Carolus Linnaeus (1707-1778), a Swiss botanist, was the first researcher to formalize the science of taxonomy. Linnaeus realized that the names given to organisms varied from language to language, and proposed that all living things be given formal, two-part names, via a process he called binomial nomenclature. The binomial would consist of a generic name (the genus) and a specific epithet (specific name) called the species. He also proposed that all naming be done in Latin, since this language was no longer used in any country, thus preventing the confusion associated with differences in the names of living things based upon the native language of the researcher.
With a working system for naming organisms,
it would now be possible to construct larger taxons to further group organisms.
These groups are placed into heirarchical order from the largest and most
general, to the smallest and most specific, such that at the most specific
level only those organisms which share many characteristics are included.
While all of these groupings, called taxons, are essentially artificial
constructs, they serve to allow investigators to differentiate between
populations in an efficient and logical manner, and gain some understanding
about how each population is related to all of the others.
Prions are bits of infective protein which can replicate in animal host cells and have been implicated in such diseases as kuru, a disease of the brain which is found only among certain tribes of New Guinea who at one time practiced cannibalism, scrapie, a disease found in sheep, bovine spongiform encephalopathy (mad-cow disease), Creutzfeldt-Jacob disease, and possibly Alzheimer's disease in humans.
Viroids are infectious RNA particles which cause plant disorders such as potato spindle-tuber disease, Chrysanthemum stunt, and tomato atypical stunt diseases.
Viruses are obligate intracellular parasites composed of a protein coat called a capsid surrounding nucleic acids (either DNA or RNA) which can infect bacteria (the bacteriophages), and have been implicated in many diseases of plants and animals. Viruses use the host cell's own biosynthetic activities to replicate themselves, which eventually leads to cell death. This occurs either through loss of cellular energy, or the destruction of the host cell by lysis (bursting), which releases all of the newly replicated viral particles, freeing them to infect new host cells. Some viruses wrap themselves in bits of the host cell plasma membrane which becomes a viral envelope that can help to prevent the virus from being recognized by the body of the host as a foreign invader and attacked by the body's defense mechanisms. Classification of the viruses is based upon their structure, including the presence or absence of an envelope, the symmetry (shape) of the capsid and the type and nature of the nucleic acid the virus contains (DNA or RNA, single-stranded, double-stranded, linear or circular).
Prokaryotes include the Archaebacteria, the Eubacteria, and the Cyanobacteria. Classification of these organisms is based upon cell morphology (shape), cell arrangement (single cells, chains, or clusters), staining reactions (Gram and acid-fast stains), cultural characteristics (appearance in broth and on media, such as colony shape, size, and pigmentation), nutrition (oxygen requirements, energy sources, carbon and nitrogen sources, modes of metabolism), biochemical characteristics (cell wall components, type of pigment chemicals, inclusion bodies, RNA type, cell antigen types), and genetic characteristics (guanine + cytosine content, percent DNA, DNA hybridization with other species). Unlike most organisms, the genetic component of prokaryote classification is highly variable, such that organisms having as much as 30% variation in their genetic code are still considered as belonging the same species. The difference in genetic makeup can be used to place these organisms into various types or strains.
Protists include unicellular and multicellular organisms which have characteristics which are similar to those found in plants, animals, and fungi. The major groups of protists are:
1. The protozoa, which are eukaryotic, unicellular organisms including the following phyla:
a. Euglenophyta- organisms such as Euglena, which
which has the ability to ingest particles of food
through a gullet, but can also undergo photosynthesis, using a chloroplast.
b. Ciliophora- organisms such as Paramecium, which
are covered with short, hairlike structures
called cilia that are used for motility.
c. Mastogophora- organisms such as Trypanosoma,
which have a long flagellum (similar to
a cilium) for motility.
d. Sarcodina- organisms such as Amoeba, which lack
cilia and flagella, and move via the
formation of pseudopodia ("false-feet"; extensions of the plasma membrane).
e. Foraminifera- organisms similar to the sarcodinids
which have a hard outer shell composed
f. Sporozoa- organisms such as Plasmodium, which are non-motile.
2. Plant-like protists, including the algaes, which are found
in both unicellular and multicellular
forms. Major phyla include:
a. Chlorophyta- green algaes such as Chlorella
(unicellular), Spyrogyra (muticellular filaments),
and Ulva (multicellular flat body called a thallus).
b. Rhodophyta- red algae such as Hypnea, Porphyra
(called Nori; used as food), Gracilaria
(used to produce agar), and Chondrus (Irish moss; used to produce carrageenans used in
c. Phaeophyta- brown algaes such as Laminaria (Kelp; used for food) and Sargassum.
d. Chrysophyta- golden brown algaes commonly called diatoms.
e. Pyrrophyta- flagellate fire algae, including Gonylaux, which causes red tide.
3. Fungus-like protists which reproduce primarily by spores including the following phyla:
a. Oomycota- these are the "water molds" including Saprolegnia
and Phytopthera (the agent of
late-blight of potato; responsible for the Irish potato famine of the Nineteenth Century).
b. Myxomycota- the acellular slime molds, including Physarum.
c. Acrasiomycota- the cellular slime molds, including
Fungi are organisms which are heterotrophic and occur in both unicellular (yeast) and multicellular (mold) forms. All fungi are dispersed in the form of spores. Fungi are classified based on the type of spores they produce and whether or not they have a sexual stage. Major fungal classes include:
1. Zygomycetes- these fungi produce zygospores and have
both sexual and asexual forms of
reproduction. Examples include Rhizopus (bread mold) and Mucor.
2. Ascomycetes- these fungi produce ascospores, have both
sexual and asexual means of
reproduction, and include yeast and mold forms. Examples include Saccharomyces (brewer's
yeast), Candida (agent of yeast infections), and Morchella (edible morel).
3. Basidiomycetes- these fungi produce basidiospores and
have both sexual and asexual means
of reproduction. Examples include mushrooms such as Amanita (the poisonous fungi called
"angel of death") and Coprinus (the common inky cap mushroom), bracket fungi which grow
on trees, rusts and smuts, which are pathogens of ornamental and crop plants.
4. Deuteromycetes- these fungi have no known means of sexual
genera such as Penicillium (P. notatum produces the antibiotic penicillin) and Aspergillus.
Animals which are often studied in microbiology
include the helminth worms which act as parasites, including Clonorchis
(a flatworm; sheep-liver fluke), Taenia (tapeworm), Enterobius
(pinworm), Ascharis and Trichina (roundworms).
Ultimately, the goal of phylogenetics is to produce
phylogenetic trees, which can be used as hypotheses to test theories concerning
the evolution of groups of organisms from a common ancestor, and can also
serve as a means of producing more meaningful systems of classification.
Phylogenetic trees separate organisms from one another based on ancestral
characteristics found in all of themembers of the taxon, and derived characteristics
which have arisen through evolution, shared by only a few organisms, or
in only one species.
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