Lwoff's Pathways - Viral Replication
Virus is from the Latin word for "poison." The term remains an apt
one since viruses constitute one of the greatest threats to human
health. One group of viruses, Herpes simplex (HSV), is the most
commonly transmitted pathogen in industrialized countries. One type
of HSV causes genital herpes - an incurable, sexually transmitted
disease suffered by 30 million people in the United States.
HSV illustrates an interesting property of certain viruses - it can
take both active and latent forms. During the active phase, the virus
interferes with the cell's normal metabolism, causing the symptoms
associated with the disease - including painful genital blisters.
During the latent phase, it's as if the virus has gone to sleep.
Although the host cells remain infected, the host is a symptom-free
carrier of the disease.
We now know the difference between the active and latent
manifestations of viral infection is a result of a switch in viral
replication patterns. Some viruses can only replicate by what is
called the lytic pathway. They enter and inject a host cell with DNA,
forcing it to make new viruses. At some point bursting at the seams
with the viruses, the host breaks open, releasing new pathogens into
the environment. Other viruses operate differently: they enter and
inject their DNA into the host cell - but instead of taking over the
host cell and using it to make viruses, the injected DNA can become
inactive for some time, until the appropriate cellular event triggers
its awakening. This latter pathway is called the temperate or
The lysogenic pathway was discovered in bacterial viruses
(bacteriophages) in the early 1920s, but was not really understood
until the 1950s, when it was explored at the cellular level by Andre'
Lwoff, a French scientist. At that time, it was known that some
bacterial cultures that grew normally and otherwise seemed perfectly
healthy were actually infected by phage. Although the phage didn't
seem to interfere with the host bacteria, such cultures had the
ability to cause the lysis or rupture of other bacteria. Thus, the
culture was described as "lysogenic."
It was not clear why such cultures were lethal to other bacteria. The
lysogenic effect didn't seem to stem from phage particles floating
around in the culture, since the cultures remained lethal even after
treatment by methods that removed any free floating phage. Nor was
the effect due to some large reserve of phage stored within the host
cells - no phage were released when the cells of a lysogenic culture
were artificially caused to burst open.
Lwoff's definitive studies on the subject relied on the time-honored
technique of persistent, careful observations. He observed the growth
of single bacterial cells of Bacillus megaterium, a really large
bacterium, in tiny droplets of medium. Close observation revealed the
secret - although free phage particles were never found floating
around in droplets that contained only single cells, they were found
in the small colonies derived from single cells. Where did they come
from? Lwoff actually observed the answer - occasionally, while a
culture in a droplet was being watched, a single cell in it would
spontaneously burst, releasing about 100 phage.
Lwoff concluded that the host cells were not really entirely immune to
the phage. When a phage did become active in a bacterium in the
lysogenic culture, it forced the host to manufacture more phage,
eventually killing the host, and releasing new phage when the cell
burst open. However, the switch from the lysogenic to the lytic
pathway was the exception rather than the rule. Most of the time, in
most of the bacterial host cells, the phage was in an inactive form.
Later, Lwoff found that it was possible to artificially induce all the
cells in a lysogenic culture to enter the lytic pathway simultaneously
by exposing the cultures to UV light, or X-rays.
Lwoff's careful experiments laid the groundwork for further research
on the interaction between the phage and their hosts in lysogenic
cultures - research that turned out to have far-reaching implications.
It was by studying this interaction that he made an important
discovery: that the genes injected into a bacterium by a phage can
actually become integrated into the host's genome - a discovery that
is the basis for much of genetic engineering.
Go to Graphics Gallery: Examples of Viral
Go to: The Institute of Molecular Virology at the University of Wisconsin, Madison
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