1. Identifying Characteristics

• Strictly aerobic.
• Straight or slightly curved rods, but not helical.
• 0.5-1.0 um x 1.5-5.0 um in size.
• No resting stages or spores are known.
• Cells stain Gram negative.
• Nearly all species are motile.
• Many species are also denitrifiers.
• Chemoorganotrophic.
• No unusual structures are formed.
• Catalase positive.
• Oxidase positive or negative.
• The percent G+C ranges from 58 to 70%.
• Colonies are usually white to cream to yellow-pigmented.
• Many species produce extracellular fluorescent pigments.

The left image shows P. fluorescens under phase contrast microscopy (100X).

There are over 230 described species in the genus Pseudomonas. Approximately 50 species are divided into five major subgroups with the fluorescent group being the largest (roughly 24 species). Because there are so many strains within these subgroups that differ only in minor biochemical properties, these are further divided into biovars instead of an endless list of closely related species. For example, P. fluorescens contains 5 biovars.

The top-left image shows a typical gram-negative stain of a Psuedomonas culture and the bottom-left image shows P. fluorescens under electron microscopy (65,000X).

The studies on the taxonomy of this complicated genus groped their way in the dark while following the classical procedures developed for the description and identification of the organisms involved in sanitary bacteriolgy during the first decades of the twentieth century. This situation sharply changed with the proposal to introduce as the central criterion the similarities in the composition and sequences of macromolecules components of the ribosomal RNA. The new methodology clearly showed that the genus Pseudomonas, as classically defined, consisted in fact of a conglomerate of genera that could clearly be separated into five so-called rRNA homology groups. Moreover, the taxonomic studies suggested an approach that might proved useful in taxonomic studies of all other prokaryotic groups. A few decades after the proposal of the new genus Pseudomonas by Migula in 1894, the accumulation of species names assigned to the genus reached alarming proportions. At the present moment, the number of species in the current list has contracted more than tenfold. In fact, this approximated reduction may be even more dramatic if one considers that the present list contains many new names, i.e., relatively few names of the original list survived in the process. The new methodology and the inclusion of approaches based on the studies of conservative macromolecules other than rRNA components, constitutes an effective prescription that helped to reduce Pseudomonas nomenclatural hypertrophy to a manageable size.

3. Isolation and Ecology

Widely distributed in nature, some species are pathogenic for humans, animals, or plants. The pseudomonads are the largest group of Gram negative aerobic heterotrophic bacteria found in soil, several species have been described as aquatic, and many are associated with the rhizosphere of plants. They have been widely studied because of the amazing metabolic diversity within this genus. The pseudomonads have been heavily used in biological disease control and bioremediation. The literature on degradation of organic compounds probably contains more citations on Pseudomonas than any other bacterial genus.

The image at the top-left shows a typical Pseudomonas culture on tryptic soy agar, while the image at the bottom-left shows P. fluorescens on a selective medium that enhances pigmentation and fluorescence. The water soluble yellow pigment is clearly visible.