Identification and Genetic characteristics

Identification

An organism is to be classified before it is to be identified- that is, to be named. This is truth even if the classification is merely the recognition that these organism are different from any known type of organism. For example, this occurred with the Legionnaires’ disease agent, which caused the famous pneumonia epidemic in 1976 in Philadelphia; this organism was different than bacteria of any established species; it has now been classified in a another bacterial genus, Legionella, and has been assigned the species name L. pneumonia. Since the organism is classified, a few of its characteristics are selected by which it can be identified by other microbiologists. In order to be helpful in identification, the combination of characteristics selected must occur only in that particular type of organism and in no other. The characteristics chosen should also be those that are easy to determine, such as shape, staining reactions, and sugar fermentations. DNA homology experiments, while very helpful for classifying an organism, would be quite unsatisfactory for the routine identification of an organism because of the complexity of the procedure.This could be an excellent example.

Many identification schemes or types are in the form of keys, which give identifying characteristics arranged in a logical order. Identification tables are also helpful and generally contain more characteristics than do keys, with the information arranged in an easy-to-read, summarized series.

The past and present state of bacterial taxonomy

The first classification order for bacteria was published in 1773, and much more have appeared since in the early schemes were based only on morphological characteristics, but as the science of microbiology evolved , other types of characteristics became increasingly important for classifications. Each step of classification scheme reflected the level of developmental research available at the time, and this continues to be true. As proved present arrangements of bacteria are only provisional, subject to modification or replacement as new information or species evolves.

Many classification criteria presently exist, but most cover only single or a few groups of bacteria.The single classification scheme is unique, however, because of its broad scope and wide agreement : Bergey’s Manual of determinative bacteriology. This international reference work is usually not for providing descriptions of all established genera and species of bacteria, but it also provides a practical arrangement knowledge of these taxa that is helpful for their identification, together with appropriate keys and tables. Eight editions of Bergey’s Manual have appeared since 1923, and a new edition is now in the process , part of which has already been documented . The title has been changed to Bergey’s Manual of systematic bacteriology to show an increased coverage of bacterial characterization, classification, and taxonomic problems, in addition to the identification aspects. Bergey’s Manual is written by hundreds of authors from all over the world, each an authority on a particular bacterial type or species .

The arrangement of bacterial taxa in the new publication of Bergey’s Manual is mainly along traditional, practical lines. Each volume is sub divided into a number of sections, each bearing a vernacular name such as “The Spirochetes” or “Gram-negative Anaerobic Cocci” instead than a formal taxonomic name. The emphasis is largely on genera and species and, due to the present incomplete and fragmentary understanding of the real relationships that exists among bacteria, no attempt is made to adhere to any comprehensive, formal taxonomic hierarchy. The present classification scheme has the considerable practical scope , but the editorial board of Bergey’s Manual regards it only as an interim arrangement that must usually give way to a new, general, comprehensive classification scheme based on genetic relatedness. This is helpful to provide greater stability and predictability, to lead to improved identification schemes, and to aid on the understanding of the origin of the present-day genera and species.

Genetic characteristics

The double-stranded chromosomal DNA of each type of microorganisms has certain characteristics that are constant and characteristics for that organism and useful for its classification:

1. DNA base composition: it is important to document that the DNA molecule is made up of base pairs; guanine-cytosine and adenine-thymine. Of the total number of nucleotide bases present in DNA, that percentage represented by guanine plus cytosine is termed the moles % G+C value (or more briefly, mol% G+C).Values for various organisms range from 23 to 75.
2. The sequence of nucleotide bases in the DNA: This sequence is different for reach type of organism and is the most fundamental of all the characteristics of an organism: it has a great importance of microbial classification.

In addition to chromosomal DNA, plasmid DNA may nevertheless be present in microbial cells. plasmids are circular DNA molecules that are capable of autonomous replication within bacterial cells, and their appearance can confer special characteristics on the cells that contain them, such as the ability to make toxins (toxigenicity), to be and possess resistant to various antibiotics, or to use different chemical compounds as nutrients.

Ecological characteristics

The living environment of microorganisms is important in characterizing that organism. For example, microorganisms normally found in marine environments generally differ from those in freshwater environments. The microbial population of the oral cavity is different from that of the intestinal tract. Some types of microorganisms are widely distributed in nature, but others may be restricted to a particular environment. The relation of an organism to its environment is often complexion and may involve special characteristics of the organism that are not yet known.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.