I had this i too i stoped sterilizing now, they no longer smell, mine have changed colour though.
probs time to get some new ones.
its the BPA free bottles I am afraid; they are more porous and thus absorb bad smells more easily. OmarsMUms idea sounds good; I also used to use a product called dr bronners sal suds its a cleaning product made out of natural ingredients, expensive but you only need a tiny drop to clean several bottles and I found it would clean absolutely any residues off (both smells and physical residues) xx
yes mine say bpa free? what does this mean?
Have you had the bottles for a while? I never had this problem with Nuk glass botlles but have it with the TT bottles that are a bit old. It is harder to get the grease out of the inside of the bottle as they get older. I just change them every few months.