Researchers say they've succeeded in obtaining microscope views of a bacterial form of life that's just about as tiny as life can be on Earth.

Researchers at the Lawrence Berkeley National Laboratory of the U.S. Department of Energy, and the University of California, Berkeley, say their work has confirmed that such ultra-small forms of bacteria exist, something that has been the subject of debate for almost 20 years.

Several kinds of bacteria -- from three microbial phyla that are poorly understood -- were studied by electron microscopy.

Found in groundwater and thought to be quite common, they surprised the researchers by being close to and in some cases smaller than what many scientists have long considered the lower size limit of life, they report in the journal Nature Communications.

The cells had an average volume 0.009 cubic microns, meaning 150 of the bacteria would fit inside a singe cell of Escherichia coli.

In excess of 150,000 of them could fit on the end tip of a human hair, the researchers said.

They are now believed to be as small as a cell can get and still possess sufficient internal material needed to sustain life, they said.

Under the microscope, the bacteria displayed densely packed spiral shapes thought most likely to be DNA, a extremely small number of ribosomes, and a minimalized metabolism that probably makes them dependent on other bacteria for many of their living needs, the study authors say.

"These newly described ultra-small bacteria are an example of a subset of the microbial life on earth that we know almost nothing about," says Jill Banfield of the Berkeley Lab who is also a UC Berkeley professor of Earth and Planetary Science.

"They're enigmatic. These bacteria are detected in many environments and they probably play important roles in microbial communities and ecosystems," she says.

However, she acknowledges, it is not yet understood exactly what these ultra-small bacteria do.

To gather these ultra-small cells for study, the researches passed groundwater through an ever-smaller series of filters, finally arriving at a filter sized at 0.2 microns, the same size used to sterilize water.

The resulting samples were anything but sterile, they found, instead being full of amazingly tiny microbes, so tiny there was some question of whether they could actually be alive.

"There isn't a consensus over how small a free-living organism can be, and what the space optimization strategies may be for a cell at the lower size limit for life," says study co-author Birgit Luef, a former Berkeley postdoctoral researcher now at the Norwegian University of Science and Technology.

Their research represents a significant first step in characterizing the size and internal structure of such ultra-small cells, she says.