Scientists have long known that the bacteria species in question, geobacter metallireducens, is commonly found in soil and consumes metal: 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. And 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.
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 found that metals that 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. 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 and Childers realized that in previous studies the ability of beobacter 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 Childers looked carefully at cultures grown on iron oxide, the cells had produced flagella and were swimming.
“This research is a wonderful demonstration of the power of genetic sequence information to predict important physiological and ecological attributes of a microorganism,” said Ari Patrinos, head of the Energy Department’s Office of Biological and Environmental Research which supported the sequencing of Geobacter.
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, Childers set up a series of microscope slides on which the bacteria needed to travel to reach the metal necessary for their survival. When she looked in the microscope, 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,” 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. “These are incredibly energy-efficient strategies, when you think about it,” said Lovley. “The geobacter doesn’t waste energy growing flagella or pili unless it genuinely needs them. But if it’s not located near metal, it somehow senses that it better get up and start moving, and the gene that governs the growth of flagella comes into play.”
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 perhaps more important, they may offer an efficient and economic solution in 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.”
Several other research projects directed by Lovley have attracted attention within recent months. One study, published in Science, found that some microbes can transform organic matter found at the bottom of the ocean to electricity. Another project, detailed in Nature, revealed an unusual community of microorganisms that offered clues about how life could survive on Mars.
Note: Derek Lovley can be reached at 413/545-9651 or dlovley@microbio.umass.edu Susan Childers can be reached at 413/545-1048 or childers@microbio.umass.edu Photos related to this research can be found at www.geobacter.org
Journal
Nature