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Microbiology

Microbiology is the study of microorganisms, including unicellular
(single-celled) eukaryotes and prokaryotes, fungi, and viruses. Today, most
of the work in microbiology is done using methods from biochemistry and
genetics. It is also related to pathology, as many microorganisms are
pathogens.

Microbiologists have made many fundamental contributions to biology,
especially in the fields of biochemistry, genetics, and cell biology.
Microbes have many traits that make them ideal model organisms:

   * They are small, therefore they do not consume many resources
   * Some have very short generation times (~30 minutes for E. coli)
   * Cells can easily survive in isolation from other cells
   * They can reproduce by mitotic division, allowing for the propagation of
     genetically identical (clonal) populations.
   * They may be frozen for long periods of time. Even if 90% of the cells
     are killed by the freezing process, there are millions of cells in a
     milliliter of liquid culture.

These traits allowed Joshua and Esther Lederberg to devise an elegant
experiment in 1951 demonstrating that adaptive mutations arise from
preadaptation rather than directed mutation. For this purpose, they invented
replica plating, which allowed them to transfer numerous bacterial colonies
from their specific locations on one agar-filled petri dish to analogous
locations on several other petri dishes. After replicating a plate of E.
coli, they exposed each of the new plates to phage. They observed that
phage-resistant colonies were present at analogous locations on each of the
plates, allowing them to conclude that the phage resistance trait had
existed in the original colony, which had never been exposed to phage,
instead of arising after the bacteria had been exposed to the virus.

The extensive characterization of microbes has allowed them to be used as
tools in other branches of biology:

   * Bacteria (especially E. coli) may be used to replicate DNA in the form
     of a plasmid. This DNA is often chemically modified in vitro then
     inserted into bacteria to select for the desired traits and isolate the
     desired product from by-products of the reaction. After growing the
     bacteria and thereby replicating the DNA, the DNA may be further
     modified and inserted into other organisms.
   * Bacteria may be used to produce large amounts of protein using genes
     encoded on a plasmid
   * Bacterial genes have been inserted into other organisms as reporter
     genes.
   * The yeast two-hybrid system combines bacterial genes with genes from
     the organism being studied and inserts them into yeast cells to study
     protein-protein interactions within a cellular environment.

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