Characterization of bacteria and fungi associated with the ISS' surfaces (2019)
microbiomejournal.biomedcentral.com
microbiomejournal.biomedcentral.com
https://en.wikipedia.org/wiki/Tersicoccus_phoenicis
https://www.jpl.nasa.gov/news/rare-new-microbe-found-in-two-...
Or these aliens experience time differently than us and have in fact traveled back from space on the various probes that used these clean rooms.
An interesting part of The Martian book was the development and maintenance of soil biota. Human microbiome seems to be a thing of active research and relative ignorance. I think it will be a critical knowledge domain for people living off-Earth for long periods both in terms of pathology and health.