I heard that probiotics taken daily have not been proven to be very useful (body attacks them as not its own flora) . true?

ProbioticsPrebiotics: It's not easy to alter intestinal flora (bacteria) with probiotics alone. Even probiotic (healthy bacteria) have to eat. Your diet, environmental exposure, antibiotic usage can all alter your "microbiome." there is some evidence that even "dead" or expired probiotic formulations are helpful. Horizontal gene transfer (swapping of genes between bacteria) can still occur, thus conferring benefit.