MadSci Network: Microbiology

Re: What is a bacteriophage?

Date: Wed Dec 6 11:44:35 2000
Posted By: James S. Dickson, Faculty, Microbiology, Iowa State University
Area of science: Microbiology
ID: 975974117.Mi

A bacteriophage is a virus which infects bacteria. Viruses can infect almost all types of life, including bacteria, yeasts and molds, plants and animals. Viruses are specific to the type of life they infect. That is, a virus which infects plants will not infect humans and animals, and a virus which infects bacteria (bacteriophage) will not infect plants or animals. If you look up "virus" in a reference book, you can find out more about them. Viruses are really interesting, in that they do not meet the requirements we normally apply to living things. Viruses are parasites. They cannot grow and reproduce without a host cell to infect. They do not have any metabolism; that is, they do not consume nutrients ("eat") and they do not consume oxygen. They cannot reproduce by themselves; they must use the host cell to reproduce. Viruses consist of only one type of genetic material (either RNA or DNA), but never contain both types of genetic material. They reproduce by infecting a cell, and then "hijack" the host cells' own metabolism to create new viruses instead of new host cells.

Bacteriophages are important for a number of reasons. Bacteriophages can carry genetic material from one type of bacterium to another. The bacteriophage can incorporate part of the DNA (or RNA) from a bacterium that it infects into its' own genetic material. When it is released by the first bacterium, it may infect a second, different bacterium, and leave the gene from the first bacterium in the second bacterium's genetic material. Some important genetic characteristics of bacteria can be moved from one bacterial species to another in this manner.

A second important issue with bacteriophages is that they can kill large populations of bacteria relatively quickly. This can be important when you are intentionally trying to produce large numbers of bacteria. For example, when we make cheese, we culture (grow) bacteria in the milk to give cheese its' distinctive flavor and texture. If the milk gets infected with a bacteriophage, the culture may die out before the cheese fermentation is completed. When this happens, the cheese manufacturers have vats of partially cultured milk which is not cheese, and which usually has to be discarded. The cheese manufacturers somtimes refer to this as a vat of cheese that has been "phaged out"; that is, the starter culture to make the cheese has been destroyed by a bacteriophage. You can understand why this would be serious when you consider that cheese is made commercially in vats that may hold 5,000 to 10,000 gallons of milk.

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