Microbes are generally classified as either eukaryotes or prokaryotes on the basis of the complexity of their cellular structure. Eukaryotic microbes comprise fungi, including yeast, algae and protozoa, while prokaryotic microbes comprise bacteria and cyanobacteria (formerly called blue-green algae). Recently it has become evident that a third group of microbes, the archaebacteria, exists(Woese, 1982). The immediately obvious difference between the eukaryotic and the prokaryotic microbes is their size, with the latter, on average, being an order of magnitude smaller in size than the former. However, the fundamental difference between eukaryotes and prokaryotes is one of cellular structure and function. The characteristic organelle of eukaryotic cells is the nucleus, which, together with the several other membrane-bounded organelles present, carries out the essential functions of the cell. In prokaryotes, these same essential functions are generally performed within the cytoplasmic membrane.