The RT article Mutant superbugs menace future space station expeditions – NASA ends with:
Microgravity is known to increase a bacterium's tendency to acquire foreign genetic material and to become resistant to metals and antibiotics, factors which could predispose the ISS E. bugadensis strains toward increased virulence in the future.
A Russian experiment concluded in July similarly found that terrestrial bacteria sent to space exhibited a marked increase in aggression and antibiotic resistance upon their return to Earth. The Biorisk experiment sent 68 different organisms into space in 2005 to see whether terrestrial or other life forms could survive interplanetary travel.
Four of the E. bugadensis samples were collected from the space station's toilet, while one was found on the Advanced Resistive Exercise Device foot platform in March 2015, part of a wider effort to conduct a microbial census of the station.
Question: Is it it really known that microgravity increases bacteria's tendency to acquire foreign genetic material and become metal & antibiotic-resistant? How is this known? How was microgravity isolated as the causative influence, and not for example radiation, or other effects?