Generally when we think about the cells of any given organism we typically think of them as so very tiny and completely invisible to the unaided naked aid. The smallest object that the human eye can see is about 0.1 mm long. Of course, permitting that the individual has perfect vision. Under optimal visual conditions, this means you might see an ameoba proteus, a paramecium and a human egg cell (shown below from left to right).
Most bacteria are too small to see by the unaided human eye. The commonly used model organism for bacterial studies, Escherichia coli is an “average” sized bacterium; only about 2 micrometers (μm) long and just 0.5 μm in diameter.
In 1993, the largest known bacteria Epulopiscium fishelsoni was discovered living in the intestinal tract of a brown surgeon fish living in the Red Sea. Due to their large size ranging from 600 μm by 80 μm, Epulopiscium fishelsoni was originally thought to be a eukaryotic protist. However, after closer inspection of their cellular morphology in the electron microscope Epulopiscium fishelsoni looked more like that of a bacterial cell than a eukaryotic cell. After analysis of the bacterium’s small subunit ribosomal RNA sequences it was discovered that Epulopiscium fishelsoni was actually a member of the low-(GC) Gram-positive bacterial group (EstherNature 362, 239 – 241). See the relative comparison in size and length of Epulopiscium, E. coli and a Paramaecium (see below).
We now know that the physical biomass of bacteria varies significantly. Reaching over more than 10 orders of magnitude ranging from the 0.2 μm wide nanobacteria to the largest and widest bacteria, Thiomargarita namibiensiss (which means “sulfur pearl of Namibia”) (see below) with a diameter measuring a whopping 750 μm (Heide N. Schulz and Bo Barker Jørgensen. Big Bacteria. Annual Review of Microbiology. (2001) Vol. 55). Though Epulopiscium fishelsoni still holds the record for being the longest massive bacterium, it’s much narrower than T. namibiensis.
Every bacterial species on the planet, even those who can swim, acquire their food through molecular diffusion. Because of diffusion limitations, bacteria which are small, in μm range, end up having a selective advantage. In addition, the limitations of diffusion put an upper limit on how big a bacterial (or prokaryotic) cells can become. Interestingly, Thiomargarita namibiensis, a colorless sulfur bacteria, which is capable of oxidizing hydrogen sulfide to sulfate with oxygen or nitrate, has the unique ability of storing large amounts of elemental sulfur and nitrate in their cells making it the largest bacterial on earth (as well as the widest).
Thiomargarita namibiensis (above) is classified as a chemolithotrophic meaning that it uses inorganic reduced compounds as a source of energy.Thiomargarita namibiensis uses nitrate as an electron acceptor in the electron transport chain; successfully oxidizing hydrogen sulfide (H2S) into elemental sulfur (S). When the sulfur is deposited into granules within the bacterium’s cytoplasm it appears opalescent like a pearl hence the name “Pearl of Namibia”. Can you see those pearly sulfur clusters in the cells above? The large cell size of T. namibiensis, and other large bacteria allows these bacteria to have more control over their position within physicochemical gradients. As well as a high storage capacity for intracellular chemicals required to fuel respiration. And though for most bacterial species on the planet, gigantism is usually a major disadvantage, T. namibiensis and Epulopiscium fishelsoni have evolved to maximize their nutrient absorption with respect to their cognate environments. Studies of these unusual bacterial giants are providing fascinating insight into the cellular adaptations testing the limits of bacterial cell size.