Pseudomonas is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 validly described species.[2] The members of the genus demonstrate a great deal of metabolic diversity and consequently are able to colonize a wide range of niches.[3] 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, P. lini, P. migulae, and P. graminis.[4][5]
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.[6][7] The latter statement was later proved incorrect and was due to refractive granules of reserve materials.[8] Despite the vague description, the type species, Pseudomonas pyocyanea (basonym of Pseudomonas aeruginosa), proved the best descriptor.[8]
Like most bacterial genera, the pseudomonad[note 1] 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 (the main authority in bacterial nomenclature) as Greek pseudes (ψευδής) "false" and -monas (μονάς/μονάδος) "a single unit", which can mean false unit; however, Migula possibly intended it as false Monas, a nanoflagellated protist[8] (subsequently, the term "monad" was used in the early history of microbiology to denote unicellular organisms). 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.[9]
Recently, 16S rRNA sequence analysis has redefined the taxonomy of many bacterial species.[10] As a result, the genus Pseudomonas includes strains formerly classified in the genera Chryseomonas and Flavimonas.[11] Other strains previously classified in the genus Pseudomonas are now classified in the genera Burkholderia and Ralstonia.[12][13]
In 2020, a phylogenomic analysis of 494 complete Pseudomonas genomes identified two well-defined species (P. aeruginosa and P. chlororaphis) and four wider phylogenetic groups (P. fluorescens, P. stutzeri, P. syringae, P. putida) with a sufficient number of available proteomes.[14] The four wider evolutionary groups include more than one species, based on species definition by the Average Nucleotide Identity levels.[15] In addition, the phylogenomic analysis identified several strains that were mis-annotated to the wrong species or evolutionary group.[14] This mis-anotation problem has been reported by other analyses as well.[16]
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 (2000), P. putida KT2440 (2002), P. protegens Pf-5 (2005), P. syringae pathovar tomato DC3000 (2003), P. syringae pathovar syringae B728a (2005), P. syringae pathovar phaseolica 1448A (2005), P. fluorescens Pf0-1, and P. entomophila L48.[9]