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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 pre adaptation 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.


A small, infectious, obligate intracellular parasite. The virus genome is composed of either DNA or RNA. Within an appropriate host cell, the viral genome is replicated and uses cellular systems to direct the synthesis of other viral components. Progeny viruses are formed by de novo assembly from the newly synthesized components within the host cell. Transmission of the progeny viruses occurs by release from the host cell, and infection of new host cells.


A very large group of microscopic fungi that live on plant or animal matter. Most are filamentous organisms and produce spores that can be air-, water-, or insect-borne. A common trigger for allergies .
Mold is a fungus that produces a superficial growth on various kinds of damp or decaying organic matter Mold is a name for certain multi cellular organisms of the various classes of the kingdom Fungi, characteristically having bodies composed of a cottony mycelium. The colors of molds are caused by the spores, which are borne on the mycelium. Most molds are saprobes and can obtain moisture and nutriment from fruits, vegetables, jelly, cheese, butter, bread, silage, and almost any dead organic matter. Among the commonest forms is the black bread mold (Rhizopus nigricans), which grows on decaying vegetables and fruits as well as on bread. Some molds, e.g., species of Penicillium, are useful in the preparation of Camembert, Roquefort, and other cheeses. Penicillin and other antibiotic substances are also obtained from molds. A few molds are pathogenic, e.g., those which cause ringworm and other skin diseases and several which cause diseases of plants. Some molds produce toxic chemicals called mycotoxins that can cause serious diseases (see ergot). Some organisms traditionally thought to be mold (e.g., slime molds) have now been placed in the kingdom Protista.