Yes, gut bacteria are very important, they serve many functions, not all of which are understood fully. For example, it is estimated that 90 percent of the body's serotonin is produced in the gut, it’s the chemical that helps with sleeping, eating, and digesting. If you want to read more, have a look on PubMed for articles relating to 'Gut Microbiome' like this, https://www.ncbi.nlm.nih.gov/pubmed/?term=gut+microbiome