CHAPTER 3. Classification of Microorganisms I. H. Siddique

 

 

3.1 CHAPTER REVIEW:

One of the important purposes of bacterial classification is the identification of pathogens. However, most bacteria are not pathogenic. Of more than l,700 species of bacteria known, only a very few are human or animal pathogens of consequence. The shared and differing properties of bacteria allow assignment of a newly identified bacterium to a group of well-characterized organisms. The methodology used for naming bacteria and their classification is known as taxonomy or systematics.

 3.2 OUTLINE

3.2.1 Introduction.

Before the discovery of microorganisms, all known living things were believed to be either plant or animal; no transitional types existed. It is now generally accepted that microorganisms have evolved from the common ancestors of plants and animals.

3.2.2 Classification of Microorganisms

Haeckel (1866) proposed that microorganisms be placed in a separate kingdom: the Protista. Members of the protista are distinguished from true plants and animals by their simple organization: they are unicellular or if multicellular, their tissues show little differentiation. The Protists can be subdivided based on their cell structure either eucaryotic or procaryotic. The eucaryotic type of cell structure (more advanced) is shared with cells of plants and animals; the procaryotic cell structure is more primitive.

A. Higher Protists: (Eucaryotic)

algae (except the blue-green)

protozoa

fungi

slime molds

B. Lower Protists: (Procaryotic)

bacteria

blue-green algae

 

Bacteria include two groups: the chlamydiae and the rickettsiae, which differ from other bacteria in being smaller (0.2-0.5 um) and in being obligate intracellular parasites. Viruses are also classified as microorganisms but are sharply differentiated from all other cellular forms of life.All living forms (except viruses) are alike in that they contain nuclear material (DNA) in the cells. The nuclear material is enclosed within a nuclear membrane in eucaryotic cells. All higher forms of life, including animals consist of eucaryotic cells. In primitive life forms, the nuclear material is not enclosed within a membrane, but rather is distributed in masses throughout the cytoplasm. These type of cells are said to be procaryotic. The nucleus of typical procaryotic cells consists of a single, very long circular, threadlike molecule of DNA not segregated from the surrounding cytoplasmic material, and never manifests any mitotic or meiotic phenomena. For additional information refer to Bergey's Manual.

3.2.3 Bergey's Manual of Systematic Bacteriology

The manual was first published in l923 to classify known bacteria and the information was presented in the form of a key. The latest ninth edition of this manual has been published in l984 in which the phylogenetic relationship among bacteria is reflected. This manual provides an important source for identification of newly isolated strains. Index Bergeyana,another companion volume contains literature index, the host and habitat index, and descriptions of organisms. The International Committee on Systematic Bacteriology publishes the approved list of valid bacterial names in the International Journal of Systematic Bacteriology. A key to the principal groups of eubacteria is presented in Tables l and 2.

3.2.4 Terms used in Classification

Species: a coherent group of related individuals

Genus: a group of related species. A genus may include a single or several species.

Tribe: a group of related genera

Family: a group of related genera one of which is usually designated as the type genus.

Order: a group of related families.

Class: a group of related orders, i.e., Eubacteriales

Phylum: a group of related classes

Kingdom: a group of related phyla