Scientists Discover the Largest Bacteria Ever Seen

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

These cells are the largest bacteria ever observed, thousands of times bigger than more familiar 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, Calif.

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

Scientists once thought bacteria were too simple to produce big cells. But Thiomargarita magnifica turns out to be remarkably complex. With most of the bacterial world yet to be explored, it is entirely possible that even bigger, even more complex bacteria are waiting to be discovered.

It has been about 350 years since the Dutch lens grinder Antonie van Leeuwenhoek discovered bacteria by scraping his teeth. When he put the dental plaque under a primitive microscope, he was astonished to see single-celled organisms swimming about. For the next three centuries, scientists found many more kinds of bacteria, all of which were invisible to the naked eye. An E. coli cell, for example, measures about two microns, or under a ten-thousandth of an inch.

Each bacterial cell is its own organism, meaning that it can grow and split into a pair of new bacteria. But bacterial cells often live together. Van Leeuwenhoek’s teeth were coated with a jellylike film containing billions of bacteria. In lakes and rivers, some bacterial cells stick together to form tiny filaments.

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

Other species’ cells can grow even bigger: The green algae Caulerpa taxifolia produces blade-shaped cells that can grow to a foot long.

As the gulf between small and big cells emerged, scientists looked to evolution to make sense of it. Animals, plants and fungi all belong to the same evolutionary lineage, called eukaryotes. Eukaryotes share many adaptations that help them build big cells. Scientists reasoned that without these adaptations, bacterial cells had to stay small.

To start, a big cell needs physical support so that it does not collapse or tear apart. Eukaryote cells contain stiff molecular wires that function like poles in a tent. Bacteria, though, do not have this cellular skeleton.

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

Eukaryotes have evolved a solution for this problem by filling cells with tiny compartments where distinct forms of biochemistry can take place. They keep their DNA coiled 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 when a cell reproduces. Each cell generates fuel inside pouches called…