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The biological identity of the genus Pseudomonas has changed dramatically in recent years during the transition between artificial classification based on phenotypic properties (e.g. Bergey's Manual of Systematic Bacteriology,1st ed., 1986) and revisionist classification based on genotypic (phylogenetic) properties (e.g. Bergey's Manual of Systematic Bacteriology, 2nd ed., 2001). However, in either scheme, the genus comprises a relatively large and important group of Gram-negative bacteria. Members of the genus are found abundantly as free-living organisms in soils, fresh water and marine environments, and in many other natural habitats. They may also be found in associations with plants and animals as normal flora or as agents of disease. For the purposes of this article, the term "pseudomonad" refers to a bacterium with ecophysiological properties similar to members of the genus Pseudomonas. Some of these bacteria were formerly in the genus Pseudomonas but have been moved to other genera, families, or orders among the alpha Proteobacteria because of their phylogenetic distance from Pseudomonas. This article will use both old and new taxonomy to identify the pseudomonads, until this period of confusion has passed.

Morphologically, members of the genus Pseudomonas (as well as most other pseudomonads) may be described as Gram-negative, non-spore forming, straight or slightly curved rods. They are typically motile by means of one or more polar flagella. These basic morphological characteristics, however, are common to many families of bacteria and so are of little value in the positive identification or diagnosis of a member of the genus Pseudomonas.

Generally, common to all constituent species of the genus Pseudomonas are certain physiological properties such as chemoorganotrophic nutrition, aerobic metabolism, absence of fermentation, absence of photosynthesis, inability to fix nitrogen, and capacity for growth at the expense of a large variety of organic substrates.

There are, of course, a few exceptions to these standardized criteria of definition or identification:

-The traditional description of a short rod-shape body does not always fit the cell morphology of all Pseudomonas species. In some of them, the cells can be extremely short, while in others (e.g. certain strains of Pseudomonas putida, and P. syringae) they may be very long. Occasionally, particularly in old cultures, the cells can be of such unusual shapes and sizes that a casual microscopic observation may cast doubt on the purity of the population.

-Some defining physiological characteristics have not been critically tested in all members of the family, and as a consequence, occasional reports of exceptional strains have been noted. Thus, nitrogen fixation has been shown to occur in Pseudomonas stutzeri, and Pseudomonas aeruginosa is capable of anaerobic respiration utilizing NO3 as a final electron acceptor (denitrification), and it can grow anaerobically, albeit slowly, with arginine and small amounts of yeast extract.

Pseudomonads are important in the balance of nature and also in the economy of human affairs. Pseudomonads are globally active in aerobic decomposition and biodegradation, and hence, they play a key role in the carbon cycle. Pseudomonas species are renowned for their abilities to degrade compounds which are highly refractory to other organisms, including aliphatic and aromatic hydrocarbons, fatty acids, insecticides and other environmental pollutants. Apparently, the only organic compounds that these pseudomonads can't attack are teflon, styrofoam and one-carbon organic compounds (methane, methanol, formaldehyde, etc.). Pseudomonads are also a regular component of microbial food spoilage in the field, in the market place, and in the home.

Pseudomonas and certain other pseudomonads include species pathogenic for humans, domestic animals, and cultivated plants. Pseudomonas species, as well as species included in the newly-created genera Burkholderia and Ralstonia (ex-Pseudomonas) are among the most important bacteria that are pathogens of plants. They cause economically significant crop disease and crop loss world-wide. Pseudomonas aeruginosa infects both plants and animals and has evolved into one of the most common and refractory nosocomial pathogens of the post-antibiotic era.

A close relative of Pseudomonas is Xanthomonas. Xanthomonas includes both phytopathogenic species and saprophytic strains. Another pseudomonad, Zoogloea is ecologically more restricted, but it has an extremely active oxidative metabolism in its natural habitat and is an important participant in the carbon cycle as a component of the microflora of activated sludge. Differentiation of the three is possible (though often not clear cut) by certain structural, physiological or ecological characteristics, some of which are summarized in the table below.

Table 1. Selected characteristics of diagnostic value for the differentiation of three genera of bacteria considered pseudomonads. This scheme of internal subdivision is reasonably consistent with separation of the genera on the basis of phylogenetic criteria. Genus Characteristic

Pseudomonas

Usually motile and oxidase-positive. Capable of growth in simple minimal media at the expense of a large variety of low-molecular-weight organic compounds. Organic growth factors are not required.

Xanthomonas

Water-insoluble yellow pigments (xanthomonadins) produced by the plant pathogenic species. Growth on nutrient agar inhibited by 0.1% triphenyltetrazolium chloride. Weak or negative oxidase reaction. Organic growth factors required.

Zoogloea

Cells actively motile when young. Production of dendritic masses of growth attaching to solid detritus in natural waters and sewage.

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The Genus Pseudomonas

The bacteriological criteria that distinguish the members of the genus Pseudomonas are given below in Table 2.

Table 2. General characteristics of the genus Pseudomonas

Gram-negative

Rod-shaped, 0.5-0.8 um x 1-3 um

Strictly aerobic; the only anaerobic activities may be denitrification and arginine degradation to ornithine

Motile by polar flagella; some strains also produce

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