PROTEOBACTERIAE

The Gammaproteobacteria is a large, diverse group that includes some of the most important microbial organisms (e.g. Escherichia, Enterobacter, Francisella, Pasteurella). By and large, all organisms in this phylum are unicellular, and most are rods. The phylum is defined by two major groups one photoautotrophic and the other heterotrophic. The purple sulfur bacteria are obligate anaerobes that utilize bacteriochlorophylls to capture light energy for photosynthetic pathways in which carbon dioxide is fixed into organic molecules. The electrons are provided by hydrogen sulfide rather than water as in the plants. Usually they occur in environments where the conditions of anoxia and light both occur. This can be seen in particular clear lakes with anoxic bottom layers. In such conditions, anaerobic photosynthetic bacteria can be very abundant.

The other great group tends to be heterotrophic and aerobic or facultatively anaerobic. It includes many taxa that are of major economic importance, particularly as disease organisms. Such organisms include Escherichia coli, Pasteurella pestis (Bubonic Plague), Francisella (Tuleremia), and Legionella (Legioner's Disease). Vibrio cholerae, the causative agent of Cholera, is a motile rod that occurs normally associated with microcrustaceans of the plankton in marine and brackish water environments. Thus, Cholera outbreaks typically follow plankton blooms. Other Vibrio and related taxa are not pathogenic, some are bioluminescent.

The methane oxidizers feed on methane and other simple carbon compounds that do not have carbon-carbon bonds. Such organisms occur in highly reduced environments on the ocean floor and as symbionts with mytelid clams and pogonophorans which live in association with geothermal vents.

Pseudomonads are motile rods, a combination of characters that is very common in the Proteobacteria, and caused many unrelated taxa to be grouped together in the former artificial classification system. Molecular methods have demonstrated that the taxa of a group now called Pseudomonadales do cluster in the Gammaproteobacteria. One of the most common species is Pseudomonas aeruginosa, normally a free-living organism. However, P. aeruginosa can be a pathogen of certain plants, and it has been found as an opportunistic pathogen in humans.

A. Purple sulfur bacteria collected from the top of an anoxic layer in the Great Salt lake

B. Thiocapsa, a micrograph of a purple sulfur bacterium.

C. Escherichia after Gram staining.

D. Abcesses in rodent lungs infected with opportunistic infection of Pasteurella.

E. TEM micrograph of Legionella.

F. A Hawaiian squid uses symbiotic bioluminescent Vibrio in its light organ.

G. TEM micrograph of Methylococcus, a methane-feeding bacterium.

D. TEM micrograph of Pseudomonas showing its flagella.

Images taken from:
A: http://aslo.org/photopost/data/502/0Purple-sulfur_bacteria_Great_Salt_Lake-med.JPG
B: http://biology.kenyon.edu/Microbial_Biorealm/bacteria/thiocapsa/thiocapsa.html
C: http://www2.mf.uni-lj.si/~mil/bakt2/jpeg/14.jpg
D: http://microbewiki.kenyon.edu/index.php/Pasteurella
E: http://upload.wikimedia.org/wikipedia/commons/7/7d/Legionella_pneumophila_01.jpg
F: http://web.uconn.edu/mcbstaff/graf/VfEs/VfEssym.htm
G: http://www.genomenewsnetwork.org/gnn_images/sequenced_genomes/methylococcus_capsulatus.jpg
H. http://www.biology.mcgill.ca/faculty/gonzalez/images/pseudomonas_sm.jpg

Stackebrandt et al. (1988), using 16S rRNA sequences, defined a seemingly unrelated group of eubacteria as Proteobacteria, the purple bacteria, which they defined as a class that they called Proteobacteria. Within that group, they defined five separate lines, each defined by a Greek letter: α, β, γ, δ, ε. The second edition of Bergey's Manual of Systematic Bacteriology (Garrity et al. 2003) adopted Proteobacteria, but raised it to phylum level with each of the five groups becoming classes. In order to bring the prokaryotes into line with kingdom-level divisions in the eukaryotes, I felt that it was necessary to raise the Proteobacteria to kingdom-level status with each of the five groups also raised to the level of phylum.

The Gammaproteobacteria has two major groups within it: the purple sulfur bacteria, and the others with the enterics at their core. I took this separation to be at the class-level (classes Chromatiae and Enterobacteriae). The ordinal structure is from Garrity et al. (2003).

SYSTEMATIC BIOLOGY					THE EUBACTERIA
HOME	SYLLABUS	WEEKLY ASSIGNMENTS	J. SYSTEMATIC BIOLOGY	TAXA OF LIFE
PHYLUM GAMMAPROTEOBACTERIA