Scientists in a mangrove forest in the Caribbean have discovered a type of bacteria that grows to the size and shape of a human eyelash.
These cells are the largest bacteria ever observed, thousands of times larger than known bacteria such as Escherichia coli. “It would be like meeting another human the size of Mount Everest,” said Jean-Marie Foland, a microbiologist at the Joint Genome Institute in Berkeley, California.
Dr.. Voland and colleagues published Their study of a bacterium called Thiomargarita magnifica is published Thursday in the journal Science.
Scientists once believed that bacteria were too simple to produce large cells. But Thiomargarita magnifica turns out to be remarkably complex. Since most of the bacterial world has yet to be explored, it is entirely possible that even larger and more complex bacteria are waiting to be discovered.
It’s been about 350 years since Dutch lens grinder Anthony van Leeuwenhoek discovered the bacteria by scraping his teeth. When he placed dental plaque under a primitive microscope, he was astonished to see single-celled organisms swimming around it. Over the next three centuries, scientists found many other types of bacteria, all of which were invisible to the naked eye. Escherichia coli cell, for example, measures about micronor less than ten thousandths of an inch.
Each bacterial cell is its own organism, which means that it can grow and divide into a pair of new bacteria. But bacteria cells often live together. Van Leeuwenhoek’s teeth are coated with a jelly-like film containing billions of bacteria. In lakes and rivers, some bacteria cells stick together to be very small strings.
We humans are multicellular creatures, our bodies are made up of about 30 trillion cells. While our cells are not visible to the naked eye, they are usually much larger than those found in bacteria. The human egg cell can reach 120 micron in diameter, or five thousandths of an inch.
Cells of other species can grow larger: the green algae Caulerpa taxifolia produces blade-shaped cells that can grow to foot length.
As the chasm between small and large cells emerged, scientists looked to evolution to understand it. All animals, plants and fungi belong to the same evolutionary lineage, which are called eukaryotes. Eukaryotes share many adaptations that help them build large cells. The scientists concluded that without these adaptations, bacterial cells would have to remain small.
To start, a large hive needs physical support so that it does not collapse or rupture. Eukaryotic cells contain rigid molecular wires that act like poles in a tent. However, bacteria do not possess this cytoskeleton.
The large cell also faces a chemical challenge: As it gets larger, the molecules take longer to get around and meet the right partners to carry out delicate chemical reactions.
Eukaryotes have developed a solution to this problem by filling cells with tiny fragments where distinct forms of biochemistry can occur. They keep the DNA wrapped in a sac called the nucleus, along with molecules that can read genes to make proteins, or proteins produce new copies of DNA when the cell reproduces. Each cell generates fuel inside sacs called mitochondria.
Bacteria do not have the parts found in eukaryotic cells. Without a nucleus, each bacterium usually carries a ring of DNA that floats freely around its interior. They also do not have mitochondria. Instead, they typically generate fuels with particles embedded in their membranes. This arrangement works well with small cells. But as cell size increases, there is not enough space on the cell surface for fuel generating molecules.
The simplicity of bacteria seems to explain why they are so small: they didn’t have the basic complexity of getting bigger.
However, that conclusion was reached hastily, according to Shalish Dett, founder of the Laboratory for Research in Complex Systems in Menlo Park, Calif., and a co-author with Dr. Volland. Scientists have made sweeping generalizations about bacteria after studying a small part of the bacterial world.
“We just scratched the surface,” he said, “but we were very dogmatic.”
That orthodoxy began to crack in the 1990s. Microbiologists have found that some bacteria have independently developed their own compartments. They also discovered species that were visible to the naked eye. Epulopiscium fishelsonifor example, appeared in 1993. When living inside a surgeonfish, bacteria grow 600 microns in length – larger than a grain of salt.
Thiomargarita magnifica was discovered by Olivier Gros, a biologist at the University of the Antilles in 2009 while surveying mangrove forests in Guadeloupe, a group of Caribbean islands that are part of France. The microbe looked like little bits of white spaghetti, forming a layer on the dead foliage that floated in the water.
At first, Dr. Gross didn’t know what he had found. It was thought that spaghetti might be a fungus, a small sponge, or some other eukaryote. But when he and his colleagues extracted DNA from samples in the lab, they discovered it was bacteria.
Dr. Gross has joined forces with Dr. Voland and other scientists to research more closely the alien creatures. They wondered if the bacteria were microscopic cells stuck together in chains.
It turns out that this is not the case. When researchers peered inside the bacterial pasta using electron microscopes, they realized that each one was its own giant cell. The average cell is about 9,000 microns in length, and the largest is 20,000 microns – long enough to span a penny in diameter.
Studies of Thiomargarita magnifica have moved slowly because Dr. Valante and his colleagues have yet to figure out how to grow the bacteria in their lab. Currently, Dr. Gross has to collect a fresh supply of bacteria every time the team wants to run a new experiment. He can find it not only on leaves, but on oyster shells and plastic bottles found on sulfur-rich sediments in the mangrove forest. But the bacteria seem to follow an unexpected life cycle.
“In the past two months, I haven’t found them,” Dr. Gross said. “I don’t know where they are.”
Inside the cells of Thiomargarita magnifica, researchers have discovered a strange and complex structure. Their membranes have different types of compartments built into them. These compartments are different from those in our cells, but they may allow Thiomargarita magnifica to grow to huge sizes.
Some of the chambers appear to be fuel plants, where the microbe can harness the energy in the nitrates and other chemicals it consumes in the mangrove forests.
Thiomargarita magnifica also contains other compartments that look remarkably like human nuclei. Each compartment, which scientists named a pepin after the tiny seeds in a fruit like a kiwi, contains a ring of DNA. While a typical bacterial cell contains only one loop of DNA, Thiomargarita magnifica has hundreds of thousands of them, each tucked inside its own pipette.
And most importantly, each Pepin contains factories for building proteins from its DNA. “They have basically small cells inside the cells,” said Petra Levine, a microbiologist at Washington University in St. Louis, who was not involved in the study.
Thiomargarita magnifica’s huge supply of DNA may allow it to make the additional proteins it needs. Each Pepin may make special groups of proteins needed in its own region of bacteria.
Dr. Voland and his colleagues hope that after they begin culturing the bacteria, they will be able to confirm these hypotheses. They will also tackle other mysteries, such as how bacteria can be so tough without a molecular skeleton.
“You can take one strand of water out with tweezers and put it in another bowl,” Dr. Foland said. “How it holds together and how it takes shape – these are questions we haven’t answered.”
Dr. Deet said there may be more giant bacteria waiting to be found, perhaps even greater than Thiomargarita magnifica.
“How much they can reach, we don’t really know,” he said. “But now, these bacteria have shown us the way.”
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