Classification of Microorganisms

 

Introduction

 

1. Taxonomy is the science of the classification of organisms, with the goal of showing relationships among organisms.
2. Taxonomy also provides a means of identifying organisms.

 

The Study Of Phylogenetic Relationships

 

1. Phylogeny is the evolutionary history of a group of organisms.
2. The taxonomic hierarchy shows evolutionary, or phylogenetic, relationships among organisms.
3. Bacteria were separated into the Kingdom Prokaryotae in 1968.
4. Living organisms were divided into five kingdoms in 1969.

 

The Three Domains

 

1. Living organisms are currently classified into three domains.  A domain can be provided into kingdoms.
2. In this system, plants, animals, fungi, and protists belong to the Domain Eukarya.
3. Bacteria (with peptidoglycan) form a second domain.
4. Archaea (with unusual cell walls) are placed in the Domain Archaea.

 

A Phylogenetic Hierarchy

 

1. Organisms are grouped into taxa according to phylogenetic relationships (from a common ancestor).
2. Some of the information for eukaryotic relationships is obtained from the fossil record.
3. Prokaryotic realtionships are determined by rRNA sequencing.

 

Classification Of Organisms

 

Scientific Nomenclature

 

1. According to scientific nomenclature, each organism is assigned two names, or binomial : a genus and a specific epithet, or species.
2. Rules for the assignment of names to bacteria are established by the International Committee on Systematic Bacteriology.
3. Rules for naming fungi and algae are published in the International Code of Botanical Nomenclature.
4. Rules for naming protozoa are found in the International Code of Zoological Nomenclature.        

 

The Taxonomic Hierarchy

 

1. A eukaryotic species is a group of organisms that interbreeds but does not breed with individuals of another species.
2. Similiar species are grouped into a genus; similiar genera are group into a family; families into an order; orders into a class; classes into a division or phylum; phyla into a kingdom; and kingdoms into domains.

 

Classification of Prokaryotes

 

1. Bergey's Manual of Systematic Bacteriology is the standard reference on bacterial classification.
2. A group of bacteria derived from a single cell is called a strain.
3. Closely related strains constitute a bacterial species.

 

Classification of Eukaryotes

 

1. Eukaryotic organisms may be classified into the Kingdom Fungi, Plantae, or Animalia.
2. Protists are mostly unicellular organisms; these organisms are currently being assigned to kingdoms.
3. Fungi are absorptive chemoheterotrophs that develop from spores.
4. Multicellular photoautotrophs are placed in the Kingdom Plantae.
5. Multicellular ingestive heterotrophs are classified as Animalia.

 

Classification of Viruses

 

1. Viruses are not placed in a kingdom.  They are not composed of cells and cannot grow without a host cell.
2. A viral species is a population of viruses with similar characteristics that occupies a particular ecological niche.

 

Methods Of Classifying And Identifying Microorganisms

 

1. Bergey' s Manual of Determinative Bacteriology is the standard reference for laboratory identification of bacteria.
2. Morphological characteristics are useful in identifying microorganisms, especially when aided by differential staining techniques.
3. The presence of various enzymes, as determined by biochemical tests, is used in identifying microorganisms.
4. Serological tests, involving the reactions of microorganisms with specific antibodies, are useful in determining the identity of strains and species, as well as relationships among organisms.  ELISA and Western blotting are examples of serological tests.
5. Phage typing is the identification of bacterial species and strains by determining their susceptibility to various phages.
6. Fatty acid profiles can be used identify some organisms.
7. Flow cytometry measures physical and chemical characteristics of cells.
8. The percentage of GC base pairs in the nucleic acid of cells can be used in the classification of organisms.
9. The number and sizes of DNA fragments, or DNA fingerprints, produced by restriction enzymes are used to determine genetic similarities.
10. The sequence of bases in ribosomal RNA can be used in the classification of organisms.
11. The polymerase chain reaction (PCR) can be used to detect small amounts of microbial DNA in a sample.
12. Single strands of DNA and RNA, from related organisms will hydrogen-bond to form a double-stranded molecule; this bonding is called nucleic acid hybridization.
13. Southern blotting and DNA probes are examples of hybridization techniques.
14. Dichotomous keys are used for the identification of organisms.  Cladograms show phylogenetic relationships among organisms.