Three years ago, we (Kris Niyogi and I) purchased some M2 seeds from Lehle
Seeds in Tucson. We had the same problem with the unsterilizable microbe.
We tried many different methods of sterilization, including 100% EtOH
followed by 95% Clorox+triton solution, all to no avail. We called the
owner of the greenhouse and she said that this was a common problem with
her (greenhouse grown) seeds. We were left using her seeds only for
screens/selections that were done on soil. Lehle is the only vendor
of A. thaliana seeds that I know of, unfortunately.