Introductory Microbiology

Composition of microbial world

• Although most microorganisms are too small to be seen, their importance cannot be ignored. Microorganisms are the foundation of the biosphere both from an evolutionary and an ecological perspective. Microorganisms were the first organisms on earth; they have lived on this planet for a period of at least 3.7 billion years of the 4.6 billon year existence of the earth. Microorganisms were living inhabitants for more than 3.0 billion years before the appearance of plants and animals. Not only did plants and animals evolve rather recently in earth’s history but they evolved from microbial life on mars also is consistent with the concept that microorganisms preceded plants and animals on earth.
• The earth’s biosphere is largely shaped by geochemical activities of microorganisms that have provided conditions both the evolution of plants and animals and for the continuation of all life on earth. Many microorganisms carry out unique geochemical processes critical to the operation of the biosphere. Therefore, it is not surprising that the diversity of microorganisms from genetic metabolic and physiological aspects is for greater than that found in plants and animals.
• In contrast to plants and animals, the diversity of microbial world is largely unknown and of that, which is known, the diversity is spectacular. Some microorganisms live at boiling temperatures, or higher in hot spring and deep sea thermal vents; other live at temperature below freezing in sea ice. Some produce sulphuric and nitric acids. Many grow without oxygen and anaerobic activities of their microorganisms are necessary for carrying out the many essential processes in the environment that cannot be accomplished by plants and animals including methane production and nitrogen fixation. Such familiar activities are leavening bread and production of yogurt, pickles, wine , beer and cheeses rely on microorganisms carrying out the key processes.
• Microorganisms also play other essential and beneficial functions for society. For example, we rely on them for production of antibiotics, antitumor agents, and a variety of biotechnology products. We use microorganisms to produce human insulin via genetic engineering and to provide enzymes for manufacturing. They are important in agriculture; their metabolic activities enhance soil fertility especially in their often unique roles in the nitrogen, phosphorus, sulphur & carbon cycles.
• A new awareness of microbial diversity has developed in recent years. Advances in molecular biology have allowed biologist to compare all living organisms to one another on the basis of highly conserved genes. Initial studies focused on those genes that code for ribonucleic acid (RNA) of ribosome, the cellular structure responsible for protein synthesis in all organisms. In particular, the sequence on the bases of the small subunit (16s or 18s) of ribosomal RNA (r RNA) has been used to map the relationship of all living organisms.

The place of microorganisms in the living world:
In biology, as in any other field, classification means the orderly arrangement of units under study into groups of larger units. Present day classification in biology was established by the work of Carolus Linnaeus (1707-1778), a Sweedish botanist. His looks on the classification of plants and animals are considered to be the beginning of modern botanical and zoological nomenclature, a system of naming plants and animals. Nomenclature in microbiology, which came much later, was based on the principles established for the plant and animal kingdoms. Until the eighteenth century, the classifications of living organisms placed all organisms into one of the two kingdoms, plant and animal. As previously stated in microbiology we study some organisms that are predominantly plantlike, others the animallike and some that share characteristics common to both plants and animals. Since there are organisms that donot fall naturally into either the plant or the animals kingdom, it was proposed that new kingdoms be established to include those organisms which typically are neither plants nor animals.

Haeckel’s kingdom Protista:
One of the earliest of these proposals was made in 1866 by a German zoologist, E. H. Haeckel. He suggested that a third kingdom, protista, be formed to include those unicellular microorganisms that are typically neither plants nor animals. These organisms the protists, include bacteria, algae, fungi and protozoa. (viruses are not cellular organisms and therefore are not classified as protists). Bacteria are referred to as lower protists; the others-algae, fungi and protozoa are called higher protists. Bacteria are procaryotic microorganisms. The eucaryotic microorganisms include the protozoa, fungi and algae (plant and animals cells are also eukaryotic). Viruses are left out of this scheme of classification.

Whittakar’s five kingdom concept:
A more recent and comprehensive system of classification, the five kingdom system, was proposed by R.H. Whittaker (1969). This system of classification, shown in figure, is based on three levels of cellular organization which evolved to accommodate three principal modes of nutrition; photosynthesis, absorption and ingestion. The prokaryotes are included in the kingdom Monera; they lack the indigestive mode of nutrition. Unicellular eukaryotic microorganisms are placed in the kingdom protista; all three nutritional types are represented here. Infact the nutritional modes are continuous; the mode of nutrition of the microalgae is photosynthetic; the mode of the nutrition of the protozoa is indigestive; and the mode of nutrition in some other protists is absorptive, with some overlap to the photosynthetic and indigestive modes. The multicellular and multinucleate eukaryotic organisms are found in the kingdoms Plantae (multicellular green plants and higher algae), Animalia (multicellular animals), and fungi (multinucleate higher fungi). Their diversified nutritional modes lead to a more diversified cellular organization. Microorganisms are found in three of the five kingdoms. Monera (bacteria and cyanobacteria), protista (microalgae and protozoa), and fungi (yeasts and molds).

Kingdom procarayote after Bergey’s manual of systematic bacteriology:
Bergey’s manual of systematic bacteriology places all bacteria in the kingdom prokaryotae which in turn is divided into four divisions as follows:
Division 1: Gracilicutes: Prokaryotes with a complex-cell wall structure characteristic of gram negative bacteria. 
Divison 2: Firmicutes: Prokaryotes with a cell-well structure characteristic of gram positive bacteria.
Division 3: Tenericutes: Prokaryotes that lack a cell wall.
Division 4: Mendosicutes: Prokaryotes that show evidence of an earlier phylogenetic origin than those bacteria includes in Division 1 and 2.
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