What’s in a name? Time to update your procedures with Pseudomonas paraeruginosa

Pseudomonas is a large assemblage of motile, rod-shaped, aerobic, non-spore forming, Gram-negative bacteria. Representatives have one or more polar flagella that assist in their movement. Today there are over 300 known species, inhabiting diverse niches and environments including soil, water, plants and animal tissues (1). One of the most commonly used species is – or has been – ‘Pseudomonas aeruginosa.’

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With the taxonomic renaming of the ‘QC strain’ Pseudomonas aeruginosa (NCTC 13628T / ATCC 9027) as Pseudomonas paraeruginosa, it is timely to review the origins and characteristics of the bacterium – one of the most abundant organisms on Earth (2).

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QC strain

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What is now known as Ps. paraeruginosa is a widely used quality control strain, being referenced in the United States Pharmacopeia chapters <51>, <60>, <61>, <62>, and <71> (and the equivalent European and Japanese harmonized chapters). These chapters relate to antimicrobial susceptibility testing, microbial limits testing and the sterility test.

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The bacterium is also used to qualify water samples and it is referenced in ISO 11930: Cosmetics — Microbiology — Evaluation of the antimicrobial protection of a cosmetic product.

History of the strain

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The bacterium was first identified in 1872 as ‘Bacterium aeruginosum’ by Schroter et al. This makes it one of the earliest known microorganisms (3). In 1894 it was reclassified by Migula as Pseudomonas aeruginosa (4).

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According to the American Type Culture Collection (ATCC), the Pseudomonas paraerguinosa strain that is held by the major culture collections is R. Hugh 813. This was isolated from an outer ear infection from a patient in Oregon.

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Reclassification

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Ps. paraeruginosa was reclassified following phylogenomic and comparative genomic studies performed by Rudra et al. that identified two distinct clades of P. aeruginosa (5).

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[Note: In phylogenetics, a clade is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree (6). By being in different clades this means the organisms are unrelated to each other (i.e., not evolved from a common ancestor)].

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According to Rudra et al, phylogenomic and comparative genomic studies were conducted on the genome sequences of 212 P. aeruginosa strains covering their genetic diversity. In a phylogenomic tree based on 118 conserved proteins, the analyzed strains formed two distinct clades.

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Clade-1, encompassing >70?% of the strains including the type strain DSM 50071T, represents the species P. aeruginosa sensu stricto.        

Clade-2, the outlier group, with NCTC 13628T as its type strain, constitutes a novel species level lineage.        

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The average nucleotide identity, average amino acid identity and digital DNA–DNA hybridization values between the strains from Clade-1 and Clade-2 are in the range of 93.4–93.7, 95.1–95.3 and 52–53?%, respectively. The 16S rRNA gene of P. aeruginosa DSM 50071T also shows 98.3?% similarity to that of NCTC 13628T.

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These values were assessed as being lower than the suggested cut-off values for species distinction, indicating that the Clade-2 strains (NCTC 13628T) constitute a new species.

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Other differences

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Rudra et al also pointed out that in contrast to swimming motility, twitching motility is only present in Clade-1 strains. The strains from these two clades also differ in their pathogenic mechanisms (based on the presence/absence of a Type III secretion system), production of biosurfactants, phenazines and siderophores, and several other genomic characteristics. This means P. paraeruginosa is less virulent than P. aeruginosa sensu stricto.

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The Rudra et al research was published in 2022 (with a follow-up paper issued in 2023) (7), and the decision to reclassify the type strain was made in 2024 by the world’s major culture collections. Consequently, strains in Clade-2 were transferred to the novel species P. paraeruginosa with ATCC 9027 designated as the type strain.

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Lineage

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For completists, the full lineage is:

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1.????? Cellular organisms

2.????? Bacteria

3.????? Pseudomonadota

4.????? Gammaproteobacteria

5.????? Pseudomonadales

6.????? Pseudomonadaceae

7.????? Pseudomonas

8.????? Pseudomonas aeruginosa group

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Culture collections

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The bacterium has been renamed in the following culture collections:

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NCTC 13628, DSM 1128, ATCC 9027, CIP 82.118, DSM 1385 (Historical Number), WDCM 00026.

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The original culture collection was ATCC (the organism being (provided by a researcher in New York named C. P. Hegarty in 1943), it was then passed onto Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ) and then to the UK National Collection of Type Culture (NCTC).

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What does the name mean?

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What does ‘Pseudomonas paraeruginosa’ mean? The specific epithet “aeruginosa” means copper-rust colored. The reference to ‘para’ refers to the bacterium being beside, alongside, near, like aeruginosa.

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Typical characteristics

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P. ?paraeruginosa grows optimally on nutrient agar. A healthy laboratory culture should produce visible colonies on solid agar within 24 hours when incubated at 37°C under aerobic conditions. It is rod-shaped (1-5 μm long and 0.5-1.0 μm wide) and Gram-stain negative.

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The temperature growth range, medium dependent, is 10-41°C. Other suitable growth media include Columbia blood medium, trypticase soy yeast extract medium and trypto casein soya agar (TSA).

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Some other characteristics are:

Flagellated and motile        

Facultative aerobe        

Produces exotoxins (exotoxin A)        

Non-spore forming        

It will not grow under halophilic conditions.        

It is capable of glucose utilization (but not fermentation).        

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Safety

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Under the UK pathogen rating system, P. paraeruginosa falls into Hazard group 2 (HG 2): Disease producing organisms which are either exotic or produce notifiable disease, but have a low risk of spread from the laboratory.

Pathogenicity

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Pseudomonas aeruginosa (and now its variants) is an opportunistic human pathogen capable of causing a wide array of life-threatening acute and chronic diseases (8).

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Ten facts about P. aeruginosa

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Hopefully, some of these facts will surprise you…

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1.????? One reason why the organism is so common on the planet is because P. aeruginosa can catabolize a wide range of organic molecules, including organic compounds such as benzoate (9).

2.????? The first scientific study on P. aeruginosa was entitled “On the blue and green coloration of bandages,” published by the pharmacist Carle Gessar in 1882 (10)

3.????? P. aeruginosa produces water-soluble pigments, which, on exposure to ultraviolet light, fluoresce under blue-green light. This is attributed to pyocyanine, a derivative of phenazine. This also gives the bacterium a distinctive grapelike odor (due to aminoacetophenone and pyocyanin production) when growing as visible colonies on many types of culture media (11).

4.????? For users of hot tubs, the bacterium is a common cause of ’Hot Tub Rash,’ in which direct contact between the skin and the infected water from the tub will make the infected skin itchy and turn it a bumpy red color (12).

5.????? With its pathogenicity, the pili and flagella of P. aeruginosa plays a vital role in the infection of burns and wounds (13).

6.????? P. aeruginosa can be transmitted to a host via fomites, vectors, and hospital workers who are potential carriers for multiply-antibiotic-resistant strains of the pathogen. P. aeruginosa can survive from six hours to 16 months on dry, inanimate surfaces in hospitals (14).

7.????? P. aeruginosa contaminated the spacecraft’s water system and infected a crew member during the Apollo era (15). The bacterium may become more virulent in space.

8.????? Under iron deficiency, a yellowish-green, fluorescent pigment develops as a result of pyoverdins (16).

9.????? P. aeruginosa communicates with other cells through quorum-sensing, using the enzyme tyrosine phosphatase. This form of communication allows the cells to regulate gene production which results in control of certain cell functions and this is important for biofilm formation (17).

10.? It can play a role in bioremediation: the bacterium can degrade aromatic hydrocarbons such as methylbenzenes, which are the by-products of petroleum industries (18).

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Tim Sandle is a pharmaceutical microbiologist. Check out Pharmaceutical Microbiology Resources and read microbiology news features like “Smart windows significantly reduce indoor pathogens”.

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References

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1.????? Palleroni, N. J. (2010). The Pseudomonas story. Environ. Microbiol. 12, 1377–1383

2.????? Costerton, W., and Anwar, H. Pseudomonas aeruginosa: The Microbe and Pathogen. Pseudomonas aeruginosa Infections and Treatment. 1994. p.1-17

3.????? Peix A, Ramírez-Bahena M-H, Velázquez E. Historical evolution and current status of the taxonomy of genus Pseudomonas. Infect Genet Evol 2009; 9:1132–1147

4.????? Sneath PHA, McGowan V, Skerman VBD. Approved lists of bacterial names. Int J Syst Bacteriol 1980; 30:225–420

5.????? Rudra B, Duncan L, Shah AJ, Shah HN, Gupta RS. Phylogenomic and comparative genomic studies robustly demarcate two distinct clades of Pseudomonas aeruginosa strains: proposal to transfer the strains from an outlier clade to a novel species Pseudomonas paraeruginosa sp. nov. Int J Syst Evol Microbiol. 2022 Nov;72 (11).

6.????? Cracraft, Joel; Donoghue, Michael J., eds. (2004). "Introduction". Assembling the Tree of Life. Oxford University Press. p. 1

7.????? Rudra, B., Duncan, L., Shah, A.J., Shah, H.N., and Gupta, R.S. "Corrigendum: Phylogenomic and comparative genomic studies robustly demarcate two distinct clades of Pseudomonas aeruginosa strains: proposal to transfer the strains from an outlier clade to a novel species Pseudomonas paraeruginosa sp. nov." Int. J. Syst. Evol. Microbiol. (2023) 73(3):005828

8.????? Lund-Palau, H., Turnbull, A. R., Bush, A., Bardin, E., Cameron, L., Soren, O., et al. (2016). Pseudomonas aeruginosa infection in cystic fibrosis: pathophysiological mechanisms and therapeutic approaches. Expert Rev. Respir. Med. 10, 685–697

9.????? Medi? A, Hüttmann N, Lje?evi? M, Risha Y, Berezovski MV, Mini? Z, Karad?i? I. A study of the flexibility of the carbon catabolic pathways of extremophilic P. aeruginosa exposed to benzoate versus glucose as sole carbon sources by multiomics analytical platform. Microbiol Res. 2022 Jun;259:126998

10.? Gessard, C. Classics in infectious diseases. On the blue and green coloration that appears on bandages. Rev Infect Dis. 1884 Sep-Oct;6 Suppl 3:S775-6. PMID: 6443777.

11.? Gilardi, G. Cultural and Biochemical Aspects for Identification of Glucose-Nonfermenting Gram-Negative Rods. Nonfermenting Gram-Negative Rods. 1985. p.17-24

12.? CDC: https://www.cdc.gov/healthy-swimming/prevention/preventing-hot-tub-rash.html

13.? Lyczak, JB, Cannon, CL, Pier, GB. Establishment of Pseudomonas aeruginosa infection: lessons from a versatile opportunist. Microbes and Infection. 2000. Volume 2. Issue 9. p. 1051-1060

14.? Stojek NM, Szymanska J, Dutkiewicz J. Gram-negative bacteria in water distribution

systems of hospitals. Ann Agric Environ Med. 2008:15:135-142

15.? Taylor GR. Recovery of medically important microorganisms from Apollo astronauts. Aerospace Med. 1974;45(8):824–882

16.? Herbert. Siderophores of the Human Pathogenic Fluorescent Pseudomonads. Current Topics in Medicinal Chemistry 1.1 (2001): 1-6. Academic Search Premiere. Web. 27 Apr. 2010.

17.? Dong, Y.H., Wang, L.-H., & Zhang, L.-H. (2007). Quorum-quenching microbial infections: mechanisms and implications. Philosophical Transactions of the Royal Society B, 362(1483), 1201-1211.

18.? Johnson, G., and Olsen, R. Multiple Pathways for Toluene Degradation in Burkholderia sp. Strain JS150. Applied and Environmental Microbiology. 1997. Volume 63. p. 4047-4052

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