4. it's a temperate
phage: being either lytic or lysogenic.
Two options are open to lambda phage upon infection
of its host bacterium, E. coli. The lambda DNA may behave in
a lytic or intemperate manner and enter the lytic cycle;
that is to say, phage genes are expressed and phage DNA is replicated
to reproduce many phage particles (± 100), which are eventually
released by the lysis of the cell induced by the phage.
Lambda may exist
in harmony with its host cell in a temporary noninfectious manner
known as lysogeny. Lysogenic (temperate)
bacteria carry a phage DNA in the form known as prophage,
when the phage genome is integrated as a part of the bacterial
chromosome. The integrated prophage is inherited in exactly the
same manner as bacterial genes; but the prophage may be induced,
for example, by ultraviolet irradiation, to enter the lytic cycle,
in which case the phage DNA is released from the bacterial chromosome.
A prophage is equivalent to a provirus.
The choice of
whether a phage should enter the lytic cycle or form a lysogen
upon infecting a host cell depends upon the conditions of infection
and the genotypes of phage and bacterium.
a bacterial culture with an excess of a phage to which it is
sensitive results in lysis of the bacteria. The ratio of phage
particles to bacteria is usually described as the multiplicity
of infection. With lytic phages, which are able only
to enter the lytic cycle and cannot establish lysogeny, addition
of a phage suspension (at a high multiplicity of infection, that
is > 1) to a culture of sensitive bacteria growing in liquid
medium results in a clearing of the culture after all the bacteria
have been infected and lysed.
incubation, the usual turbidity may be regained; this is due
to the presence in the population of bacteria resistant to the
phage, which were not susceptible to infection and thus can reproduce
to repopulate the culture. With temperate phages, however, the
clearing of the culture is only transient and is rapidly overtaken
by the reappearance of turbidity, in this case due to the presence
of cells in which lysogeny was established; because of their
immunity to subsequent lytic infection, the lysogenic bacteria
reproduce and thus restore the turbidity. The activity of a phage
preparation is usually assayed by examining its ability to form
plaques in a bacterial culture grown on agar.
Phage particles are mixed with an excess of bacteria in suspension
(that is, at a very low multiplicity of infection), so that each
phage particle infects a single bacterium (this bacterium is
surrounded by numerous uninfected ones). When the mixture is
poured onto an agar plate, it hardens so that the bacteria are
immobilized in fixed positions. Lytically infected bacteria burst
at the end of the lytic cycle to release a large number of progeny
phage particles; these particles infect neighboring bacteria,
which suffer the same ultimate fate of lysis. The spread of phage
generations through neighboring cells finally is halted by a
decline in bacterial metabolism, the result being the production
of clearings in the confluent bacterial culture; these clearings
are known as plaques. you can learn how to pick plaques.