THREE DOMAIN CLASSIFICATIONS

Introduction

• This system was proposed by Carl Woese in 1978 on the basis of molecular biology and biochemistry.
• This classification is entirely dependent on the differences in the nucleotides sequences of rRNA in the cells and also differences in cell membrane lipids structure.
• The sequence similarity in the rRNA molecule provided a strong basement to predict the evolutionary classification of microbes.
• According to this classification system and ancestor cell give rise to three different cell types.
• Each representing a domain viz; Archaea, the Bacteria (prokaryotes) and Eukarya (eukaryotes) which includes algae, fungi Protozoa, plants and animals.

The Archaea (archaebacteria)

• Salient features of Archaea:
• Archaea bacteria represent a unique group of microorganisms that are related to bacteria, but might have deviated from the evolutionary live of bacteria very early during the evolution of Monera.
• They are considered as the primitive bacteria.
• The cell wall lack (peptidoglycan) (psedopeptidoglycan).
• The membrane consist of characteristic lipids i.e. the lipids have branched hydrocarbon that increase the fluidity of the membrane.
• In some Archaea bacteria the plasma membrane is a monolayer composed of glycerol tetraether lipids.
• The genome consists of single covalently closed circular DNA.
• Some of the Archaea bacteria can survive in extreme environment such as high temperature (Thermophiles) extremely halophilic (Salt Lakes, tidal pools) and anaerobic environments (methanogenic bacteria).
• The archaea are insensitive to certain antibiotics (ex: chloramphenicol) but are sensitive to diphtheria toxin.
• Members of domain Archaea are distinguished from bacteria by many features, most notably their distinctive rRNA sequences,
• Some have unusual metabolic characteristics, such as the methanogens, which generate methane (natural) gas.
• Although some archaea are members of a community of microbes involved in gum disease in humans, their role in causing disease has not been clearly established.

The Bacteria (eubacteria)

• Salient features of bacteria:
• They are unicellular prokaryotes.
• The bacterial cell wall contains peptidoglycan (murein).
• The cell membrane is composed of phospholipids.
• Bacteria are sensitive to some common antibiotics like tetracycline, ampicillin, and penicillin.
• The cytoplasm contains double stranded covalently closed circular DNA.
• Bacteria contain rRNA that is unique to the bacteria, as indicated by the presence of molecular region distinctly different from the rRNA of archaea and eukarya.
• Bacteria include mycoplasmas, cyanobacteria, gram positive and gram negative bacteria.
• Members of domain Bacteria are usually single-celled organisms.
• Although most bacteria exhibit typical prokaryotic cell structure (i.e., they lack a membrane-bound nucleus), a few members of the unusual phylum Planctomycetes have their genetic material surrounded by a membrane. This inconsistency is another argument made for abandoning the term “prokaryote.”
• Bacteria are abundant in soil, water, and air, including sites that have extreme temperatures, pH, or salinity.
• Bacteria are also major inhabitants of our bodies, forming the human microbiome.
• Indeed, more microbial cells are found in and on the human body than there are human cells. These microbes begin to colonize humans shortly after birth. As the microbes establish themselves, they contribute to the development of the body’s immune system.
• Those microbes that inhabit the large intestine help the body digest food and produce vitamins. In these and other ways, the human microbiome helps maintain our health and well-being.
• Unfortunately, some bacteria cause disease, and some of these diseases have had a huge impact on human history. In 1347 the plague (Black Death), an arthropod-borne disease, struck Europe with brutal force, killing one-third of the population (about 25 million people) within four years. Over the next 80 years, the disease struck repeatedly, eventually wiping out 75% of the European population. The plague’s effect was so great that some historians believe it changed European culture and prepared the way for the Renaissance. Because of such plagues, it is easy for people to conclude that all bacteria are pathogens, but in fact, relatively few are. Most play beneficial roles.
• In addition to maintaining human health by forming our micro biomes, they break down dead plant and animal material and, in doing so, cycle elements in the biosphere. Furthermore, they are used extensively in industry to make bread, cheese, antibiotics, vitamins, enzymes, and other products.

The Eukarya (eukaryotes)

• Salient features of bacteria:
• Domain Eukarya includes microorganisms classified as protists or fungi. Animals and plants are also placed in this domain.
• Protists are generally unicellular but larger than most bacteria and archaea. They have traditionally been divided into protozoa and algae.
• The major types of protists are algae, protozoa, slime molds, and water molds. Algae are photosynthetic. They, together with cyanobacteria, produce about 75% of the planet’s oxygen and are the foundation of aquatic food chains.
• Protozoa are unicellular, animal-like protists that are usually motile.
• Many free-living protozoa function as the principal hunters and grazers of the microbial world.
• They obtain nutrients by ingesting organic matter and other microbes.
• They can be found in many different environments, and some are normal inhabitants of the intestinal tracts of animals, where they aid in digestion of complex materials such as cellulose.
• A few cause disease in humans and other animals.
• Slime molds are protists that behave like protozoa in one stage of their life cycle but like fungi in another. In the protozoan phase, they hunt for and engulf food particles, consuming decaying vegetation and other microbes.
• Water molds are protists that grow on the surface of freshwater and moist soil. They feed on decaying vegetation such as logs and mulch.
• Some water molds have produced devastating plant infections, including the Great Potato Famine of 1846–1847 in Ireland, which led to the mass exodus of Irish to the United States and other countries.
• Fungi are a diverse group of microorganisms that range from unicellular forms (yeasts) to molds and mushrooms.
• Molds and mushrooms are multicellular fungi that form thin, threadlike structures called hyphae.
• They absorb nutrients from their environment, including the organic molecules they use as sources of carbon and energy.
• Because of their metabolic capabilities, many fungi play beneficial roles, including making bread dough rise, producing antibiotics, and decomposing dead organisms. Some fungi associate with plant roots to form mycorrhizae.
• Mycorrhizal fungi transfer nutrients to the roots, improving growth of the plants, especially in poor soils.
• Other fungi cause plant diseases (e.g., rusts, powdery mildews, and smuts) and diseases in humans and other animals.
• The microbial world also includes numerous acellular infectious agents.
• Viruses are acellular entities that must invade a host cell to multiply.
• The simplest virus particles (also called virions) are composed only of proteins and a nucleic acid, and can be extremely small (the smallest is 10,000 times smaller than a typical bacterium).
• However, their small size belies their power: they cause many animal and plant diseases and have caused epidemics that have shaped human history.
• Viral diseases include smallpox, rabies, influenza, AIDS, the common cold, and some cancers. Viruses also play important roles in aquatic environments, and their role in shaping aquatic microbial communities is currently being explored.
• Viroids are infectious agents composed only of ribonucleic acid (RNA).
• They cause numerous plant diseases. Satellites are composed of a nucleic acid enclosed in a protein shell.
• They cause plant diseases and some important animal diseases such as hepatitis.
• Finally, prions, infectious agents composed only of protein, are responsible for causing a variety of spongiform encephalopathies such as scrapie and “mad cow disease.”
• The field of microbial evolution, like any other scientific endeavor, is based on the formulation of hypotheses, the gathering and analysis of data, and the reformation of hypotheses based on newly acquired evidence.
• That is to say, the study of microbial evolution is based on the scientific method. To be sure, it is sometimes more difficult to amass evidence when considering events that occurred millions, and often billions, of years ago, but the advent of molecular methods has offered scientists a living record of life’s ancient history. This section describes the outcome of this scientific research.