A Microbial Biorealm page on the genus Shewanella denitrificans
Higher order taxa
Bacteria, Proteobacteria, Gammaproteobacteria, Alteromonadales, Shewanellaceae
Description and significance
The Shewanella genus is a diverse group of marine gamma-proteobacteria, ranging from psychrophilic, to piezophilic, to psychrotolerant, to mesophillic (Kato et al.). Some species are noted for their pathogenicity, still others are studied for their diversity of electron transport systems. They are capable of anaerobic respiration using a large spectrum of electron acceptors in addition to aerobic respiration.
Shewanella denitrificans is noted primarily for its ability to vigorously denitrify nitrate and nitrite, converting these molecules to gaseous nitrogen. Although there are many other members of the genus that are denitrifiers, S. denitrificans is unique in its enthusiastic denitrification. It was first described in 2002 by Brettar et al., who aligned it with the genus and distinguished it as a novel species based on physiological and 16S rRNA comparisons, among others. Like the other organisms in its genus, S. denitrificans is a rod shaped, Gram-negative bacterium with polar flagella. It is found at the oxic-anoxic interface and was first isolated from the Gotland Deep, a basin in the Baltic Sea. It was first isolated as three strains, called OS217T, S220 and OS226, which were phylogenetically identified as a single species. S. denitrificans is a mesophilic organism with an optimum growth temperature of 20-25 C. Based on its fatty acid composition and genome sequence, S. denitrificans is most related to S. baltica, S. putrefaciens and S. frigidimarina within the Shewanella genus, but has a different growth optimum from these species. In addition, its enzymatic activities, electron acceptors, and utilization of substrates distinguish it from the others in its genus.
The genome for Shewanella denitrificans was completed on April 11, 2006 by the DOE Joint Genome Institute/Pacific Northwest National Laboratory. It is a circular genome that is 454906 bases long and code for 3905 genes. The genome has a GC content of 45% and can be found in more detail at NCBI
Cell structure and metabolism
The marine bacteria Shewanella denitrificans is rod shaped and approximately 0.5-0.8 micrometers wide by 2-3 micrometers long. It is Gram-negative and has flagella located at its poles (Brettar et al. 2002). It powers its flagella using both a proton and a sodium ion driven pump (Mitchell et al.).
Although this species is facultatively anaerobic, its denitrification capabilities are what make it important in the marine environment. Denitrification is an important part of the nitrogen cycle; it is the counterpart to nitrogen fixation (Zumft). Denitrification is found in a wide variety of bacteria as well as a few Archaea and fungi. It is a complicated process and requires a suite of proteins for it to occur. Shewanella denitrificans can reduce nitrate or nitrate and use them as terminal electron acceptors, in the process converting the nitrogen oxides to nitrogen gas. For S. denitrificans the process of denitrification occurs in the periplasm. Some Shewanella species have even been shown to be chemotactic towards a nitrite or nitrate source (Nealson et al.).
Members of the Shewanella genus are also remarkable for their ability to reduce a variety of metals. For bacteria, there is a hierarchy of electron acceptors, such that there are some acceptors that are preferred and utilized over others. However, it is possible for some bacteria, including Shewanella, to reduce metals, such as iron, manganese oxide (Stapleton et al.), chromium (Guha et al.), and magnetite (Kostka et al.). In addition to iron reduction, specific members of the Shewanella genus have been shown to be able to reduce humic substances as part of their energy harvesting strategies (Lovley et al.). This suggests the possibility of unusual and interesting electron transport systems because of the paucity of organisms that are capable of the reduction of insoluble metal oxides (NCBI). This ability lines up well with another well-studied metal reducer, Geobacter. The two genera are different, however, in their actual mechanism of iron (III) reduction. Instead of using an outer membrane cytochrome, Geobacter uses a pilus, a nanowire, to establish contact with the iron surface about to be reduced (Reguera et al.). Shewanella, however, has been shown to excrete quinones and quinone-like substances to act as "electron shuttles" in the process of iron reduction (Newman and Kolter), (Childers et al.).
The Shewanella genus includes members found at a variety of pressures and temperatures (Chilukuri et al.), but S. denitrificans is a midrange organism found at the oxic-anoxic interface in the marine environment. It is an important species in this region because most of the denitrification that occurs happens at this interface, and S. denitrificans contributes significantly to the total denitrification of the system (Brettar et al., 2001).
Itself, Shewanella denitrificans does not cause any known human pathogen. However, other members of its genus have pathogenic characteristics. Shewanella putrifaciens, is significant in the fisheries industry due to its pathogenicity of numerous fish (Taylor et al.). Shewanella alga, in an unusual case, even caused tonsilitis in a healthy child (Liu et al.). In an even more rare and isolated event, Shewanella septicemia caused the death of an older man with a damaged liver (Otsuka et al.). However, these incidences are very exceptional.
Application to Biotechnology
Shewanella denitrificans does not make any substances that are in current use for biotechnological applications.
Research into the Shewanella genus is rapidly expanding as organisms are found at a variety of environments, including the surfaces of sponges (Lee et al.)and deep-sea sediments (Miyazaki et al.); and as new characteristics such as being obligatory aerobic are discovered (Zhao et al.). The genus is also being examined from a taxonomic point of view; with the discovery of new species has come the proposal to define two sub-genera based on psychrotolerance and piezophily.
Brettar, I.; Moore, E.R.B.; Hofle, M.G. Phylogeny and Abundance of Novel Denitrifying Bacteria Isolated from the Water Column of the Central Baltic Sea. Microbial Ecology, VOL. 42, NO. 3, pp. 295-305; 2001 ISSN: 0095-3628 
Brettar, I.; Christen, R.; Hofle, M.G. Shewanella denitrificans sp. nov., a vigorously denitrifying bacterium isolated from the oxic--anoxic interface of the Gotland Deep in the central Baltic Sea. International Journal of Systematic and Evolutionary Microbiology [Int. J. Syst. Evol. Microbiol.]; vol. 52, no. 6, pp. 2211-2217; 2002 ISSN: 1466-5026 
Childers, S.E.; Ciufo, S.; Lovley, D.R. Geobacter metallireducens accesses insoluble Fe(III) oxide by chemotaxis. Nature, vol. 416, no. 6882, pp. 767-769; 2002 ISSN: 0028-0836 
Chilukuri, L.N; Bartlett, D Isolation and characterization of the gene encoding single-stranded-DNA-binding protein (SSB) from four marine Shewanella strains that differ in their temperature and pressure optima for growth. Scripps Institution of Oceanography Contributions; 67, B127; 1999 
Genome project: Shewanella denitrificans OS217 OS-217 project at DOE Joint Genome Institute 
Guha, H.; Jayachandran, K.; Maurrasse, F. Microbiological reduction of chromium(VI) in presence of pyrolusite-coated sand by Shewanella alga Simidu ATCC 55627 in laboratory column experiments. Chemosphere: chemistry, biology and toxicology as related to environmental problems, vol. 52, no. 1, pp. 175-183; 2003 ISSN: 0045-6535 
Kato, Chiaki; Nogi, Yuichi. Correlation between phylogenetic structure and function: examples from deep-sea Shewanella. F E M S Microbiology Ecology, VOL. 35, NO. 3, pp. 223-230; 2001 ISSN: 0168-6496 
Kostka, J.E.; Nealson, K.H. Dissolution and reduction of magnetite by bacteria. Environmental Science & Technology, VOL. 29, NO. 10, pp. 2535-2540; 1995 ISSN: 0013-936X 
Lee, On On; Stanley C. K. Lau, Mandy M. Y. Tsoi, Xiancui Li, Ioulia Plakhotnikova, Sergey Dobretsov, Madeline C. S. Wu, Po-Keung Wong, Markus Weinbauer and Pei-Yuan Qian. Shewanella irciniae sp. nov., a novel member of the family Shewanellaceae, isolated from the marine sponge Ircinia dendroides in the Bay of Villefranche, Mediterranean Sea. International Journal of Systematic and Evolutionary Microbiology (2006), 56, 2871–2877 
Liu, Min-Chang; Gau, Shiow-Jen; Wu, Hsien-Cheng. Acute exudative tonsillitis caused by Shewanella algae in a healthy child. Scandinavian Journal of Infectious Diseases. VOL. 38; NO. 11-12, pp. 1104-1105, November 2006
Lovley Derek R., Jocelyn L Fraga, John D. Coates, Elizabeth L. Blunt-Harris (1999) Humics as an electron donor for anaerobic respiration Environmental Microbiology 1 (1), 89–98 
Miyazaki, Masayuki; Yuichi Nogi, Ron Usami, and Koki Horikoshi. Shewanella surugensis sp. nov., Shewanella kaireitica sp. nov. and Shewanella abyssi sp. nov., isolated from deep-sea sediments of Suruga Bay, Japan. Int J Syst Evol Microbiol, Jul 2006; 56: 1607 - 161
Nealson, KH, Moser, DP, Saffarini, DA. Anaerobic Electron Acceptor Chemotaxis in Shewanella putrefaciens Appl. Environ. Microbiol. 1995 61: 1551-1554 
Newman Dianne K and Roberto Kolter. A role for excreted quinones in extracellular electron transfer. Nature 405, 94-97. 4 May 2000 
Otsuka, Taiga; Noda, Takahiro; Noguchi, Akinori; Nakamura, Haruki; Ibaraki, Kazuo; Yamaoka, Kotaro. Shewanella infection in decompensated liver disease: a septic case. Journal of Gastroenterology. VOL. 42, NO. 1, pp. 87-90; 2007 
Mitchell, J.G.; Barbara, G.M. High speed marine bacteria use sodium-ion and proton driven motors. Aquatic Microbial Ecology, VOL. 18, NO. 3, pp. 227-233; 1999 ISSN: 0948-3055 
Reguera, Gemma; Kevin D. McCarthy, Teena Mehta, Julie S. Nicoll, Mark T. Tuominen and Derek R. Lovley. Extracellular electron transfer via microbial nanowires. Nature 435, 1098-1101. 23 June 2005 
Stapleton, R.D., Jr.; Sabree, Z.L.; Palumbo, A.V.; Moyer, C.L.; Devol, A.H.; Roh, Y.; Zhou, J. Metal reduction at cold temperatures by Shewanella isolates from various marine environments. Aquatic Microbial Ecology, VOL. 38, NO. 1, pp. 81-91; 2005 ISSN: 0948-3055
Taylor, P.W. Multiple Antimicrobial Resistance in a Chronic Bacterial Infection of Koi Carp. North American Journal of Aquaculture; N. Am. J. Aquacult.; North American Journal of Aquaculture [N. Am. J. Aquacult.]; vol. 65, no. 2, pp. 120-125; 2003 ISSN: 1522-2055 
Zhao, Jian-Shen; Dominic Manno, Cindy Leggiadro, David O'Neil, and Jalal Hawari Shewanella halifaxensis sp. nov., a novel obligately respiratory and denitrifying psychrophile Int J Syst Evol Microbiol, Jan 2006; 56: 205 - 212. 
Zumft, W.G. (1997): Cell biology and molecular basis of denitrification. In: Microbiol. Mol. Biol. Rev. Bd. 61, Nr. 4, S. 533-616. PMID 9409151 
Edited by Karen Rossmassler, student of Rachel Larsen and Kit Pogliano