Microgravity is one of the important environmental conditions during spaceflight. A series of studies have shown that many kinds of bacteria could be detected in space station and space shuttle. Space environment or simulated microgravity may throw a certain influence on those opportunistic pathogens and lead to some changes on their virulence, biofilm formation and drug tolerance. The mechanism of bacteria response to space environment or simulated microgravity has not been defined. However, the conserved RNA-binding protein Hfq has been identified as a likely global regulator involved in the bacteria response to this environment. In addition, microgravity effects on bacterial pathogenicity may threaten astronauts' health. The present paper will focus on microgravity-induced alterations of pathogenicity and relative mechanism in various opportunistic pathogens.