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A team of microbiologists affiliated with the University of Massachusetts at Amherst (UMass) has uncovered the unusual survival strategies used by a common bacterium. The finding could have implications in cleaning up contaminants ranging from petroleum to uranium. Results of the National Science Foundation (NSF)-funded study by UMass microbiologist Derek Lovley and colleagues will be published in the April 18 issue of Nature. The research was also funded by the U.S. Department of Energy.
Scientists have long known that the bacteria species, Geobacter metallireducens, is commonly found in soil and consumes metals - specifically, iron and manganese oxides. The new findings detail the microorganisms' intriguing survival tactics. First, the species is apparently able to locate and home in on the metal that serves as its food source. "This is the first microorganism found to have a built-in sensor that allows it to essentially 'sniff out' metals," said Lovley. If a source of iron or manganese is not nearby, the bacterium - which was previously believed to be incapable of movement - can essentially decide to grow flagella, the whip-like structures that enable bacteria to swim.
"This work demonstrates again that basic research, in addition to answering the questions for which it was specifically designed, can also produce totally unexpected insights into the natural world," says Susan Porter Ridley, program director in NSF's division of molecular and cellular biosciences. "In this case, the sequence of bases in a microbe's DNA led to the discovery that the microbe can actually sense and locate chemicals it needs. This knowledge, in turn, shows great promise for helping us solve the problem of environmental pollution."
Scientists were already aware that some bacteria species, such as the well-studied E coli, are able to sense and swim toward sugars. But scientists had never seen Geobacter swim, leading Umass researchers to wonder how it is that metals serve as its energy source.
Clues to the puzzles were found as Geobacter's genome was sequenced in collaboration with The Institute for Genomic Research in Rockville, Maryland. The Geobacter's genetic code revealed a startling discrepancy: "We looked at the complete genetic code, and saw clear evidence of genes for flagella, so we realized this bacterium does indeed have the genetic potential to swim," said Lovley. "The question then was, 'Does this have anything to do with Geobacter's growth on metals?'"
Lovley realized that in previous studies the ability of Geobacter to swim had always been analyzed when it had been grown on soluble metals, which are often used in laboratory experiments because they are easy to work with. No one had looked carefully at growth on the metal oxides that Geobacter actually uses in natural environments. When the researchers looked at cultures grown on iron oxide, the cells had produced flagella and were swimming.
The genome also contained genes which suggested that Geobacter might be able to sense chemicals in the environment. To see if this was also related to growth on metals, the scientists set up a series of microscope slides on which the bacteria needed to travel to reach the metal necessary for their survival. It turned out that the bacteria were growing flagella and swimming to the metal source. "These bacteria really do grow flagella in order to search for, and establish contact with, the soluble iron or manganese oxides they need," paper co-author Susan Childers said. Under the microscope, the microbes are cigar-shaped, and one to two microns long; 10,000 of them would measure an inch.
Once the bacterium reaches the metal, it is able to grow the short, hair-like structures called pili, which allow the bacterium to anchor itself to the metal source, ensuring growth.
The finding represents more than just an intriguing look at how microbes survive and thrive: these microbes can be used to clean up petroleum spills, and they may offer an efficient and economic solution to removing uranium from contaminated groundwater. Previous efforts at flushing uranium from the soil involved pumping water out of an area and removing the soil - a process which has proven to be both expensive and inefficient, Lovley said.
"Geobacters don't actually remove the uranium," explained Lovley, "but they do transform the metal from a soluble form to an insoluble form, so that it is no longer able to leach into the groundwater and eventually contaminate rivers."