Animal and bacterial viruses interact with their host cells in several different ways. Some viruses take over the metabolism of the host cell completely with resulting lysis of the cell. An example is the phage T4, which infects E. coli. Others use the host cells' enzymes and metabolic machinery while the host cell continues to multiply and the virions leak out of the cell. Examples are the filamentous phages. Other viruses live in harmony with the host cell and multiply as the host cell multiplies. An example is the phage lambda (A), which also infects E. coli. This phage confers new properties on the host bacterium.
Although viruses infect all kinds of cells, the relationships between even the most highly studied animal viruses and the animal host cells they invade are poorly understood in comparison with phage-bacteria systems. This is largely because the eukaryotic host cells of animal viruses are far more complex and grow much more slowly than the prokaryotic host cells of the phage.
We will first focus on bacteriophages, which serve as excellent models for all other viruses. What you learn about them will help immensely in understanding similar relationships between viruses and the animal cells they infect, described in chapter 14.
How phages affect the cells they infect depends primarily on the phage. Some phages multiply inside the cells they invade and escape by lysing the host cell. Since more virus is produced, this interaction is termed a productive infection and the phages that lyse the cell are termed lytic. These viruses take over the metabolism of the cell and direct it exclusively to phage replication. Another type of productive infection is carried out by phage that multiply inside the cells they invade, and then leak out or extrude without killing the host cells. These phages partially take over all the metabolic processes of the cell. An example is the filamentous phage, M13. Still other phages, termed temperate (temperatus means controlled), integrate their DNA into the genome of the bacteria they infect or the DNA replicates as a plasmid. The phage replicates as the bacterial DNA replicates. The infection is termed latent because there may be no sign that the cells are infected. The bacterium carrying the phage DNA is a lysogen and the cell is in the lyso-genic state (lysis means dissolution). The phage DNA often codes for proteins that modify the properties of the host, the phenomenon of lysogenic conversion. An example of a temperate phage that infects E. coli is lambda (A). These different relationships are shown in figure 13.4. ■ plasmid, p. 66
Some of the phages that undergo these three kinds of relationship with their host bacteria are listed in table 13.2.
Genetic alteration of host cell
Disease of host cell
Genetic alteration of host cell
PRODUCTIVE INFECTION More virus produced
LATENT STATE Nucleic acid of virus becomes part of host cell DNA
Lysis of cells— release of virions
Release of virions— non-lysis of cells
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