Classification of Microorganisms

PHYLOGENY: THE STUDY OF EVOLUTIONARY RELATIONSHIPS OF LIVING ORGANISMS

§  Over 1.7 million different organisms have been identified to date.

§  Many similarities among living organisms:

·         Made up of cells surrounded by a plasma membrane.

·         Use ATP as energy source.

·         Store genetic information as DNA.

·         Ribosomes are the site of protein synthesis.

§  Both differences and similarities among organisms are caused by natural selection (Darwin, 1858).

§  Organisms can be classified into taxonomic categories (taxa), based on the differences and similarities among them.

§  Ancient Greeks (on to 1700’s) classified all living organisms into two groups:

1.           1. Kingdom Plantae

2.           2. Kingdom Animalia

§  1735---Linnaeus also used 2 kingdom systems, but established other groups (taxa) and classified most known organisms into all his groups. Microorganisms didn’t get a clear place in this system, but they were fairly new and not well known anyway.

§  In 1850s bacteria and fungi were incorrectly placed in the Plant Kingdom.

§  In 1860s Kingdom Protista was proposed to include bacteria, fungi, algae, and protozoa, but many scientists’ still classified bacteria and fungi as plants.

§  In 1866, Haeckel proposed adding the kingdom Protista for all microorganisms.

Three Kingdom Systems

1.      Plant

2.      Animal

3.      Protista

§  Intense disagreement over classification of bacteria and fungi persisted over 100 years.

§  In 1930s electron microscopy made it clear that bacterial cells lacked a nucleus. The term procaryote was introduced in 1937.

§  In 1959 Kingdom Fungi was established.

§  In 1961 the current definition of the term procaryote was established.

§  In 1968 the Kingdom Procaryotae was accepted by biologists.

§  In 1969 Robert Whitaker proposed a five-kingdom system of biological classification for all living organisms.

Five Kingdom Systems Proposed in 1969 by Robert Whitaker:

1.      Kingdom Procaryotae (Monera): Oldest known cells. Lived over 3.5 billion years ago. Lack a nucleus and membrane bound organelles.

The other four kingdoms are eucaryotes. Have a true nucleus and membrane bound organelles.

2.      Kingdom Protista: Mostly unicellular, lack tissue organization. Most have flagella during life.

3.      Kingdom Fungi: May be unicellular (yeasts) or multicellular (molds). Many are saprotrophs.

4.      Kingdom Plantae: Multicellular, photosynthetic.

5.   Kingdom Animalia: Multicellular, heterotrophs that ingest food through a mouth or oral cavity.

Differences between Eucaryotic and Procaryotic Cells

	Procaryotes	Eucaryotes
Cell size	0.2-2 um in diameter	10-100 um in diameter
True Nucleus	Absent	Present
Membranous Organelles	Absent	Present
Cell Wall	Chemically complex When present	simple
Ribosome	Smaller (70S)	Larger (80S) in cell 70S in organelles
DNA	Single circular	Multiple linear chromosome chromosomes (histones)
Cell Division	Binary fission	Mitosis
Figure	Figure a	Figure b

Figure A: Detail cell structure of Procaryotic cell

Figure B: Detail cell structure of Eucaryotic cell

The Three Domain System:

Domain: In 1978 Carl Woese proposed this level of classification above kingdom.

There are three domains based on the following distinguishing criteria:

·         Cell wall composition

·         Membrane lipids

·         RNA sequence

·         Protein synthesis

·         Antibiotic sensitivity

I. Domain Eubacteria: “True bacteria”.

II. Domain Archaeabacteria: “Ancient bacteria”

III. Domain Eucarya: All eucaryotes: Protista, Fungi, Plantae, and Animalia.

Recent developments in molecular biology and biochemistry have revealed that there are two types of procaryotic cells, based on differences in their ribosomes, cell walls, and metabolism.

1. Eubacteria: “True bacteria”.

·         Cell wall contains peptidoglycan.

·         Sensitive to antibiotics.

2. Archaeabacteria: “Ancient bacteria”

• Differences in structure of membrane lipids
• Differences in RNA
• Cell wall structure (NO peptidoglycan)
• Unusual metabolic processes-----the ability to metabolize unusual substrates and the production of unusual end products
• Ability to thrive in extreme physical conditions

·         The archae are placed on to three groups:

1. Methanogens: Strict anaerobes that produce methane. E.g.     Methanococccus

2. Extreme Halophiles: Require high salt concentrations. E.g. Halobacterium

3. Thermoacidophiles: Live in hot, acidic environments. E.g. Acidocaldarius

3. Eucarya: It contains four kingdoms

• kingdom:  protista---unicellular eukaryocytes. in the future, this kingdom may be split.
• kingdom:  fungi---unicellular yeasts, multicellular molds, macroscopic fungi--these all absorb organic matter through their plasma membranes
• kingdom:  plantae---plants--macroscopic algae, mosses, ferns, conifers, flowering plants--all are multicellular, all carry on photosynthesis
• kingdom:  animalia---animals--sponges, worms, insects, vertebrates--all ingest nutrients 

Figure C: The Three Domain System

The two empire system:

The two-empire system (two-superkingdom system) was the top-level biological classification system in general use before the establishment of the three-domain system. It classified life into Prokaryota and Eukaryota. When the three-domain system was introduced, some biologists preferred the two-superkingdom system, claiming that the three-domain system overemphasized the division between Archaea and Bacteria. However, given the current state of knowledge and the rapid progress in biological scientific advancement, especially due to genetic analyses, that view has all but vanished.

		Two superdomains	Three domains	Six kingdoms
Biota / Vitae life	Acytota / Aphanobionta (Viruses, Viroids, Prions?, ...) non-cellular life
	Cytota cellular life	Prokaryota / Procarya (Monera)	Bacteria	Eubacteria
			Archaea	Archaebacteria
		Eukaryota / Eukarya		Protista
				Fungi
				Plantae
				Animalia

 Figure D: The Two Empire System