Bacteria Domain

Carl Woese et co. established the three-domain approach to biological categorisation in 1990. It classifies cellular life forms into three domains: Archaea, Bacteria, and Eukarya. Archaea and Bacteria were classified as prokaryotes. Archaea is a class of organisms that includes primitive prokaryotes that exist in harsh settings known as archaebacteria. Archaebacteria are included in this kingdom. Bacteria is a domain that includes typical prokaryotes, which lack membrane-bound organelles. Eubacteria are included in this kingdom. Eukaryotic creatures are included in the domain Eukarya. Protista, Fungi, Plantae, and Animalia are all included in this kingdom.

Three-Domain System

This categorization method categorises life according to the structure of the 16S ribosomal RNA (rRNA), as well as the lipid structure of the cell membrane and its antibiotic sensitivity. The primary distinction from previous categorization systems is the separation of archaea and bacteria.

Evaluating the rRNA structure is quite beneficial. Because rRNA molecules serve the same job throughout nature, their structure remains relatively constant throughout time. Subsequently, similarities and dissimilarities in rRNA nucleotide sequences provide an excellent indicator of the degree to which cells and species are connected or unrelated.

Carl Woese utilises 16S ribosomal RNA (rRNA) as a ‘Chronometer’ in this categorization because of

• It is globally distributed, meaning it is present in all species. 
• It is functionally comparable in all organisms.
• The sequences of two species can be aligned or matched.
• It can change its sequence slowly.

Domain of Archaea

Single-celled creatures are found in the Archaea domain. Archaea share genes with bacteria and eukaryotes. Due to their striking resemblance to bacteria, they were initially mistaken for bacteria.

Archaea, like bacteria, are prokaryotic organisms that lack a membrane-bound nucleus. Additionally, they lack internal organelles and many are nearly the same size and shape as bacteria. Archaea reproduce by binary fission, contain a single circular chromosome, and, like bacteria, employ flagella to move around in their surroundings.

Archaea have a different cell wall composition than bacteria and a different membrane composition and rRNA type than both bacteria and eukaryotes. These distinctions are significant enough to justify archaea having their own domain.

Archaea are severe creatures that thrive in some of the most inhospitable environments. This includes areas beneath Arctic ice, hydrothermal vents, and acidic springs. The Archaea are classified into three phyla: Crenarchaeota, Euryarchaeota, and Korarchaeota.

• Crenarchaeota has a diverse range of hyperthermophiles and thermoacidophiles. These archaea survive in situations with extremes of temperature (hyperthermophiles) as well as in highly hot and acidic conditions (thermoacidophiles.)
• The phylum Euryarchaeota contains the Archaea known as methanogens. They create methane as a byproduct of metabolism and require an oxygen-free environment.
• Little is known about Korarchaeota archaea due to the fact that only a few species have been discovered thriving in hot springs, hydrothermal vents, and obsidian pools.

Domain of Bacteria

Bacteria are classed as a domain. These organisms are feared by the general public due to the fact that some are dangerous and capable of causing sickness.

Bacteria, on the other hand, are necessary for life, as some are found in the human microbiome. These bacteria provide critical services, such as allowing humans to digest and absorb nutrients from our meals. Bacteria found on the skin help prevent harmful germs from invading the region and also aid in the immune system’s activation.

Bacteria are also critical for the global ecosystem’s nutrient recycling, as they are the principal decomposers. The majority of known harmful prokaryotic species are bacteria. As a result of this, and since Archaea are notoriously difficult to cultivate in the laboratory, Bacteria are now more thoroughly investigated than Archaea.

Bacteria have their own cell wall composition and kind of rRNA. They may be classified into five broad categories:

• Proteobacteria: This phylum contains the majority of bacteria, including E.coli, Salmonella, Heliobacter pylori, and Vibrio.
• Cyanobacteria: These bacteria are photosynthesis-capable. Due to their hue, they are sometimes referred to as blue-green algae.
• Firmicutes: Included in this group of gram-positive bacteria are Clostridium, Bacillus, and mycoplasmas (bacteria without cell walls.)
• Chlamydiae: These parasitic bacteria replicate within the cells of their hosts. Chlamydia trachomatis (causes chlamydia STD) and Chlamydophila pneumoniae are two organisms (causes pneumonia.)
• Spirochetes: These bacteria in the form of a corkscrew demonstrate an unusual twisting behaviour. Borrelia burgdorferi (which causes Lyme disease) and Treponema pallidum are two examples (cause syphilis.)

Domain of Eukarya

Eukaryotes, or creatures with a membrane-bound nucleus, are included in the Eukarya domain.

This dominion is split further into kingdoms.

• Protista
• Fungi
• Plantae
• Animalia

Eukaryotes have a unique rRNA sequence in comparison to bacteria and archaea. Cell walls of plants and fungi are composed differently than those of bacteria. Antibacterial antibiotics are often resistant to eukaryotic cells.

Protists, fungi, plants, and animals are all included in this realm. Algae, amoeba, fungus, moulds, yeast, ferns, mosses, blooming plants, sponges, insects, and mammals are just a few examples.

Conclusion

The three-domain system adds a level of categorization (the domains) “above” the five- or six-kingdom systems previously utilised. This categorization scheme highlights the essential distinction between the two prokaryotic groupings, with Archaea appearing to be more closely linked to Eukaryotes than to other prokaryotes — bacteria-like creatures lacking a cell nucleus. The approach categories previously recognised kingdoms as Archaea, Bacteria, and Eukarya.