The Martian (2015)‘s Mark Watney (Matt Damon) might like to goof off when it comes to watching The Dukes of Hazzard and joking about poop fertilizer, but not when it comes to science. After the storm that separates him from the rest of his crew early in the story, he finds himself in the unfortunate circumstance of being impaled by a large antenna. When patching himself back up, part of his regimen involves the use of antibiotics to protect against infection. His other innovative scientific resourcefulness would be moot if he were killed by a festering wound, and since medical supplies are in no short supply on his voyage, he makes use of them.

But does he need an antibiotic? There are no bacteria in space, so is an infection something he needs to worry about? The short answer is yes—plus, it’s simply better to be safe than sorry.

There may be no bacteria in space, but bacteria still survives in three places that could contribute to a wound infection: the antenna itself, Watney’s space suit, and Watney’s own skin.

The antenna is an Earth-produced item. Since it existed on Earth, it could (theoretically) have transported bacteria with it to space. The same goes for the space suit. The antenna plunged directly into him, which would have pushed bits of the space suit in the wound as well. That adds up to several foreign objects with possible bacteria making its way into Watney.

Plus, human skin harbors tons of bacteria. Generally it lives in a pretty symbiotic way, where it doesn’t hurt us and we’re not really bothered by it. But when there’s an open wound, things change. Even as his wound is healing, when he is within the confines of the HAB (another Earth/man-created object), he runs the risk of contamination without an antibiotic.

Gut wounds are particularly gross. There is a high risk of infection in an abdominal wound if an antibiotic is not administered. Watney proves throughout The Martian that he is meticulous and calculated, not to mention extremely bright. It stands to reason that he’d know about the possibility of infection, however slim, and wouldn’t ignore available treatment. The most likely scenario, then, is that (off camera) he treated himself.