I would like to take exception to the recent explanation for high
particle/pfu ratios as merely reflecting an excess of noninfectious particles.
This explanation presumes that the efficiency of infection is 100%- and
that is clearly untrue for most if not all viruses. Clearly, a number
of cxritical events must progress for the final outcome of an infection
to yield infectious progeny, the absolute requirement for plaque formation.
A viable particle that infects a cell and kills it (or doesn't kill it)
fails to produce progeny virus particles will not yield a plaque. I
believe experiments have been done in which the virus containing supernatant,
after the usual excess adsorption period, was still found to have pfu's
in it. In fact, we published a paper on a urea-resistant poliovirus mutant
which attached, eluted, but remained infectious.
Particle/pfu ratios can be a real pain in devising or explaining
certain kinds of experimental results.