Read more: 鈥�Not so simple: Bugs that break all the rules鈥�
Some species of bacteria grow big enough to be seen without a microscope
If you know nothing else about bacteria you鈥檒l know this: they are minuscule, far too small to be seen with the naked eye. There are good reasons for this. The complex cells of plants and animals have sophisticated internal transport systems for moving molecules around. By contrast, bacteria rely mainly on diffusion to move things around the cell. Since diffusion only works well over distances of up to a few millionths of a metre, bacteria cannot grow too big.
Advertisement
Yet a handful of species do grow very large. In fact, at least three are so large that they can be seen without a microscope. The first giant was discovered in 1985 but because of its size . This rod-shaped bacterium, Epulopiscium fishelsoni (pictured right), lives in the gut of surgeon fish in the Red Sea and is up to 0.7 millimetres long, hundreds of times longer than the E. coli in our guts, which is around 0.002 millimetres long.
E. fishelsoni busts the 鈥渕aximum鈥� size limit by having tens to hundreds of thousands of copies of its DNA. This means it can produce proteins in many different places in its cell, so the proteins need only diffuse a short distance to get to where they are needed. show the ratio of DNA to volume is actually the same as for a normal-sized bacterium.
E. fishelsoni held onto its title as the biggest bacterium until 1997, when Thiomargarita namibiensis came along (). T. namibiensis, which means 鈥渟ulphur pearl of Namibia鈥� for the shimmering sulphur granules inside it, is up to 0.75 millimetres across. It relies on the same trick as E. fishelsoni, with thousands of copies of its DNA. While it has a volume up to 100 times greater than E. fishelsoni, most of this space is used for storage: up to 98 per cent of the cell is taken up by an enormous membrane-bound sac, or vacuole. This contains up to three months supply of nitrate, which it uses to oxidise the hydrogen sulphide it feeds on. The store is needed because the supply of nitrate to its sea-floor home is patchy, depending on dead animal material falling from above.
An almost identical sulphur bacterium which, at 0.5 millimetres across, is also visible to the naked eye, was discovered in Mexico in 2002. There are probably more gargantuan bacteria out there waiting to be found.
Read previous article: 鈥�Not so simple: Bacteria with backbone鈥�
Read next article: 鈥�Not so simple: Bugs that hunt in packs鈥�