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Antibiotic resistance is a growing problem on Earth 鈥� and it could occur in space too. Bacteria exposed to trace quantities of an antibiotic while growing in simulated microgravity developed resistance to the drug. What鈥檚 more, they retained that resistance for more than 100聽generations after the antibiotic was removed.

With talk about future crewed missions to the moon and Mars, biologists are keen to know more about how microbes behave in space. What鈥檚 of particular concern is evidence that microgravity, radiation exposure and sleep deprivation weaken astronauts鈥� ability to fend off infection. If bacteria on board spacecraft develop antibiotic resistance, keeping astronauts healthy might be a real challenge.

To explore that possibility, a team led by Madhan Tirumalai and George Fox at the University of Houston, Texas, grew a harmless strain of Escherichia coli in a simulated microgravity environment that had been sterilised with a broad-spectrum antibiotic called chloramphenicol.

Growing resistance

After the bacteria had grown for 1000 generations, the team tested whether they had developed antibiotic resistance. The E. coli cells were not only resistant to chloramphenicol, but also to several other antibiotics.

This makes sense, says Tirumalai. 鈥淭he genome of a bacterium is not very large, and the number of genes actively involved in drug export and import is small,鈥� he says. This means that no matter which antibiotic is used, it targets a limited number of genes. As a result, says Tirumalai, if the bacteria develop resistance to one drug, they will probably develop resistance to other antibiotics too.

The E. coli gained multiple changes to its genome that are known to be associated with antibiotic resistance. Perhaps more significantly, the team discovered that the bacteria were able to hold on to their antibiotic resistance. After a further 110聽generations of growth with no聽antibiotic present, the bacteria were still resistant to chloramphenicol and some other drugs.

Tirumalai says the results will be relevant for space agencies or companies launching crewed missions. 鈥淲hen you鈥檙e going to be in space for more than 6聽months, bacteria are subject to several thousand generations of growth,鈥� he says.

Because resistance emerges even in response to trace amounts of antibiotic, it could occur on a spacecraft if astronauts or equipment are exposed to antibiotics immediately before a mission, says Tirumalai. And if even small quantities are present in the craft itself, resistance is likely to develop and be even more persistent.

Earlier studies have suggested that some pathogenic bacteria are more virulent when grown in space. If this greater virulence is coupled with antibiotic resistance, that could spell聽deadly trouble for future crewed missions.

But Jason Rosenzweig at Texas Southern University stresses that not every pathogen seems to become more virulent in space. 鈥淚t鈥檚 not a one size fits all type of response,鈥� he says.

Tirumalai agrees: 鈥淭his is just one model organism 鈥� it would be presumptuous to extrapolate these results to other bacteria,鈥� he says.