Scientists are discovering the biggest bacteria ever seen

In a Caribbean mangrove forest, scientists have discovered a bacterial species that grows to the size and shape of a human eyelash.

These cells are the largest bacteria ever observed, thousands of times larger than more well-known bacteria such as Escherichia coli. “It would be like meeting another human the size of Mount Everest,” said Jean-Marie Volland, a microbiologist at the Joint Genome Institute in Berkeley, California.

Dr. Volland and his colleagues published their study of the bacteria, called Thiomargarita magnifica, on Thursday in the journal Science.

Researchers once thought that bacteria were too simple to produce large cells. But Thiomargarita magnifica turns out to be remarkably complex. With most of the bacterial world not yet explored, it is entirely possible that even larger, even more complex bacteria are waiting to be discovered.

It is about 350 years ago that the Dutch lens grinder Antonie van Leeuwenhoek discovered bacteria by scraping his teeth. When he put the tooth plaque under a primitive microscope, he was surprised to see single-celled organisms swimming around. Over the next three centuries, scientists discovered many more kinds of bacteria, all of which were invisible to the naked eye. For example, an E. coli cell measures about two microns or less than ten thousandths of an inch.

Each bacterial cell is its own organism, which means that it can grow and divide into a few new bacteria. But bacterial cells often live together. Van Leeuwenhoek’s teeth were coated with a gel-like film containing billions of bacteria. In lakes and rivers, some bacterial cells stick together to form small filaments.

We humans are multicellular organisms, our bodies are made up of about 30 trillion cells. While our cells are also invisible to the naked eye, they are typically much larger than bacteria. A human egg cell can reach about 120 micrometers in diameter, or five thousandths of an inch.

Cells of other species can grow even larger: The green alga Caulerpa taxifolia produces leaf-shaped cells that can grow a foot long.

As the gap between small and large cells emerged, scientists looked at evolution to understand it. Animals, plants and fungi all belong to the same evolutionary genus, called eukaryotes. Eukaryotes share many adaptations that help them build large cells. Researchers reasoned that without these adaptations, bacterial cells would have to remain small.

To start, a large cell needs physical support so that it does not collapse or tear apart. Eukaryotic cells contain rigid molecular wires that act as rods in a tent. However, bacteria do not have this cell skeleton.

A large cell also faces a chemical challenge: as its volume increases, it takes longer for molecules to drift around and meet the right partners to perform precise chemical reactions.

Eukaryotes have developed a solution to this problem by filling cells with small spaces where various forms of biochemistry can take place. They keep their DNA rolled up in a sac called the nucleus along with molecules that can read genes to make proteins, or the proteins produce new copies of DNA as a cell multiplies. Each cell generates fuel inside bags called mitochondria.

Bacteria do not have the spaces found in eukaryotic cells. Without a nucleus, each bacterium typically carries a loop of DNA that flows freely around its interior. They also do not have mitochondria. Instead, they typically generate fuel with molecules embedded in their membranes. This arrangement works well for small cells. However, as the volume of a cell increases, there is not enough space on the cell surface for enough fuel-generating molecules.

The simplicity of bacteria seemed to explain why they were so small: They just did not have the complexity that was crucial to becoming large.

However, this conclusion was drawn too quickly, according to Shailesh Date, the founder of the Laboratory for Research in Complex Systems in Menlo Park, California, and a co-author with Dr. Volland. Researchers made extensive generalizations about bacteria after studying just a small part of the bacterial world.

“We’ve just scratched the surface, but we’ve been very dogmatic,” he said.

That dogma began to crack in the 1990s. Microbiologists found that some bacteria have independently developed their own space. They also discovered species that were visible to the naked eye. Epulopiscium fishelsoni, for example, came to light in 1993. The bacterium that lives inside surgical fish grows to be 600 microns long – larger than a grain of salt.

Olivier Gros, a biologist at the University of the Antilles, discovered Thiomargarita magnifica in 2009 while studying the mangrove forests of Guadeloupe, a cluster of Caribbean islands that are part of France. The microbe resembled miniature pieces of white spaghetti forming a layer on dead tree leaves floating in the water.

First, Dr. Do not grasp what he had found. He thought the spaghetti could be mushrooms, small mushrooms or another eukaryote. But when he and his colleagues took DNA from samples in the laboratory, it revealed that they were bacteria.

Dr. Gros went with Dr. Volland and other scientists to take a closer look at the strange organisms. They wondered if the bacteria were microscopic cells put together in chains.

That turned out not to be the case. When the researchers looked into the bacterial noodles with electron microscopes, they realized that each one was its own giant cell. The average cell measured about 9,000 microns long, and the largest was 20,000 microns – long enough to span the diameter of a penny.

Studies of Thiomargarita magnifica have been moving slowly because Dr. Vallant and his colleagues have not yet figured out how to grow the bacteria in their laboratory. For now, Dr. Gros assemble a fresh supply of the bacteria each time the team will run a new experiment. He can find them not just on leaves, but oyster shells and plastic bottles sitting on the sulfur-rich sediments of the mangrove forest. But the bacteria seem to follow an unpredictable life cycle.

“In the last two months, I can not find them,” said Dr. Gros. “I do not know where they are.”

Inside the cells of the Thiomargarita magnifica, scientists have discovered a bizarre, complicated structure. Their membranes have many different kinds of spaces embedded in them. These spaces are unlike those in our own cells, but they can allow Thiomargarita magnifica to grow to enormous sizes.

Some of the rooms appear to be fuel-producing factories where the microbe can drain the energy into nitrates and other chemicals it consumes in the mangrove.

Thiomargarita magnifica also has other spaces that resemble human kernels remarkably. Each of the compartments, which the researchers call pepines after the small seeds in fruits such as kiwis, contains a loop of DNA. While a typical bacterial cell has only one loop of DNA, Thiomargarita magnifica has hundreds of thousands of them, each tucked inside its own pepin.

Even more remarkably, each pepin contains factories for building proteins from its DNA. “They have essentially small cells in the cells,” said Petra Levin, a microbiologist at Washington University in St. Louis. Louis, who was not involved in the investigation.

Thiomargarita magnifica’s huge supply of DNA can make it create the extra proteins it needs. Each pepin can make a special set of proteins needed in its own region of the bacterium.

Dr. Volland and his colleagues hope that after they start cultivating the bacteria, they will be able to confirm these hypotheses. They will also tackle other mysteries, such as how the bacterium manages to be so tough without a molecular skeleton.

“You can take a single filament out of the water with tweezers and put it in another tub,” said Dr. Volland. “How it’s connected and how it’s taken shape – these are questions we have not answered.”

Dr. Date said there may be more giant bacteria waiting to be found, perhaps even larger than Thiomargarita magnifica.

“How big they can get, we don’t really know,” he said. “But now this bacterium has shown us the way.”

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