I don't think preons are technically mirror molecules, but it's close enough from a risk perspective. If rogue AI were to do us in through bioengineering, it might be by creating a novel prion.
Scrapie is incredibly durable in the environment; the prion toxins can remain transmissible in the soil for a decade, causing disease years after it was shed by its previous host.
https://en.wikipedia.org/wiki/Transmissible_spongiform_encep...
"Cattle are believed to have been infected by being fed meat-and-bone meal that contained either the remains of cattle who spontaneously developed the disease or scrapie-infected sheep products...
"The British Government enquiry took the view that the cause was not scrapie, as had originally been postulated, but was some event in the 1970s that could not be identified."
https://en.wikipedia.org/wiki/Bovine_spongiform_encephalopat...