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Summary Chapter 10: Classification of microbes (MBY161)
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- University Of Pretoria (UP)
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CHAPTER 10: CLASSIFICATION OF MICROORGANISMS The science of classification, especially the classification of living forms, is called taxonomy
The objective of taxonomy
o is to classify living organisms—that is, to establish the relationships between one
group of organisms and another and to differentiate them
o Taxonomy also provides a common reference for identifying organisms already
classified.
characteristics of that isolate are matched to lists of characteristics of
previously classified bacteria to identify the isolate
o Finally, taxonomy is a basic and necessary tool for scientists, providing a universal
language of communication
In 2001, an international project called the All Species Inventory was launched. The project’s
purpose is to identify and record every species of life on Earth in the next 25 years
Similiarities between organisms
o For example, all organisms are composed of cells surrounded by a plasma membrane,
use ATP for energy, and store their genetic information in DNA. These similarities are
the result of evolution, or descent from a common ancestor.
Why we use taxnomy
o To facilitate research, scholarship, and communication
taxon
o subdivisions used to classify organisms eg domain, kingdom, phylum
systematics or phylogeny
o the science organizing groups of organisms into a hierarchy
1735 Carolus Linnaeus Plantae and Animalia
1857 Carl von Nageli Bacteria and fungi in
Plantae (Fungi is closer
related to animals than
plants)
1866 Ernst Haeckel Protista: Bacteria,
protozoa, algae and fungi
1968 Robert Murray Kingdom Prokaryotae
1969 Robert Whittaker Monera as part of 5
kingdom
1978 Carl Woese 3 domains (eukarya,
bacteria and archaea
2 types of prokaryotic cells and one type of eukaryotic cell
Three domains
o The discovery of three cell types was based on the observations that ribosomes are
not the same in all cells
Comparing the sequences of nucleotides in ribosomal RNA from different
kinds of cells shows that there are three distinctly different cell groups: the
eukaryotes and two different types of prokaryotes—the bacteria and the
archaea
In addition to differences in rRNA, the three domains differ in membrane
lipid structure, transfer RNA molecules, and sensitivity to antibiotic
o 3 domains are Domain Eukarya (all eukaryotes; animals, plants, fungi and protists),
Domain Bacteria (domain of prokayotic organisms, characterized by peptidoglycan
cell walls) and Domain Archaea (domain of prokaryotic cells lacking peptidoglycan)
, o Domain Archaea often lice in extreme environments and carry out unusual metabolic
processes
o Archaea include three major groups:
Methanogens, strict anaerobes that produce methane (CH4) from
carbon dioxide and hydrogen
Extreme halophiles, which require high concentrations of salt for
survival
Hyperthermophiles, which normally grow in extremely hot
environment
o According to the endosymbiotic theory, eukaryotic cells evolved from prokaryotic
cells living inside one another, as endosymbionts
A phylogenetic tree
o grouping organisms according to common properties implies that a group of
organisms evolved from a common ancestor; each species retains some of the
characteristics of the ancestor
o when fossils cannot be used as evidence (due to not all organisms being readily
fossilised) other evidence must be used like similiarities in genomes
o molecular clock --- an evolution timeline based on nucleotide sequences in organisms
because mutations accumulate at a constant rate we can compare the
organisms and provide an estimate of when the two diverged from a common
ancestor
use DNA hybridization and rRNA sequencing to gain a better understanding of the
evolutionary relationships among prokaryotic groups.
CLASSIFICATION OF ORGANISMS
Scientific nomenclature
o it is used because common names can be misleading and are different in different in
different languages
o every organism is assign two names or a binomial
these are the genus --- the first name of the scientific name, and the specific
epithet --- the second or species name in a scientific binomial
genus is usually a noun
specific epithet is usually an adjective
this system is called binomial nomenclature --- the system of having
two names for each organism; also called scientif nomenclature
o enables them to shae knowledge efficiently and accurately
Taxonomic Hierarchy
o eukaryotic species--- a group of closely related organisms that can interbreed
o family --- a taxonmic group between order and genus
o order--- a taxonomic classification between class and family
o class— a taxnomic group between phylum and order
o phylum--- a taxonomic classification between kingdom and class
o kingdom --- a taxonomic classification between domain and phylum
o domain --- a taxonomic classification based on rRNA sequences; above the kingdom
level
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