WASHINGTON (Reuters) - The largest known bacteria, discovered in shallow mangrove swamps in the Caribbean and large enough to be seen with the naked eye, is redefining what properties are possible for bacteria, the oldest life form on Earth.

Scientists on Thursday said the bacterium, called Thiomargarita magnifica, is notable not only for its size - the gigantic single-celled organism that can reach nearly two centimeters in length - but also because its internal structure differs from that of any other bacteria.

The DNA does not move as freely within the cell of this species as in most bacteria, but is contained within many small sacs attached to the cell membrane. The structures attached to the cell membrane in cells are called organelles.

"They are thousands of times larger than normal-sized bacteria. Discovering these bacteria is like facing a human being the length of Mount Everest," said marine biologist Jean-Marie Voland of the US Department of Energy's Joint Genome Institute and Research Laboratory in California Complex Systems.

The bacteria were found in several locations in Guadeloupe, the French archipelago in the Caribbean. It was first detected in sulfur-rich seawater in the Guadeloupe swamp, a microbiologist at the University of the Antilles and co-lead author of the study, Olivier Gros.

"In 2009, I found long white threads attached to a sunken mangrove leaf. I found these threads very interesting. I took them back to the lab for analysis... and it was a big surprise to me to find a huge bacteria living in the mangroves of Guadeloupe," Gross said.

Normal bacteria are from one to five micrometers in length. The average length of Thiomargarita magnifica is ten thousand micrometers (one centimeter), with some species reaching twice that length.

"It's much larger than we thought was the maximum possible size for a single bacterium," Foland said. "It's roughly the size and shape of an eyelash."

The maximum length of the bacteria known so far was about 750 micrometers.