General Microbiology - Schlegel H. 1987

Prokaryotes: A Systematic Review
Pseudomonads and other Gram-negative rods (groups 7, 12, and 3)

The term "pseudomonads" is commonly used to describe all Gram-negative, rod-shaped Bacteria with polar flagella. Consequently, this grouping often encompasses highly specialized eubacteria such as Nitrosomonas, Methylomonas, thiobacilli, and even phototrophic bacteria (Rhodopseudomonas). Because of this broad application, the name reflects only morphological characteristics rather than any recognized taxonomic category.

The family Pseudomonadaceae comprises Gram-negative, non-spore-forming, aerobic bacteria that appear as straight or slightly Curved Rods with polar flagella. They derive energy through aerobic Respiration, and in some species, Anaerobic respiration (such as denitrification or nitrate respiration), but not via Fermentation. While pseudomonads are primarily chemoorganotrophs, certain species can optionally grow as chemolithotrophs. The genus Pseudomonas serves as the prototype for the family. This genus, much like Group 7 as a whole, is characterized by distinct metabolic versatility. Its representatives are capable of utilizing a vast array of organic substrates, including heterocyclic and Aromatic Compounds that other bacteria cannot metabolize. Sugars are typically degraded via the Entner-Doudoroff Pathway (Section 7.2.3). Certain Pseudomonas species oxidize sugars incompletely, resulting in the accumulation of sugar acids (such as gluconic and 2-oxogluconic acids).

Thanks to their undemanding nutritional requirements, pseudomonads are ubiquitous: they are found in soil, aquatic environments, wastewater, and the air. If an open nutrient medium containing mineral salts and organic acids or sugars is left exposed, pseudomonads typically colonize it first. They are often easily identified by the Water-soluble pigments they produce, such as pyocyanin (a blue-green phenazine derivative) and pterin derivatives exhibiting a yellow-green fluorescence. Several of these fluorescent pigments function as siderophores (Section 7.7).

Pseudomonad species. Pseudomonas aeruginosa (formerly known as P. pyocyanea) is primarily an aquatic bacterium. However, it exhibits remarkable adaptability and can act as an opportunistic human pathogen, causing conditions such as otitis media and wound infections characterized by blue-green pus, as well as bacteremia in immunocompromised individuals. P. fluorescens and P. putida are widespread water and soil bacteria capable of oxidizing an extraordinarily diverse range of Organic compounds. Many plant-pathogenic strains have recently been consolidated into a single species, P. syringae. The Glucose Catabolism via the Entner-Doudoroff pathway was originally discovered using the hydrogen bacterium P. saccharophila as a model.

Xanthomonads. Plant-pathogenic pseudomonads that produce a yellow pigment have been classified into the genus Xanthomonas. Their pigment is a bromine-containing polyene compound. Strains of Xanthomonas campestris secrete extracellular Polysaccharides that resist enzymatic degradation and are produced industrially; their aqueous solutions are widely used as thickening agents (e.g., in puddings, dietetic soups, and printing inks).

Other aerobic Gram-negative rods. Genera such as Alcaligenes, Agrobacterium, Rhizobium, the acetic acid bacteria Acetobacter and Gluconobacter (Section 10.1), and free-living nitrogen-fixing bacteria (Azotobacter, Beijerinckia, Derxia; Section 13.2) share significant metabolic similarities with pseudomonads.

The genera Alcaligenes, Rhizobium, and Agrobacterium group together bacteria that resemble aerobic pseudomonads in their METABOLISM/26.html">Energy Metabolism (respiration), yet differ in flagellar arrangement: their flagella are subpolar or limited in number to 2–6 (degenerate peritrichous flagellation) rather than strictly polar. The genus Alcaligenes includes the facultatively autotrophic hydrogen bacterium A. eutrophus (Section 11.4). Bacteria of the genus Rhizobium act as endosymbionts of leguminous plants and possess The ability to fix molecular nitrogen (Section 13.1). Agrobacterium tumefaciens induces crown galls and tumors on the ROOT collars, stems, or leaves of various plants (Section 4.3).

Chemolithotrophic bacteria (Group 12). Aerobic chemolithoautotrophic bacteria (Chapter 11) utilize inorganic ions or compounds as hydrogen sources or electron Donors, respectively. Carbon dioxide serves as their carbon source, which they assimilate via the ribulose bisphosphate cycle. However, the majority of these bacteria are merely facultative autotrophs, retaining the ability to utilize organic substrates as well. Autotrophy is found across numerous genera, including Pseudomonas, Alcaligenes, Aquaspirillum, Xanthobacter, Mycobacterium, Bacillus, and Nocardia.

In Bergey's Manual, Group 12 is restricted to bacteria that utilize ammonia, nitrite, sulfur compounds, or iron (e.g., Nitrosomonas, Nitrobacter, Thiobacillus); some of these are obligate autotrophs. They are discussed in detail in Sections 11.1–11.3.

Chlamydobacteria (Group 3). The most prominent representative is the filamentous bacterium Sphaerotilus natans. This bacterium thrives in polluted flowing waters, sugar factory settling basins, on dams, and in cooling towers. It forms filaments and flocs or, when firmly attached to a substrate, grows into long, fur-like sheets or sheaths. In a short period of time, S. natans can severely clog pipes, settling basins, and drainage ditches.

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Fig. 3.8. Sphaerotilus natans with Cells inside a partially vacated sheath. (Stokes J. L., J. Bacteriol., 67 [1954], 278.)

Sphaerotilus natans is a unicellular, Gram-negative bacterium featuring a polar tuft of flagella, which places it among the pseudomonads. It is characterized by a distinctive growth habit: it forms long filaments consisting of chains of cells held together within a slender tubular sheath (Fig. 3.8). This sheath is composed of a heteropolysaccharide and Functions as a capsule. The bacteria multiply within the sheath via transverse division and can eventually emerge as motile cells. On Agar media, cultures frequently undergo "dissociation," giving rise to two distinct colony types—rough colonies, which consist primarily of filaments, and smooth colonies, composed predominantly of individual cells. Numerous variants are distinguished based on Cell dimensions, filament thickness, and other morphological traits.

Filamentous bacteria are widely distributed in iron-rich waters [containing Fe(OH)₂], such as ditches, springs, drainage pipes, and bogs. Historically, they were referred to as "ochre bacteria" (Leptothrix ochracea). Their natural habitats are typically poor in available organic nutrients yet rich in iron. Consequently, their sheaths become permeated and surrounded by iron oxide particles. Following the pioneering work of Winogradsky (1888), these bacteria were long regarded as autotrophic iron-oxidizing microorganisms. However, since neither physiological nor biochemical studies have substantiated this hypothesis, L. ochracea and the morphologically distinct Cladothrix dichotoma are now considered mere ecological variants of Sphaerotilus.