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particle to pfu ratios

Steven Enkemann enkemans at dc37a.nci.nih.gov
Wed Jul 26 16:17:48 EST 1995

In article <demers-2407952306580001 at demers.cts.com>, demers at cts.com (Bill
Demers) wrote:

> Suppose a purified virus prep has a particle number:pfu ratio of 50:1.
> Does this mean that 49 out of 50 particles are non-infectious and most of
> the prep is "junk"
> or
> that pfu is a concentration of virus required to detect a single
> infectious event, ie. 50 infectious particles exposed to cells has the
> chance of forming a single plaque.

The answer is perhaps a little of both.  It depends on the virus you're
talking about, how late in infection the sample was obtained, and how the
virus was cultured to create the sample you're measuring.  Many viruses
produce more and more defective virions as an infection progresses.  (A
virion is a completely assembled virus particle.)  These defective virions
may be indistinguishable from viable virions in electron micrographs (or
other methods used to count particles).  For some types of viruses these
defective virions can also inhibit the infectivity of viable virions.
   But the presence of defective particles is not the whole story.  There
are many defense mechanisms that prevent an infective virion from
initiating or otherwise completing an infection.  Again what those are
depends on the system and the virus.  Obviously plants have different
mechanisms than animals.  The simplist system to illustrate this is our
own immune system. An individual who has been immunized against a virus
(say Polio) can by innoculated with 100-fold more virion particles than
would cause disease in an individual who has not been immunized.  Same
number of particles different infectivity.  There are many other levels of
barriers to infection that could be labeled defense mechanisms.
   There is also a third underlying cause for the discrepency between the
measused number of virions and the number of plaque forming units.  That
is the difficulty of initiating the infectious process itself.  A virus is
very tiny.  During the course of an infection it must initiate a series of
virally directed biochemical reactions at the same time as it usurps the
biochemical machinery of it's host.  Any biochemical reaction has a finite
probability of failing.  For a tiny virus, packaging only one or two of
the components required to initiate an infection, this becomes a form of
'Russian Roulette'.  If the initial reactions fail, or the components
required for that reaction never meet because they became lost within the
vast sea of the host, the viral infection fails.

Steven A. Enkemann PhD.

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