Reality → Scale → Micro world → Microbes
We can, arbitrarily, define a 'world of microbes' that starts at 0.1 millimeter, a size below which we can no longer distinguish a subject with the naked eye and which may be represented by the size of the human egg, and ends at 10 nanometer, about the size of the smallest known viruses [1] . In the upper size range we still encounter microscopic varieties of complex life forms [2] , followed by the realm of bacteria [3] and, finally, viruses [4] . Bacteria form a large share of all living mass on our planet, about as much as plants, much larger than all animals combined [5] . Viruses form the last category of microbial life (or non-life? [6] ) before the world of molecules and atoms begins.
To visualize the size of this arbitrary world of microbes we might imagine the human egg enlarged to the size of a stadium and then see a small virus at the size of a golf ball.
A significant portion of Microorganisms is smaller than 0.1 mm. Among the known species are even multi-cellular animals, a crustacean (see Stygotantulus) and some rotifers. Abundant marine phytoplankton contains single-celled algae that can be as small as a few μm.
The typical size of bacteria is a few μm (see E.coli), while the smallest bacteria measure about 0.1 μm (see mycoplasma).
Viruses are normally one order of magnitude smaller than bacteria, with sizes generally ranging from 300 nm down to less than 20 nm.
Bacteria are everywhere: each gram of soil contains 40 million bacterial cells and each gram of water one million (see Life, Note 1 and PNAS article). Living bacteria have been found in such inhospitable environments as polar ice, acid hot springs and radioactive material (see Extremophile).
Viruses show some characteristics of living organisms, e.g., they replicate, incur mutations and undergo evolution). On the other hand, viruses also show characteristics of inert matter, e.g., they lack cell structure and metabolism. The origin of viruses is subject of various hypotheses.