Bacterial Kingdoms (World of Microbiology and Immunology)
Bacterial kingdoms are part of the classification scheme that fits bacteria into appropriate groupings based on certain criteria. The kingdom is the broadest classification category.
There are two kingdoms of prokaryotes. These are the bacteria (or eubacteria) and the archaebacteria (or the Archaea). The members of these two kingdoms appear similar in shape and appearance, even under the extreme magnification of the electron microscope. However, they are very different from each other in a number of molecular and biochemical aspects. It is these differences that have resulted in the microorganisms being grouped into separate kingdoms.
For example, eubacteria contain the rigid, stress-bearing network known as the peptidoglycan. The only exceptions are the bacteria from the genera Mycoplasma and Chlamydia. Archaebacteria do not contain peptidoglycan. Instead, they contain a different structure that is called pseudomurein.
Another major difference in the prokaryotic kingdoms is in the sequence of a species of ribonucleic acid (RNA) known as 16S ribosomal (r) RNA. The 16 S rRNA is found in many prokaryotic and eukaryotic cells. The function it performs is vital to the life of the cell. Hence, the RNA species has not been altered very much over evolutionary time. The 16s rRNA species of eubacteria and Archaebacteria are very different. Thus, these microorganisms must have taken different evolutionary paths long ago.
Within the eubacterial kingdom are other divisions also known as kingdoms. These divisions are again determined based on the differences in the sequences of the 16S rRNA of the various bacteria. These sequence differences within the eubacterial kingdom are, however, not as pronounced as the sequences differences between the eubacteria and Archaebacteria kingdoms.
The first eubacterial kingdom is referred to as protobacteria. This designation encompasses most of the bacteria that are Gram-negative. Because a great many bacteria are Gramnegative, the protobacterial kingdom is extremely diverse in the shape and the biochemical characteristics of the bacteria. Examples of protobacteria include the photosynthetic purple bacteria, Pseudomonas, and bacteria that dwell in the intestinal tract of warm-blooded animals (e.g., Escherichia coli, Salmonella, and Shigella.
The second eubacterial kingdom is comprised of the Gram-positive bacteria. This group is also diverse in shape and chemical character. The kingdom is further split into two major groups, based on the proportion of the nucleic acid that is composed of two particular building blocks (guanosine and cytosine). One group contains those bacteria whose DNA is relatively low in G and C (e.g., Clostridium, Staphylococcus, Bacillus, lactic acid bacteria, Mycoplasma). The other group is made up of bacteria whose DNA is relatively enriched in G and C (e.g., Actinomyces, Streptomyces, Bifidobacterium. The latter group contains most of the antibiotic-producing bacteria that are known.
The various eubacterial kingdoms, and the Archaebacterial kingdom, are markedly different in 16S rRNA sequence from the eukaryotic kingdoms (plants, fungi, animals). Thus, following the establishment of these life forms, the eukaryotes began to diverge from the evolutionary paths followed by the eubacteria and Archaebacteria.
See also Life, origin of; Microbial taxonomy