> But the idea that N. ceranae might have been a mutation of N. apis prompted by Fumagillin use seems extremely farfetched to me.

Yes, I admit it is farfetched. But the concept is not farfetched. We have seen that bacteria and fungi are rapidly evolving organisms. This is especially true under the following circumstances: 1) host switch. 2) competition from similar species. 3) pressure from antibiotics and other novel compounds. It has been shown that very small levels of antibiotics can cause mutations  to increase. This may have an evolutionary basis in that microorganisms frequently try to poison each other. Many of these same organisms are the source of modern antibiotics. But in nature, the best defense microorganisms can employ is the ability to mutate. This can occur by recombination, random changes or incorporation of  genetic material from other organisms. 

* * *

Antibiotics target essential functions and therefore impose a
strong selective force for genetic resistance. At subinhibitory con-
centrations, antibiotics can also act as mutagens, driving the
emergence of drug-resistant mutants, a phenomenon that has
been demonstrated in vitro for different bacterial species and a
variety of antibacterial classes.

Journal of Antimicrobial Chemotherapy Advance Access published September 26, 2013

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