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particle to pfu ratios

ryan at mbcf.stjude.org ryan at mbcf.stjude.org
Wed Jul 26 10:44:16 EST 1995

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

short answer: 
yes, it means that 98% of the particles are  non-infectious "junk".

somewhat longer answer: 
The strategy of the plaque assay is to isolate (by limiting dilution)
individual infectious virus particles. Because of this limiting dilution, 
a scorable event (a plaque) is interpreted as the result of a single
_independently infectious_ particle. Noninfectious particles (includes the 
totally dead, as well as viruses with any defect that prevents the generation 
of infectious progeny without complementation) don't show up. So, to go beyond
the short answer above, the junk particles which show up in the total particle
count but don't show up in the PFU titre may include ones that are completely
nonviable (e.g. empty capsids lacking any genetic material) and may also
include the merely defective (ts mutants, small or large deletions, whatever)
that could replicate with a little wt assistance. The plaque assay is designed
to avoid that assistance: an individual particle must make the plaque on its
own to be scored as an infectious hit. Bottom line: junk particles might be
able to replicate if complemented by other defectives or by wt, but if they
can't infect alone they don't score as PFUs.

Cheers, Kevin.

	Kevin W. Ryan
	Department of Virology & Molecular Biology
	St. Jude Children's Research Hospital
	Memphis, Tennessee 38101-0318, U.S.A.
	phone: (901) 495-3411
	fax:   (901) 523-2622
	Internet: ryan at mbcf.stjude.org

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