In the weird and wonderful world of bacteria, it sometimes seems best to expect the unexpected. Take their size, for instance. Most bacteria are only a few micrometers, or millionths of a meter, long. It might take several thousand of them laid end-to-end to span across a penny, and still be too thin to see with the naked eye.
But in the warm waters of the Red Sea, and off the coast of Australia, the largest bacteria ever seen have been discovered in the guts of a fish. Epulopiscium fischelsoni is a bacterium of mammoth proportions. It can sometimes grow as large five hundred micrometers, or as about the size of the period at the end of this sentence, which is a remarkable size for bacteria.
"Epulos" as they are more commonly called, have been found in the intestines of several kinds of surgeonfish. These surgeonfish live in and around coral reefs, feeding on the algae and plants that grow there. When researchers studying the fish looked into their guts, they found tens of thousands of these bacteria there.
Their discovery came as a surprise, and their size even more so. In fact, for several years after their discovery, they were believed to be protists - a completely unrelated group of microbes that are evolutionarily closer to humans. Research was slow because unlike some other bacteria, epulos could not be grown in the laboratory. Finally, the analysis of a portion of their DNA revealed that they are indeed bacteria, and not protists as previously thought.
However, the large size of the epulos was still unexpected. The small size of most bacteria is owed to their limited abilities. Since they have few ways to transport nutrients across their cell membranes, they rely on diffusion to move food into their cells, and wastes out of them. This process of diffusion is limited by the surface area of a cell, which is the space that a cell's surface would occupy if it were stretched out flat. As a cell gets bigger, both its volume and surface area increase, but its surface area increases more slowly than its volume. Above a certain size, there is not enough surface area to absorb all of the nutrients that the increasing cell volume needs. So, the limit of a bacterium's size is related to the proportion of its surface area to its volume.
One can imagine a bacterium as a paper bag. Each bag can hold a certain amount (its volume), while it is made up of only a certain quantity of paper (its surface area). While a large bag might hold the same amount as several small bags, if cut up and laid out flat, a large bag would take up much less paper than the many small bags. This is the same case with bacteria, and it is this balance of surface area and volume that keeps most bacteria so small.
So how do epulos get so big? Detailed microscopic analysis revealed the answer. When the bacteria were shown in cross-section, it was apparent that the cell membrane didn't stretch smoothly around the cell. Instead, it was convoluted and wrinkled, with many pockets and folds. These wrinkles and folds dramatically increase the surface area of the cell membrane. So, spread out, the cell membrane would occupy much more space than simply the surface of the cell, but the volume it contains is still small (but very large for a bacterium). This would be the same as wadding up a paper bag into a ball. It still has the same surface area if spread out, but now it has a much smaller volume. It is this increase in surface area, while maintaining a large volume, that keeps the biggest bacterium in the world alive.