First . . .
Microbes are often non-descript rods, or spheres.
More about how to do that later, but on to more history. Even today the conception of a species of bacteria is hard to define, in the same way an animal species is defined. We are all probably familiar with Charles Darwin, 19th century father of natural selection and evolution, but he also called for a framework or geneology of organisms in order to study evolution. Darwin helped to develop the concept of a geneology of life with branching lineages. This is still a common assumption, but it makes sense with modern molecular evidence as well.