NASA uploaded a pretty neat video about understanding gut bacteria. The video starts by stating there are 3 pounds of bacteria in the human gut. Wow! Dr. Fred Turek of Northwestern Universities Center for Sleep and Circadian Biology is studying that bacteria in astronauts on the International Space Station.
When Scott Kelly went to live in the orbital complex for a whole year, one of areas scientists were studying was the bacteria inside Kelly's gut and how they changed (or didn't) over the yearlong stay in microgravity.
Scientists want to know what the effect the space environment has on the development of the bacteria. They want to see if certain bacteria out-compete others to upset the balance.
If it turns out there were no changes in the microbiome of Kelly's gut, then a human mission to Mars would likely also not produce any major changes. However, if there are changes – say one bacteria gets stronger than another and puts the ecosystem out of balance – then ways to counteract the changes will have to be developed.
Video courtesy of NASA Johnson