Probiotics won't do you any harm, except to your wallet. The bacteria therein may not even get past your stomach acid. There's a massive multi-million pound industry which tries to persuade us that our gut health can be improved by drinking small amounts of inexpensively-produced watery cultures. The evidence is thin, and always will be, as these products are classed not as medicines but as foods, which means that they will never be subjected to the rigour of clinical trials. Mmmm, Danone...

Prebiotics are all about substances which we can't digest but which our gut bacteria can. I haven't found any evidence that focusing on weird and wonderful (and expensive) "prebiotics" will do you any more good than seeking out lovely, inexpensive wholefoods that are rich in fibre.   

As for faecal transplants, there is huge money to be made. A shitload, in fact. Whilst anecdotal evidence from a few people looks promising (if you can believe that the poo-recipients are objective and not just weirdos who will try anything, and never admit that they've wasted their money), there has been nowhere near enough research on this for it to be anything like recommended. Still, if anyone feels like jumping the gun, I can provide the required material, at considerable cost, of course, and on the understanding that shipping is at your own risk...