Dilian, I work with species of Bacillus that are endosymbionts of plant and animals (humans). I don't have experience with tardigrades. But Bacillus is so widely endosymbiotic with all kinds of eukaryotes I think they are probably there. You could probably do a Google to see if they have been reported to inhabit tardigrades.  If bacillus is there then it certainly may survive the extreme conditions in its endospore phase. Some of the endospores are virtually indestructible. We often heat them up to temperatures that kill other microbes to stimulate germination of the endospores. I've seen the endospores surviving autoclaving under sterilization conditions. Look for Bacillus in your tardigrades. 

Dear Dilian,

there are no recent publications known to me, but some very old ones are reparted from the beginning of last century, for both bacteria and some ectosymbionts. If you want to have detrails about that you may search reserachgate for Ralph Schill University of Stuttgart and contact him directly. He is an expert in tardigrades.

Best Regards,

Mike

Dear Colleagues, Thanks a lot for your answers! Have a nice work!

Cheers!

Dilian

Provided below is the information on recent discussions in the horizontal gene transfer from bacteria etc to tardigrade Hypsibius dujardini.

https://www.ncbi.nlm.nih.gov/pubmed/26598659

https://www.ncbi.nlm.nih.gov/pubmed/27035985

https://www.ncbi.nlm.nih.gov/pubmed/27069789

https://www.ncbi.nlm.nih.gov/pubmed/27173900

https://www.ncbi.nlm.nih.gov/pubmed/27173901

Hi all, if you take the time to read this publication (https://www.ncbi.nlm.nih.gov/pubmed/27069789) you will learn that some bacterial genomes are detected in the Tardigrade genomic sequencing data from multiple projects. We discussed the possibility of a symbiosis and functional interactions. However, this has not been demonstrated, to the best of my knowledge.