The extremely small size of microorganisms is difficult to appreciate. A useful illustration is to point out that, in a drawing or photograph showing a typical rod-shaped bacterium 1 µm in length, the organism has been magnified some 60 000 times. On this scale, an average human adult would be well over 50 miles tall. Three-dimensional (volume) aspects should also be appreciated. An approximately spherical bacterium 1 micrometre (µm) across has ten times the diameter of a similarly-shaped virus 0.1 micrometre across, but its volume is 1000 times greater. A corresponding relationship exists, in turn, between bacteria and the animal cells which they may infect.

Micrographs of various microorganisms, together with the scale magnification used appear in many standard texts and a commercially set of such Photographs is mentioned in 'Other Resource Materials'. Students' ability to calculate the actual size of organisms shown in such material may be tested in the examination. The micrometre could usefully be introduced to students though awareness of this unit will not be obligatory and, if it is used in questions, an explanation in terms of SI units will be provided (eg 1000 µm = 1 mm}.

The significance of structural simplicity of microorganisms, in comparison with humans in allowing maximum use of resources for rapid growth. reproduction and product formation should be appreciated.