Does taking olive leaf extract effect the good bacteria?

That probably has not been studied: olive leaf extract is a relatively ‘new’ supplement. Often with such supplements any research is of poor quality, in vitro (test tube/ petri dish) not in vivo (in real people, uses few subjects or very short duration.

Research into far more established supplements or wholefoods and the gut flora is still very much a work-in-progress.

Authors like Dr Michael Mosley or Professor Tim Spector are distilling the latest research: if they don’t discuss a product it is likely because there is no evidence base.