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Bacteriophages, also known as phages, are viruses that attack bacteria. Phages are among the most complex viruses. Their most common structure consists of a long nucleic acid molecule (usually DNA) coiled within a polyhedral head. Many phages have a tail attached to the head. The phage may use fibers extending from the tail to attach to a bacterium.
A viral reproductive cycle can be lytic or temperate. In a lytic cycle, the virus lyses (destroys) the host cell. When the virus infects a susceptible host cell, it forces the host to use its metabolic machinery to replicate viral particles. Viruses that have a lytic cycle are described as virulent (lethal).
Five steps are typical in viral reproduction.
1. Attachment (or absorption)- The virus attaches to receptors on the host cell wall.
2. Penetration- The nucleic acid of the virus moves through the plasma membrane and into the cytoplasm of the host cell. The capsid of a phage remains on the outside. In contrast, many viruses that infect animals cells enter the host cell intact.
3. Replication- The viral genome contains all the information necessary to produce new viruses. Once inside, the virus induces the host cell to synthesize the necessary components for its replication.
4. Assembly- The newly synthesized viral components are assembled into new viruses.
5. Release- Assembled viruses are released from the cell. Generally, lytic enzymes destroy the host cell. The new viruses infect other cells, and the process begins all over again. The time required for viral reproduction from attachment to the bacterium to the release of new viruses is approximately 30-35 minutes.
Temperate viruses do not always destroy their hosts. In a lysogenic cycle the viral genome becomes integrated into the bacterial DNA and is replicated along with the host DNA. The first two steps of this process are exactly the same as the first two steps of the lytic cycle (attachment and penetration). Those steps are then followed by integration and replication. In the case of some bacterial viruses, the phage DNA becomes integrated into the host bacterial DNA and is then referred to as a prophage. When the bacterial DNA replicates, the prophage also replicates. The viral genes that code for viral structural proteins may be repressed indefinitely. Bacterial cells carrying prophages are called lysogenic cells. Certain external conditions, such as ultraviolet light and x-rays, can cause temperate viruses to revert to a lytic cycle and then destroy their host.
Sometimes temperate viruses become lytic spontaneously. Bacterial cells containing certain temperate viruses may exhibit new properties. This is called lysogenic conversion. An interesting example involves the bacterium that causes diphtheria. Two strains of this species exits, on that produces a toxin (and causes diphtheria) and one that does not. The only difference between these two strains is that the toxin-producing bacterial contain a specific temperate phage. The phage DNA codes for the powerful toxin that causes the symptoms of diphtheria. In the same way, the bacterium that causes botulism (a serious form of food poisoning) is harmless unless it contains certain prophage DNA that induces synthesis of toxin.
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