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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?

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As detailed in this article, an experiment found that "microgravity" gave bacteria a permanent genetic boost (as compared to bacteria not in microgravity). The experiment was conducted in a *rotating vessel designed to simulate microgravity", therefore the environment of the experiment was controlled (with microgravity being the independent variable) and the effect of "radiation or other effects" would be extremely small (if any). As a result, it can be reasonably concluded that by itself, microgravity can increase "bacteria's tendency to acquire foreign genetic material and become metal & antibiotic-resistant".

The simulation is described as follows:

To assess that risk, Madhan Tirumalai at the University of Houston in Texas and his colleagues placed E. coli in a rotating vessel designed to simulate microgravity. They kept them there for 1,000 bacterial generations, much longer than in previous studies.

After giving the cells time to adapt to microgravity, the researchers combined them with another strain of E. coli that hadn’t been subjected to microgravity and allowed them to grow together. The adapted cells grew about three times as many colonies as the others.

Even after the cells were taken out of microgravity for up to 30 generations before being combined with the control strain, they maintained 72 per cent of their adaptive advantage, pointing to permanent mutations in the genes rather than merely a temporary adjustment.

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