosynthetic chemicals and drugs|
|_Pseudomonas fluorescens_|Used in the food and leather industries|
|_Pseudomonas oleovorans_|Used in the production of polyesters|