Classification is important. It is an activity essential to all scientific work. It would be impossible to make any generalizations about microorganisms and their role in nature if we could only refer to each strain by a different and arbitrary name. We must arrange microorganisms into groupsthat share common properties, so that we can talk about sets of strains that have properties in common.Identification Identification is a branch of systematics. It is, however, different from classification, which tests of the way living organisms are grouped together into taxonomic groups or taxa (singular taxon).

Identification deals with the process of allocating a new specimen (an ‘unknown’) to the correct and previously described taxon.Importance: Identification is a very important practical activity, which will concern most microbiologists from time to time. Large areas of microbiological work are heavily dependent on good identification. Some areas, such as hospital microbiology, are almost entirely concerned with identification and are collectively referred to as diagnostic microbiology. It may be emphasized that numerous kinds of microbiological work..Culture of Microorganisms Many microorganisms, especially those with simple morphology, must first be isolated in pure culture before they can be identified.

The importance of the purity of these cultures cannot be overemphasized: most cultural tests become grossly misleading with mixed cultures. The commonest cause of admixture is isolating a microorgnaism from a selective medium that inhibits, but does not kill, unwanted species.The main classes of data used in microbial identification are morphological, chemical, and serological. —From Professor J.

R. Norris and Professor M. H.

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Richmond, 1978, Essay in Microbiology, 9/1-10/1 Ribosomal genes Contain regions that are conserved among all self replicating organisms, together with highly variable regions Are large enough to contain adepqate amount of information Phylogenetic relationships among prokaryotes.Carl Woese and his colleagues demonstrated that the nucleotide sequences occurring in ribosomal RNA could be used to suggest phylogenetic relationships among prokaryotes. In other words, Woese reasoned that changes in rRNA sequences that still permitted protein synthesis would occur so slowly that such changes could be used as a clock to compare one organism with another.

Thus, if six different genera of bacteria all contained the same specific changes in a segment of their rRNA, one could suppose that all had a common ancestral origin. To put it another way, the more widespread the existence of a specific rRNA sequence, the longer ago it occurred.Why are rRNA sequences so highly conserved? Keep in mind that such RNA must maintain a fairly restricted secondary structure in order to function in protein synthesis and that this structure is dependent on base pairing within the RNA molecule. Any mutational change incompatible with such secondary structure will be lethal to the organism. In addition, rRNA must be able to bind to protein within the ribosome, and any changes that interfere with such binding would also be lethal.Woese’s sequnce analyses led to the conclusion that all living organisms could be fitted into one of three distinct kingdoms. One, comprising all of the eucaryotic cell types was termed the EUCARYTOTES and the other two kingdoms, contaning prokaryotic cells, were designated the Eubacteria (true bacteria) and the Archaebacteria (ancient bacteria). Moreover, it was subsequently shown that rRNA existing in mitochondria and chloroplasts (obtained from eucarytoic cells) is closely related to that found in the Eubacteria.

In addition, at least one protein sequence occurring in eucaryotic cells is closely related to a similar protein found in the Archaebacteria. Thus , eucaryotic cells appear to contain genes derived from both eubacteria and archaebactria as well as distinct gene sets that have evolved in eucaryotic cells.The general structure of ribosomes from prokaryotes and eukaryotes is summarized. Figure Although ribosomes from prokaryotes such as bacteria are smaller (70S) than those from eukarytoes (80S), all share the same basic structure. They are composed of two major subunits of different size and with different sedimentation coefficients.

Ribosomes can be completely dissociated into their component parts in solutions of urea or detergents.Each subunit contains at least one single-stranded RNA molecule and numerous proteins. Naturally enough, the RNA’s structural components of the ribosomes, are referred to as ribosomal RNA’s or rRNA’s.

In both prokaryotes and eukaryotes the larger ribosomal subunit contains two rRNA molecules,……

The small ribosomal subunit contains one single-stranded rRNA molecule-16S in prokaryotes and about 1500 nucleotides long.