All bacterial noted to be Non-Pathologic in Human Gut ,they are so because their structure and EXOCyTOTIC molecules are in symbiotic relationship with host or different specie ,therefore may not cause harm but could be dangerous to their host  when their microbiota population overwhelm the system and the symbiotic relationship between otherwise different neighboring species   or with  the host is distorted ,or there is antibiotics  genetic resistant that  resulted from treatment of pathogenic organisms.Therefore arming once self with this information,there is no safe or good gut flora.  except flora less virulent  with gut symbiosis.

Please check the below given link.

http://www.sciencedirect.com/science/article/pii/S2352304214000130

The best bacteria in gut of warm blood animals is Bafidiobacteria or Lactobacilli as Lb.acidophilus and B.bifidium

Dear Paola,

Please check the following.

Regards

SM Najim

http://www.drdavidwilliams.com/restore-and-improve-gut-bacteria/

http://articles.mercola.com/sites/articles/archive/2011/09/24/one-of-the-most-important-steps-you-can-take-to-improve-your-health.aspx

http://www.everydayhealth.com/columns/therese-borchard-sanity-break/ways-cultivate-good-gut-bacteria-reduce-depression/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3539293/pdf/10.1177_1756283X12459294.pdf

i hope this paper will help you.

http://gut.bmj.com/content/65/11/1906.full.pdf+html

Thank your very much.

Kind regards

Jeanne