Rust-dissolving bugs may chow down with variety of utensils
THOMAS J. DICHRISTINA
To attach to surfaces, many The deletion mutant has an acShewanella oneidensis, an anaerobacteria have adapted serine cumulation of exopolysaccharides bic bacterium found in lake and proteases into adhesion molon its surface, suggesting that ocean sediments, has attracted ecules. Examples include BordaSO3800’s normal function is to interest from microbiologists and tella pertussis, which uses a somehow remove the exopolysacgeochemists for its ability to disserine protease to attach to hucharides. “Under an electron misolve and chemically transform man epithelial cells in the croscope, deletion mutant cells metal oxides. throat, and Treponema denticola, are covered with this gooey subThis versatile organism embeds stance, while metal-reducing wild-type cells enzymes in its are relatively outer memplain,” DiChrisbrane, allowing tina says. the enzymes to The results directly access suggest that the mineral surbacteria are still faces. Yet muable to chemitant bacteria cally transform deficient in adFe(III) oxide hering to iron Mutant S. oneidensis (right panel) have a deficiency in adhering to Fe(III) oxide surfaces. even if there is (Fe)(III) oxide some distance surfaces are still Some cells have an accumulation of polysaccharides, which appears in pink. between the able to reduce mineral surface and reducing enwhich sticks to the surfaces of Fe(III) at normal rates, Tom zymes, he says. human teeth via a serine DiChristina and colleagues at the Some possible mechanisms inprotease. Georgia Institute of Technology in clude secreting Fe-chelating molWorking with DiChristina, graduAtlanta report in Environ. Sci. ecules, thus bringing the Fe to the ate student Justin Burns identified Technol. 2009, DOI 10.1021/ bacteria by dissolving it, or proa protein (SO3800) loosely bound es9018699. ducing “electron shuttles,” comto the bacterial outer membrane The behavior of the mutant pounds like flavins or quinones whose sequence suggested that it bacteria suggests that S. oneidenthat bring bacterial electrons to was a serine protease. To test the sis may be using a variety of tacthe Fe. Evidence exists for these hypothesis that this protein might tics to bring metals and metalmechanisms in other bacteria, be an adhesion molecule, they proreducing enzymes together. including relatives such as Shduced bacteria carrying a deletion Researchers studying S. ewanella putrefaciens. for SO3800. Colleagues from Emory oneidensis say that it “breathes Alfred Spormann, who studies University and the University of metals.” This is because metals S. oneidensis at Stanford UniverGeorgia’s Savannah River Ecology can play the role of electron acsity, notes that the new data still Laboratory contributed to analysis ceptor in its metabolism in the leaves open whether the bulk of of this mutant. same way that oxygen (O2) does electron transfer from cell to minMutant bacteria retained only for the mitochondria inside anieral surface proceeds via direct 35% of their normal ability to mal cells. contact or indirectly via soluble adhere to hematite (Fe2O3), a S. oneidensis’ properties could metabolites. make it useful for bioremediation, form of Fe(III) oxide. In conHe says he found the SO3800 defending equipment against cortrast, the mutants could still mutant phenotype to be interestrosion, or even driving a micro“breathe” the Fe(III) (and other ing, adding that other mutants bial fuel cell. Knowing more metal salts and electron accepdeficient in exopolysaccharide about S. oneidensis’ metal-transtors) just as well. production in fact have reduced forming biochemistry could help Although SO3800 exhibits prometal uptake rates. He points out scientists enhance its capabilities tease activity in the lab, what it that the mutant studied in the as a cleanup (bio)technology. does for the cell is not clear: it paper is only partially deficient in “Thequestion we were trying to may help the bacteria bind Fe(III) adhesion, and that single cell answer was: do these bacteria oxide surfaces directly, or it may studies could reveal more. need to directly attach to the iron shape the surface of the bacteria Kenneth Nealson at University oxides to breathe on them?” to clear away obstacles to of Southern California, who disDiChristina says. binding. 10.1021/es9035545
2010 American Chemical Society
Published on Web 12/08/2009
January 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 9
covered S. oneidensis in the 1980s, says the paper makes a valuable point that S. oneidensis may have several mechanisms for reducing metal oxides, including reduction at a distance. However, he said he was disappointed that the mutant
bacteria were not tested for their ability to bind other forms of Fe oxides besides hematite. “This basically shows that attachment is not required for chemical action,” Nealson says. “But attachment is not the same
10 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / January 1, 2010
as contact. If the bacteria can get close enough to a particle of iron oxide, they might be able to act on it, even if they’re not attaching to it.” —QUINN EASTMAN
