Basis of Classification

Bergey’s Manual of Systematics of Archaea and Bacteria maintains a list of prokaryotic species and their evolving characteristics. As new research and discoveries come to light, certain bacterial species may be reclassified or renamed. In this section, we will cover taxonomical classification of clinically important bacteria.

Bacteria are classified based on

Phenotypic - observable traits such as cell morphology, cell wall type, evolutionary relationships.
Genotypic - genetic similarities and differences.

Phylogenetic Trees are used to visualize large number of species and their relationships.

Bacteria are grouped based on phenotypic and/or genotypic similarities and differences . The groups (phyla and class) are arranged in a branching tree-like arrangement. The closer the classes are on the tree, more similar they are to each other than the classes at a different or further branch of the tree.

For example, in the following image, the phylum 'Gram positives' are more similar to phylum 'Spirochetes' than they are to phylum 'Entamoebae'. Though when we take a closer look at 'Gram-positive' and 'Spirochetes', we find a lot of differences, but both have characteristics of bacteria (such as no nucleus and reproduction via binary fission). Entamoebae belongs to Kingdom Protista under Domain Eukarya which indicates it contains a nucleus and reproduces via mitosis and cell division.

Tree of Life

Figure 4.24

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