Microbial Taxonomy
Chapt. 10
I. Taxonomy - The science that names, identifies, and arranges organisms into catagories
1. Nomenclature - the naming of organisms.
2. Classification - arrangement of organisms into groups or taxa.
3. Identification - The process of determining to which taxon an organism belongs.
Importance:
1. Before a clinical or industrial microbiologist can identify a microbe, it must already have been described, named, and classified by a microbial taxonomist.
2. Communication among professional microbiologists around the world is facilitated by a common system of nomenclature.
II. Classification
A. Carolus Linnaeus - in the mid-1700’s, devised a system for naming and classifying organisms that is still used today.
1. Linnaeus divided organisms into 2 catagories: Animalia and Plantae.
a. These catagories were subdivided further
into 7 levels of classification.
2. Levels of Classification:
Kingdom Procaryotae (Monera)
Division (Phylum) Gracilicutes
Class Scotobacteria
Order Enterobacteriales
Family Enterobacteriaceae
Genus Escherichia
species E. coli
3. Basis for classification
a. The first classification systems were based on similarities in structure and physiology.
b. After 1859, when Charles Darwin published, On the Origin of Species, taxonomists began basing their classification systems on evolutionary relatedness (phylogeny).
B. Kingdoms
1. After Linnaeus set up his 2 kingdom system, many organisms were discovered that did not fit in either catagory.
a. Carl Von Nageli - 1857 - proposed placing bacteria in the plant kingdom.
b. Ernst Haekel - 1866 - suggested placing all unicellular organisms in the Kingdom Protista.
c. Edward Chatton - 1937 - Proposed the term procaryote to describe cells with no nucleus.
d. R.G.E. Murray - proposed the Kingdom Procaryotae for organisms w/o a nucleus.
2. Robert Whittaker - 1969 - proposed a five kingdom system.
a. Procaryotae (Monera) - All procaryotes.
b. Protista - Mostly single-celled organisms that are much more primitive than higher plants, animals, and fungi.
c. Fungi - includes yeasts, molds, and mushrooms.
d. Plantae - includes flowering and non-flowering plants.
e. Animalia - Multicellular heterotrophs which includes animals with backbones (vertebrates) and invertebrates.
3. Carl Woese - 1978 - proposed that all organisms be grouped into 3 kingdoms.
a. He based his classification on nucleotide sequences of ribosomal RNA..
1) If two groups are closely related, then they shared a common ancestor which passed on a similar sequence of bases in the rRNA
b. By comparing rRNA, Woese discovered two very dissimilar groups of bacteria: the Archaeobacteria and the Eubacteria.
c. When rRNA of all eucaryote groups were compared, the differences were much smaller compared to the differences between either the Eubacteria and Eucaryotes or between Eucaryotes and Archaeobacteria.
1)
Thus Woese concluded that the eucaryotes should all be placed in the same kingdom, the Eucaryotes.d. The Archaeobacteria is a very diverse group of organisms.
1) Characteristics:
a) Cell walls never contain peptidoglycan
b) Live in extreme environments; eg. deep sea vents, hot and acidic environments, and very salty habitats.
c) Carry out unusual metabolic processes.
III. Nomenclature
A. Binomial Nomenclature - also developed by Linnaeus.
1. Linnaeus gave each organisms that he described a two-part name (binomial):
a. Genus - Escherichia
b. specific epithet - coli
2. Scientific Name - E. coli
B. Taxonomic Rules:
1. Genus is always capitalized.
2. Specific epithet is never capitalized.
3. Both the genus and specific epithet are underlined or italicized.
4. Higher taxonomic levels are capitalized but not underlined.
C. To name a new species, descriptions must be published, along with the binomial in the International Journal of Systematic Bacteriology.
D. The entire bacteriological classification scheme has been published in Bergey’s Manual of Systematic Bacteriology.
E. Another useful reference is Bergey’s Manual of Determinative Bacteriology.
1. It is useful for identification purposes, but doesn’t list the entire classification for bacteria.