Not just any probiotic - there's a lot of unresearched crap being sold, some even dangerous (e.g. some soil based organisms can become opportunistic). I can recommend L. Plantarum 299v (this strain reportedly does not produce d-lactic acid, which is really desirable, as probiotic use could lead to acidosis [2], [3]), E. Coli Nissle 1917, and maybe a well researched soil-based mix (Prescript Assist maybe, they claim that it's the best researched probiotic in its class). Testing your biome at ubiome could provide you with info on what is missing, or what group of organisms is underrepresented. I haven't tried that yet though.
Sorry for not providing links to studies for all my claims, as I'm not at home, but all that should be easy to back up by searching at pubmed.
[1]: https://www.ncbi.nlm.nih.gov/m/pubmed/23384445/
[2]: https://med.virginia.edu/ginutrition/wp-content/uploads/site...
[3]: http://hkjpaed.org/pdf/2006%3B11%3B246-254.pdf
Update: Added another link. Why the downvote?