Classification of living organisms is referred to as taxonomy and it aims to classify living organisms by differentiating them and establishing relationships between groups of organisms.
The kingdom of living organisms was termed as domain by Carl R. Woese in 1978.Organisms are grouped in three domain systems namely eukarya, archaea and bacteria, primarily on the basis of cell type. Other than the cell type, differences in rRNA, the three domains differ in membrane lipid structure, transfer RNA molecules, and sensitivity to antibiotics also account for classification of organisms.
- The Domain bacteria includes all of the pathogenic and non-pathogenic prokaryotesThey have a cell wall composed of peptidoglycan and muramic acid. They also have membrane lipids with ester-linked, straight-chained fatty acids that resemble eukaryotic membrane lipids. Bacteria also have plasmids, which are small, double-stranded DNAmolecules that are extrachromosomal.
- Domain archaea include three major groups namely methanogens, extreme halophiles and hyperthermophiles. They lack muramic acid in the cell walls.
- Eukarya includes the eukaryotic organismsas animals, plants, fungi, and protistsand have a defined nucleus and membrane bound organelles
The taxonomic classification scheme for prokaryotes is elaborated in Bergey’s Manual of Systematic Bacteriology which describes the classification of prokaryotes into two domains: Bacteria and Archaea.
Further classification of domain is as follows-
The binomial (scientific) nomenclature assigns each microbe 2 names – Genus – noun, always capitalized and species – adjective, lowercase. Both are written italicized or underlined. Ex. Staphylococcus aureus (S. aureus), Escherichia coli (E. coli)
The basic taxonomic group in microbial taxonomy is the species. Taxonomists working with higher organisms define their species differently than microbiolo- gists. Prokaryotic species are characterized by differences in their phenotype andgenotype. Phenotype is the collection of visible characteristics and the behavior of a microorganism. Genotype is the genetic make up of a microorganism
Bergey’s Manual provides classification scheme for bacteria as well as a reference for identification of different bacteria in the laboratory. According to Bergey’s manual of determinative bacteriology, the bacteria are identified on the basis of morphological characteristics staining, biochemical tests, serology, G-C content, DNA probesand different molecular tests. In addition to the properties listed in Bergey’s, the source and habitat of microorganisms is also important to consider when identifying microorganisms.
Points to remember:
- Taxonomy is the science that deals with classificationof living organisms on the basis of characteristics that are similar to and different from other organismsand establishing relationships between groups of organisms.
- All living organisms are grouped in three domain systems namely eukarya, archaea and bacteria, primarily on the basis of cell type.
- The taxonomic classification scheme for prokaryotes is elaborated in Bergey’s Manual of Systematic Bacteriology according to which the prokaryotes are divided into two domains: Bacteria and Archaea