Even bacteria can get sick, usually as the result of infection by a virus. Bacterial viruses are sometimes referred to as bacteriophages, or phages for short. Phage comes from a Greek word meaning to eat. When bacteria catch a virus, they do not merely get a mild infection, like a cold, as humans usually do. They are doomed. The bacteriophage takes over the bacterial cell and fills it up by manufacturing more bacteriophages, as shown in Figure 2.29. Then the bacterial cell bursts and liberates the new crop of bac-teriophages to infect more bacteria. This takes only about an hour or so. In a matter of hours, a bacteriophage epidemic could wipe out a culture of bacteria numbering several times the earth's human population.
Bacterial viruses infect only bacteria. Some have relatively broad host ranges, whereas others infect only a single species or even just a few particular strains of bacteria. Generally speaking, any particular disease, whether caused by bacteria or by viruses, infects only a closely related group of organisms.
bacteriophage A virus that infects bacteria rickettsia Type of degenerate bacterium that is an obligate parasite and infects the cells of higher organisms
Viruses are packages of genes that are not alive by themselves but may take over living cells. Once in control the virus uses the cell's resources to manufacture more viruses.
FIGURE 2.29 into a Cell
Components of a new virus are synthesized under the direction of vi ral DNA but using the synthetic machinery of the host cell. First (1) a virus binds to the host cell and then (2) inserts its nucleic acid into the host cell. The synthetic machinery of the host cell then manufactures the viral proteins and nucleic acids according to the genetic information carried by the viral DNA (3). Finally, the virus causes the cell to burst, releasing the newly synthesized viruses (4) that seek a new host. The host cell dies as the result of the viral infection.
An immense variety of viruses exists (see Chapter 17 for more details). Vi ruses infect every other life-form, from bacteria to eukaryotes, including humans.
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