Because of their simple morphology, bacteria can’t be divided into orders, families, genera, and species on the basis of their structure alone. The biochemical, physiological, and cultural characteristics are to be considered for the classification of bacteria.
Erwin F. Smith, a pioneer worker in this field at first accepted a system of classification proposed by Migula which was based on the number and arrangement of flagella. It was as follows:
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1. Bacterium included all atrichous rod-like forms.
2. Pseudomonous included polar flagellate forms.
3. Bacillus included peritrichous rod-like forms.
In 1905, Smith revised this system with slight modifications in the name of the divisions. (i) Aplanobacter was given for atrichous, rod-like bacteria. (ii) Bacterium for polar flagellate bacteria and (iii) Bacillus for peritrichous bacteria.
The commonly used and more widely accepted divisions of bacteria are:
1. Bacterium or Bacillus included all rod-like and kidney-shaped forms.
2. Coccus includes all spherical bacteria.
3. Spirillum includes spiral, comma, and variously curved bacteria.