Species Classification PDF

Species Basic taxonomic group is species, collection of strains having similar characteristics. Bacterial species consist of special strain called type strain together with all other strains similar to type strain. Each species of microorganism has only official has one officially accepted name. The...

Species Basic taxonomic group is species, collection of strains having similar characteristics. Bacterial species consist of special strain called type strain together with all other strains similar to type strain. Each species of microorganism has only official has one officially accepted name. The name of species is in Latin. Some species are named after persons, for example Escherichia coli –the organism of the colon is named after Theoder Escherich (German bacteriologist) or Clostridium barkeri – the spindle shaped organism is named after H.A. Barker (American biochemist). Some names are nonsensical (e.g. Runella slithyformis- the organism whose shape resembles runes and which is slithy (the later term taken from Lewis Carroll’s poem) The species name is in italics. The nomenclature is by International Code of Nomenclature. The International Code of Nomenclature of Bacteria includes: -Each distinct kind of organism is designated as species. -The species is designated by a Latin binomial to provide a characteristic international label. (binomial system of nomenclature) -Regulation is established for the application of names. -A law of priority ensures use of the oldest available legitimate name. -Designation of categories is required for classification of organisms. -Requirements are given for effective publication of new specific names, as well as guidance in coining new names. Methods of classifying bacteria a)The initiative method- Microbiologist is familiar with the properties of organisms he or she is studying for many years which decide that the organisms represent one or more species or genera. b)Numerical taxonomy- Scientist determines many characteristics (100 to 200) to group bacteria for each strain studied, giving each characteristic equal weight. Then using a computer he/she calculates % similarity (%S) of each strain to every strain.For any two strains this is: %S = NS/ NS + ND Where, NS= no. of characteristics that are positive or negative for the two strains. ND= no. of characteristics that are different. Those strains having a high %S to each other are placed into groups, those groups having a high %S to each other are in turn placed into larger groups and so on. This system is useful as it has great practical usefulness and it also yields classifications that have a high degree of stability and predictability. c) Genetic relatedness- This method is objective and is based on most fundamental aspect of organisms, their hereditary material (DNA). At first only crude comparisons could be made based on mol% G+C values. It is true that the two organisms of same or similar species that are very closely related will have very similar mol% G+C values., and it is also true that two organisms having different mol% G+C values are not closely related. The organisms that are completely unrelated may have similar mol% G+C values. Therefore, precise methods are of comparison are needed – viz., methods by which the DNA molecules from various organisms could be compared with respect to the sequence DNA homology experiments: The ds DNA molecules from two organisms are heated to convert into single strands. The single strands from one organism when mixed with those of other organism and allowed to cool. If the strands are closely related heteroduplexes will form. If the two organisms are not closely related, no heteroduplexes will form. RNA homology experiments and rRNA oligonucleotides The degree of similarity is used as a measure of relatedness between organisms, but at a level beyond that of species. RNA homology and RNA oligonucelotide cataloging are two modern methods to determine degree of similarity between rRNA cistrons of different organisms.