Microorganisms are tremendously diverse in size, shape, physiology and lifestyle. Microbial taxonomy is a means by which the microorganisms can be grouped together. Organisms having similarities with respect to the criteria used are in the same group, and are separated from the other groups of microorganisms that have different characteristics.
In a broader sense taxonomy (biological classification) consists of three separate but interrelated parts: classification, nomenclature, and identification. Once a classification scheme is selected, it is used to arrange organisms into groups called taxa (s., taxon) based on mutual similarity. Nomenclature is the branch of taxonomy concerned with the assignment of names to taxonomic groups in agreement with published rules. Identification is the practical side of taxonomy, the process of determining if a particular isolate belongs to a recognized taxon.
One of the oldest classification systems, called a natural classification, arranges organisms into groups whose members share many characteristics and reflects as much as possible the biological nature of organisms. Carolus Linnaeus, developed the first natural classification, based largely on anatomical characteristics, in the middle of the eighteenth century. However this approach of classification does not necessarily provides information of evolutionary relatedness in microbes. Our recent understanding of the evolutionary relationships among microbes now serves as the theoretical underpinning for microbial taxonomic classification.
In practice, determination of the genus and species of a newly discovered prokaryote is based on polyphasic taxonomy. This approach includes phenotypic, phylogenetic, and genotypic features. To understand how all of these data are incorporated into a coherent profile of taxonomic criteria, we must first consider the individual components and determine how they are assessed quantitatively through numerical taxonomy.
- Phenetic Classification
Phenetic system, groups organisms together based on the mutual similarity of their phenotypic characteristics. This classification system succeeded in bringing order to biological diversity and clarified the function of morphological structures. Although phenetic studies can reveal possible evolutionary relationships, they are not dependent on phylogenetic analysis. They compare many traits without assuming that any features are more phylogenetically important than others, that is unweighted traits are employed in estimating general similarity.
- Phylogenetic Classification
Phylogenetic or phyletic classification systems compare organisms on the basis of evolutionary relationships of species. Scientists observed differences and similarities between organisms as a result of evolutionary processes, which put insight into the history of life on earth. This is usually based on direct comparison of genetic material and gene products and is widely accepted in the world now. The power of rRNA as a phylogenetic and taxonomic tool rests on the features of the rRNA molecule that make it a good indicator of evolutionary history and the ever-increasing size of the rRNA sequence database.
- Genotypic Classification
Genotypic classification seeks to compare the genetic similarity between the organisms. Individual genes or whole genomes can be compared. There are currently many ways in which the genotype of a microbe can be evaluated for taxonomic terms.
Peter H. A. Sneath and Robert Sokal have defined numerical taxonomy as “the grouping by numerical methods of taxonomic units into taxa on the basis of their character states.” Information about the properties of organisms is converted into a form suitable for numerical analysis and then compared by means of a computer. The resulting classification is based on general similarity as judged by comparison of many characteristics, each given equal weight.
The results of numerical taxonomic analysis are often summarized with a tree like diagram called a dendrogram. Essentially, a dendrogram appears as a tree oriented on a horizontal axis. The dendrogram becomes increasingly specialized—that is, the similarity coefficient increases—as the dendrogram moves from the left to the right. The right hand side consists of the branches of the trees. Each branch contains a group of microorganisms which share so many similarities.