Ecophysiology of the Genus Shewanella
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Ecophysiology of the Genus Shewanella KENNETH H. NEALSON AND JAMES SCOTT
Introduction The genus Shewanella is comprised of more than 20 species inhabiting a wide range of environments including spoiled food, oil field wastes, redox interfaces in marine and freshwater, cold waters and sediments of the deep sea, and mesophilic ones all around the planet. Much of the recent interest in the shewanellae stems from their almost notorious abilities in the area of anaerobic respiration; these bacteria appear to be able to use nearly any electron acceptor more electronegative than sulfate, including oxygen. However, the genus is much more than just a group of respiratory specialists, as will be discussed here. As with many contemporary genera, the Shewanella have experienced a rocky road to their present status, as evidenced by the species S. putrefaciens. Originally isolated as an active agent in food spoilage (Derby and Hammer, 1932), this organism was first called “Achromobacter putrefaciens,” then Pseudomonas putrefaciens (Shewan et al., 1960), Alteromonas putrefaciens (Lee et al., 1977), and finally Shewanella putrefaciens (MacDonell and Colwell, 1985). One notable strain of S. putrefaciens called “MR-1” was isolated from Oneida Lake, New York, as a metal reducer, and after genome sequencing was begun, the epithet was changed to S. oneidensis on the basis of molecular data, including 16S rRNA sequence analyses, DNA gyrase sequence analyses, lipid analyses, and DNA/DNA hybridization (Venkateswaran et al., 1999). Several Shewanella species have been found in Antarctic Continental shelf sediments (Bozal et al., 2002) as well as in sea-ice microbial communities (adapted to grow at temperatures below 4°C) isolated from McMurdo, Antarctica (Brown and Bowman, 2001). Some of these (like S. livingstonensis; Bozal et al., 2002) are cultivated strains, while others (such as McMurdo.10) are as yet uncultivated and exemplify psychrophilic Antarctic strains identified only via sequence analysis of 16S rDNA (Fig. 1). Some of the psychrophilic strains are also tolerant of high
pressures, i.e., piezotolerant (Kato and Nogi, 2001). To date, only a few genera are known to have piezophilic members, and the regulation and growth under high pressure conditions of two notable species S. benthica (Kato et al., 1998) and S. violacea (Nogi et al., 1998; Nakasone et al., 1999) are under intensive study (Tamegai et al., 1998; Yamada et al., 2000). Finally, the carbon source of many Shewanella strains is quite restricted, mainly fermentation end products such as lactate, some amino acids, formate, and hydrogen. Members of this group thus live a syntrophic lifestyle, in rich environments, and in association with fermentative communities that supply them the needed nutrients. The placement of the various species within the genus Shewanella has largely been via the similarity of 16S rRNA sequences (Fig. 1), and while the organisms are indeed taxonomically linked by this criterion, they repr
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