Chapter 10: Classification of Microorganisms

Classification of Microorganisms begins by explaining taxonomy as the science of naming and organizing organisms, originally developed by Carl Linnaeus and now refined with molecular biology. The chapter introduces the three-domain system—Bacteria, Archaea, and Eukarya—based on ribosomal RNA sequencing, which provides a more accurate reflection of evolutionary relationships than older five-kingdom models. Within these domains, organisms are grouped into phyla, classes, orders, families, genera, and species, forming a hierarchical structure of classification. Microbial species are defined differently than plant or animal species. For prokaryotes, a species is typically a population of cells with similar characteristics, and a strain is a genetic variant of a species. The chapter outlines binomial nomenclature and the standard format for writing genus and species names (e.g., Escherichia coli). It highlights Bergey’s Manual of Systematic Bacteriology as a key reference for bacterial classification. Various methods for identifying microbes are discussed, including morphological characteristics, differential staining (Gram and acid-fast), and biochemical tests that detect metabolic properties. The chapter emphasizes the importance of molecular techniques in modern taxonomy, including DNA base composition (GC content), DNA fingerprinting, polymerase chain reaction (PCR), nucleic acid hybridization, and ribotyping. Serological methods—like ELISA and slide agglutination tests—are introduced for identifying microbes based on antigen-antibody reactions. Techniques such as phage typing, fatty acid profiles, and flow cytometry also support identification. A major focus is the use of rRNA sequencing, especially 16S rRNA, to establish phylogenetic relationships among bacteria. Cladograms and dichotomous keys are explained as tools for classification and identification. The chapter concludes by reinforcing how accurate microbial identification is essential for diagnostics, epidemiology, and biotechnology. With a blend of classical and molecular tools, this chapter equips students to understand how scientists organize and differentiate the vast diversity of microbial life.