Initially scientists believed that all living organisms were either plants or animals. Later it became clear that this was an over-simplification; and Robert Whittaker proposed a 5-kingdom system that is the currently accepted taxonomic system. The five kingdoms include animalia, plantae, fungi or mycetae, protista (including protozoans and most algae), and Procaryotae or Monera.

Since 1923, the Bergey's Manual of Systematic Bacteriology and other taxonomic texts have been used to help us better understand the groups of micro-organisms. Modern equipment and new discoveries require that microbiology students understand that organizing and grouping microbes will be in a constant state of change. Watch for the changes during your time in this course or colleege.