> > As Allen points
> > out taking colonies off treatment does not necessary result
> > in an outbreak of AFB.

 My guess is that long term use of antibiotic should reduce the spore
count.
 As spores become active the antibiotic present gets them. I guess
there  is
 some density of the bacteria that would need to be present in the gut
of a
 larvae to kill it before it became an adult. If you stay below that
level
 you don't start the "kill larvae"/"clean scale from cell"/"reinfect
larvae"
 cycle with the house bees.
Adding to the above:
 Bucher(1958) determined the LD50* of bacillus larvae to be 35 spores
in  one-day-old honey bee larvae.
*LD 50 The dose(number of microorganisms) that will kill 50% of the
animals(honey bee  larvae) in a test series.
Bob