Likely a sterile medium means an uncontaminated culture (food free of living organisms) that will grow only the new bacteria introduced for analysis. Source(s): http://www.thefreedictionary.com/sterile
Stonepelt was Bluestar's first mentor and a very well-respected senior warrior of ThunderClan. It is revealed in this book that he took her to the Gathering on her second day as an apprentice because she had caught a squirrel very well for a new apprentice, and the squirrel had been as big as she was.
Note: During one of the sets for the daily quest, the third daily quest hub for Golden Lotus (only able to be seen at honored+ rep), these stonebark tricksters seem to be replaced with mogu's to kill for the daily quests for the Golden Lotus. This can be irritating when farming them for sky shards and motes, but it doesn't happen too often.