Research has shown that bacteria are more lethal and resistant to antibiotics when exposed to microgravity (a lack of gravity), such as in space.
Microgravity has been shown to promote biofilm formation of bacteria. This is when bacteria grow and stick to each other, as well as stationary surfaces, from a matrix of polymer substances produced by a densely-packed cell colony.
Biofilms are known not only to increase antibacterial resistance among bacteria but their ability to infect. They have already been known to grow on and attach to equipment on space stations, causing them to degrade.
For example, biofilms have affected the Russian Mir space station's navigation window, air conditioning, oxygen electrolysis block, water recycling unit, and thermal control system. If left unresolved, prolonged exposure to these biofilms can lead to malfunction- something that could have particularly devastating effects during long-haul space flights, like those to Mars.
Microgravity also seems to distort the structure of bacteria. Reducing certain bacteria in size, their smaller surface area leads to fewer molecule-cell interactions, which then lessens the effectiveness of antibiotics.
A lack of gravity may also alter their behavior- such as sedimentation and buoyancy, and thus change the way they take in nutrients and are targeted by drugs. This could lead both to increased drug resistance and infectiousness in space.
Although difficult problems to overcome, microgravity also has some benefits. For example, under a lack of gravity, it is easier to grow molecular crystals both larger and more symmetrically than on Earth, making them more effective at treating certain illnesses, including various cancers and Parkinson's disease.
Although space as an environment poses many challenges, the more humans spend time there and are able to conduct experiments, the more likely we will be to overcome any problems. Although bacteria may be more deadly in space, perhaps drugs created there will be more potent.
Sources: Science Alert, Environmental Microbiology, Nature