With the development and exploration of space flight technology, the space environment has become a new area of human activity. On Earth, microbes and humans have co-existed for millions of years, and microbes are widely present in the air, water, soil, and other ecosystems. Therefore, as part of human space exploration, microbes are introduced directly or indirectly into the space environment by human activities. Although outer space is a very extreme and complex environment, microorganisms exhibit high adaptability, including to microgravity, strong radiation, low temperature, high pressure, and low-nutrient environments, via phenotypic and genetic changes. These changes may affect the astronauts in the space environment as well as on Earth because mutant microbes will inevitably return with the spacecraft. Microbes usually change their morphology, growth rate and adaptability to adapt to extreme environment, but the role and significance of these phenotypic changes and the underlying mechanisms are important unresolved questions in the field of space biology. Space microbiology is in its infancy worldwide, and the influence of the space environment on microorganisms and the relevant specific mechanisms are cutting-edge issues that require further research and attention.