Hunting for new microbes is not as easy as taking a jeep trip in the outback with a pair of binoculars. By definition, microbes are invisible without the aid of a microscope, so the challenge to find new ones is great. The difficulty is compounded when one does look under the microscope to see two apparently similar bacteria which later prove to be not at all similar. For example: two bacteria may have the same rod shape, but one thrives in the presence of oxygen whereas the other one is killed by oxygen.
Because microbes are so difficult to observe, they are the last organisms to be catalogued with fewer than one percent yet described.
To see how microbes are related to other life forms, check out the Tree of Life web site at:
http://tolweb.org/tree/phylogeny.html
Heywood, V. H. (ed.) 1995. Global Biodiversity Assessment, p.1140. Cambridge University Press, Cambridge.