I'm guessing you're asking about the specific ATB UR which is sold by bioMerieux. The paper linked by Maureen will give you a good overview of MIC testing procedures, so do read it. The ATB system is a simplified method of checking susceptibility, and here's a brief how-to: http://www.techmicrobio.eu/documentation_fabricants/Biomerieux%20et%20API/ATB/14039b_ATB%20UR.pdf The PDF contains references that should explain the system further.