I'm not sure about chocolate cravings. The scientist I heard talk had done some experiments on her self that eating chocolate for a while produced cravings but I guess that could be psychology rather than bacteria. There have been solid studies on gut bacteria making people fat and thin, presumably by effecting their desire to eat. See eg http://www.nytimes.com/2013/09/06/health/gut-bacteria-from-t...

The experiments basically fed gut bacteria from thin creatures, (mice / humans) to fat ones (mice) and they lost weight and visa versa for fat to thin. I guess you could extend that experiment quite easily to mice and chocolate though I'm not sure it's been done yet. Maybe if anyones got some experimental mice and bars of chocolate knocking around they could report back?