Just to provide an alternate anecdote, I take probiotics fairly regularly (one to three in sequence every 1-2 weeks, or whenever I get out of whack) and have only had neutral or positive results. The flora in the probiotic is really nothing that shouldn't already be in a healthy gut, although of course you do want to be very careful as it's clear that your gut micro-biome is very important to your overall health. Just another data point.