>We have heard a number of theories, including blaming extender patties,
however I have never seen any proof.

I haven't seen any investigation into the mechanism(s) by which AFB gained
resistance.  However, it appears likely that the constant application of
extender patties strongly selected for resistant strains, no matter where
they actually obtained the genetic material (by mutation, upregulation, or
transferrence of plasmids).
For more info google the title of a free download "Mechanisms of
Antimicrobial Resistance in Bacteria"

Randy Oliver

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