Major subgroups

The work of the early bacteriologists the developer Cohn, Koch, and Beijerinck was focused on classification and identification of bacteria. Through, this work was based around the  morphology of the  organism and  since  most bacteria are rod-shaped when viewed  under the  microscope and  form  cream or white  colonies on agar  plates the  success of this  approach is limited. The development of the Gram stain in 1884 through the Danish physician Hans Christian Gram allowed the separation of the prokaryotes into two classes that was later found to be based around cell wall structure. Gram-positive bacteria retained the purple dye while the Gram-negative bacteria cell wall allowed the stain to be washed away. The Gram-positive Bacteria have remained a valid taxonomic group, but the Gram-negative Bacteria have been proved to be more genetically and phenotypically diverse and for a while included members of the Archaea.

While we can  now separate the  Bacteria from  the  Archaea and  further subdivide the Bacteria on  the  basis  of phylogeny several physiological properties are  widely spread throughout the kingdom. The Bacteria with the capability of causing a human pathology are not restricted to any single subgroup nor are the abilities to denitrify grow photosynthesize or anaerobically. The majority of Bacterial species have unique properties that may be very different from members of the similar genus but superficially same to those which could be considered to be phylogenetically very distinct.