> Apparently one spore won't do the job alone 

This has always been the case.  I remember a study that I think found "11
spores" to be the minimum number to infect a single larvae, but I don't
remember the name or author.  Anyway, so many spores is hard to transmit on
a hive tool, and even harder to transmit on "shaken bees", be they in
packages or in a AFB-prompted "shake down", as I explained in a prior post.

So, all those years of squeaky-clean hive tools, and buying dentistry
autoclaves to clean them... maybe it was more for morale.

> and some strains are harder to infect.

I would have to see some data on this, as one has to wonder what would stop
a spore from culturing and growing in the hemolymph of bee larvae, yet not
be fatal to the larvae.  And how could such a variation in the hemolymph
exist between strains, and yet have been overlooked to date?  

Everyone uses "MYPGP agar" for culturing AFB, as it is so difficult to get
it to culture in the other common media types, but the recipe does not have
to be very accurately followed to get spores to culture and grow well.  This
also tends to make me think that AFB would be tolerant of minor variations
in hemolymph due to hybridization.

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