Title 2. DIFFERENT METHODS OF CLASSIFICATION OF MICROBES.pdf ✅

PHARMD GURU Page 1 INTRODUCTION:  Taxonomy is the science that deals with the logical arrangement of living things into categories.  Aristotle (4th Century B.C.) was probably the first to group all organisms and categorised them as either plants or animals.  In 1735, Carolus Linnaeus (Swedish botanist) named thousands of plants and animals and classified them in the Kingdoms Plantae and Animalia. He also devised the binomial (Genus and species) scheme of nomenclature.  Until the eighteenth century, the classification of living organisms was placed into two Kingdoms, Plant and Animal.  The German Zoologist, E. H. Haeckel in 1866 proposed a third Kingdom 'Protista', to include those unicellular microorganisms that are typically neither plants nor animals.  Protista contains bacteria, algae, fungi and protozoa but viruses are not classified as protista (non-cellular organism).  Bacteria are classified as lower protista and algae, fungi and protozoa are classified as higher protista.  With the introduction of electron microscopy, it was made possible to observe internal cell structures. It was discovered (1940) that in some cells (e.g. typical bacteria), the genetic material was not enclosed by a nuclear membrane.  In other cells (algae, fungi, protozoa), the nucleus was enclosed in a membrane. This resulted in the division of these organisms into Procaryotes and Eukaryotes.  Bacteria are prokaryotic (absence of nuclear membrane) microbes. Fungi, algae, protozoa, plant and animal cells are eukaryotic (presence of nuclear membrane) (Fig. 2.1). A comparison of prokaryotes and eukaryotes is given in Table 2.1. DIFFERENT METHODS OF CLASSIFICATION OF MICROBES
PHARMD GURU Page 2
PHARMD GURU Page 3 CLASSIFICATION OF MICROORGANISMS: WHITTAKER'S FIVE-KINGDOM CONCEPT: Many biologists have accepted the system proposed by Robert H. Whittaker in 1969 for the classification of living organisms. He proposed that all living beings can be classified under five kingdoms based on the nutrition and absorption of food materials. The prokaryotic organisms are included under the kingdom monera (lower protista). These organisms do not have the ingestive mode of nutrition. The unicellular eukaryotic microorganisms are placed in kingdom protista (higher protista). In microalgae it is photosynthetic, in protozoa it is ingestive and some other protista it is absorptive. Whittaker explained that some organisms assimilate carbon dioxide by means of photosynthesis while some obtaining through nutrients absorbed from other organisms and some obtaining organic nutrients by way of ingestion. He classified plants, fungi and animals in three separate kingdoms based on the mode of nutrition. In the scheme of Whittaker, microorganisms are accommodated in three of the five kingdoms. This system places all living things (except viruses) into five kingdoms (Fig. 2.2) based on cellular organisation and nutritional patterns; the Monera, Protista, Fungi, Animalia and Plantae. Kingdom Monera includes all unicellular prokaryotes
PHARMD GURU Page 4 (e.g. bacteria, cyanobacteria) and Kingdom Protista includes unicellular eukaryotic cell (e.g. microalgae, protozoa). The multicellular and multinucleate eukaryotic organisms are found in the Kingdom Fungi, Kingdom Plantae and Kingdom Animalia which mainly utilize nutrients by absorption, photosynthesis and ingestion, respectively. EIGHT KINGDOM SYSTEM OF CLASSIFICATION:  Cavalier-Smith (1987) classified protists into eight kingdom on the basis of ultrastructure of cell and rRNA sequencing.  All microorganisms are divided into two empires (Fig. 2.3) like Bacteria and Eukaryota. The empire Bacteria includes two Kingdoms (Eubacteria and Archaeobacteria) and empire Eukaryota contains six kingdoms (Archezoa, Protozoa, Plantae, Chromista, Fungi and Animalia).  The Kingdom Chromista includes diatoms, brown algae, cryptomonads and oomycetes. The Chromista are photosynthetic and have their chromoplast within the lumen of endoplasmic reticulum.