Cecile, I reached out to Martin with your question, and he had this to say:

Hi, Cecile! Thanks for your question. I’m unfamiliar with Jim’s method — did you start a preferment with yeast and then continue feeding it with only flour and water? I do believe that over time (if you started with yeast) that bacterial populations might also take hold. However, I can’t point to specific scientific literature which might confirm this. Years ago I think I made a starter following a method which included a little yeast — I remember that it worked well and did contribute good flavor to my loaves. I say go for it!
Happy baking,
Martin

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