I have some experience in inactivation of mycobacteria. We usually inactivate 2 loops of colonies, in 2 ml of sterile destiled water at 90°C for 30 minutes, once it is at room temperature we store it at -20°C, and after 48 hours you can save it in a -80°C. We work with this protocol, and we havent get problems, maybe you can try this with some cultures. I wish this could help a little. Finally, you have to consider the complex of the wall cell in mycobacteria, maybe you have to adjust time and temperature a little.

I guess that depends on whether you have a pure culture or if you want to selectively inactivate Gram positive bacteria in a mixed consortium. For the former you could heat inactivate as suggested above or fix them in an alcohol (ethanol or methanol work well). If it's the latter then you would have to consider something specific to Gram positives like a lysozyme treatment in an osmotically balanced buffer. 

Did you purify DNA after these protocols and how you test sterility of sample??

Yes I purified DNA from the ethanol fixed samples, I just freeze dried the alcohol treated sample and suspended the pellet in the first reagent from the DNA extraction kit I was using. I imagine speed vac would work just as well. I wasn't looking for sterility per se, we just wanted to prevent changes in the microbial community during transport. If you mean sterility as inactivation of the organism then culture it alongside an untreated (positive) control. 

we purify DNA with a home method (sorry i dont kinow the right translation) based in Chlorophorm method (from RFLP extraction method), but we have tryed kits for purifying, and i think that the quality of DNA depends of what you want to do with the material, beceause there are so many ways to extract the DNA. always depending of what you want to do with.  The method we use it is long and is not easy getting the reagents actually we are trying with kits.

Do you plan to sequence the whole bacterial genome? If yes, heating method to 90°C may not give you the quality of materials for whole genome sequencing, it may be okay with  partial sequencing, otherwise 65°C  for 20-30min is enough for inactivation.

Thank you all. But heating method give me nice results for NGS and inactivation when I heat at 90 degree for 20 min in a presence of strong salts from Hofmeister series

I found DENATOR T1 stabilizer absolutely great for bacteria inactivation

http://www.denator.com/products/stabilizor-system