Domain: Bacteria, Phylum: Proteobacteria, Class: Deltaproteobacteria, Order: Desulfuromonadales, Family: Geobacteraceae,
Geothermobacter ehrlichii gen.nov.,sp.nov.
Description and Significance
Geothermobacter ehrlichii is a Gram negative bacterium. It is shaped like a rod and is 1.2 to 1.5 µm in length and 0.5 µm in diameter. It does not form spores. This microorganism expresses a flagellum up to 6 µm long allowing it to be very motile.
It also has pili all along its outer surface which are thought to aid it in biofilm interactions . Geobacteracae are usually mesophiles, but strain SS015 is a thermophile. This ability to live at higher temperatures is incorporated into its nomenclature. Geothermobacter literally means “a rod from hot earth” which seems to perfectly describe this microorganism for its capacity to live in temperatures up to 65°C. Geothermobacter ehrlichii was isolated from a hydrothermal chimney named Bag City. It is a strict anaerobe and its main metabolic process is reduction of Fe(III) and decomposition of organic material.
The complete genome sequence of this novel bacterium is not yet available so very little is known about the size and content of the genome. Most of what is known about the DNA sequence of this bacterium is from isolated strain SS015 . It has a 62.6% G+C content and phylogenetic analysis of its 16S rRNA has shown that it is most similar to other members of the family Geobacteraceae. However, the 16S rDNA sequence of this strain is less than 94% similar to sequences of other members of Geobacteraceae. Therefore Geothermobacter ehrlichii represents a new genus in this family. Other interesting facts include resistance to antibiotics like trimethoprim and tetracycline.
Cell Structure, Metabolism and Life Cycle
Geothermobacter ehrlichii is a rod-shaped gram negative bacterium. It can be cultured in its pure form from a fluid sample by use of serial dilution. This method specifically cultures and identifies strain SS015 for further analysis of cell structure and study of its metabolism. Cell structure of Geothermobacter ehrlichii was discovered to be rod-shaped existing in either single cell form or in chains.
These cells have morphologic features when grown under conditions that deviate from normal optima. When grown under the presence of antibiotics, Extracellular Polymeric Substances (EPS) can be observed, predominantly a characteristic of biofilms. This matrix provides protection for the thin peptidoglycan membrane for the maintenance of the cell’s structure.
Geothermobacter ehrlichii contains c-type cytochromes that are membrane bound hemoproteins containing heme group c responsible for electron transport. The c heme group surrounds a metal ion which allows for the intraconversion between Fe(II) and Fe(III) oxidation states, and the metabolic potential increases when Fe(III) is present as an electron acceptor in a hydrothermal environment. Because metabolic potential increases when Fe(III) is used as a terminal electron acceptor, Geothermobacter ehrlichii has the ability to locate Fe(III) oxides through the use of chemotaxis to Fe(II). Geothermobacter ehrlichii uses malate and other organic acids as electron donors in the environment.
Not much is known about the life cycle of Geothermobacter ehrlichii. Like most other members of the Geobacteraceae family it is thought to form biofilms around iron clusters which will be reduced. Pili are essential in the formation of these biofilms. Thanks to its flagellum, Geobacter ehrlichii is also known to be very motile helping it locate electron donors and acceptors around the hydrothermal vents.
Ecology and Pathogenesis
These bacteria play an important role in the ecosystem because they are involved in the decomposition of both natural and contaminant organic compounds present in marine and freshwater. Fe(III) reducing microorganisms such as Geothermobacter ehrlichii can fully oxidize organic electron donors such as acetate. Acetate is a key intermediate that is necessary to complete the process of anaerobic organic-matter degradation in the environment. Its habitat is that of marine hydrothermal vents ranging in temperatures 35-65 degrees Celsius with pH ranging from 5-8 with a pH of 6 being optimum. Under environmentally stress conditions Geothermobacter ehrlichii produced large amounts of extracellular polysaccharide. This was evident in the “Bag City” hydrothermal vents where copious amounts of the polysaccharide was found on the surface of the vent.
The gram negative bacteria cell wall consists of a thin peptidoglycan layer. The outer membrane consists of lipopolysaccharides (LPS) and other polysaccharides. Although this species of bacteria does not come into contact with human under normal environmental conditions, it could however activate an innate immune response of the body due to cytokines in the body reacting to the LPS surface of the bacteria. This type of pathogenesis would be the only known form of toxicity to its host due to the inflammatory response of an organism’s immune complex to the endotoxin. Unlike other members of Geobacteraceae, strain SS015 is able to produce the EPS matrix and have antibiotic resistance.
Unique physiological capabilities of Geothermobacter ehrlichii enables it to survive at higher temperatures and use different sources as electron donors; it gives this bacterium important ecological significance of decomposition of contaminant organic matter. Further research and laboratory work such as culturing without Fe(III) as the electron acceptor under anaerobic conditions, is still underway to discover the ecological tolerance capabilities of Geothermobacter ehrlichii in the environment.
(1) Kashefi, Kazem, Holmes, Dawn E., Baross, John A., Lovley, Derek R. "Thermophily in the Geobacteraceae: Geothermobacter ehrlichii gen. nov., sp. nov., a Novel Thermophilic Member of the Geobacteraceae from the "Bag City" Hydrothermal Vent". Applied Environmental Microbiology. 2003. Volume 69. p. 2985-2993.
(2) Kashefi, Kazem, Shelobolina, Evgenya S., Elliott, W. Crawford, Lovley, Derek R. "Growth of Thermophilic and Hyperthermophilic Fe(III)-Reducing Microorganisms on a Ferruginous Smectite as the Sole Electron Acceptor". Applied Environmental Microbiology. 2008. Volume 74. p. 251-258.
Page authored by Rhonda Sohocki and Carlos Salgado, student of Prof. Jay Lennon at Michigan State University.