Microgravity and Swimming Motility Increase Biofilm Formation by Pseudomonas ­aeruginosa During Spaceflight

The microgravity environment encountered during spaceflight has been reported to alter a wide range of bacterial phenotypes, including cell growth, gene expression, antibiotic resistance, and virulence. However, the exact mechanism of how spaceflight alters these behaviors remains largely unknown. The goal of this work was to build upon the current understanding of microbial responses to spaceflight, focusing on the effect of microgravity on community behaviors, such as biofilm formation and quorum sensing. Bacterial biofilms were abundant on the MIR space station, where they were responsible for increasing corrosion and blocking a water purification system. Biofilms continue to be a challenge on the International Space Station (ISS). Health and safety hazards linked to the development of biofilms are also of great concern because decreased immune function has been observed in space travelers. We characterized biofilm formed by Pseudomonas aeruginosa PA14 during the STS-132 space shuttle mission in May 2010. We chose to study P aeruginosa biofilm formation during spaceflight because it forms biofilms both inside and outside of the human body, and can switch between benign and pathogenic interactions with humans. Further, P. aeruginosa caused astronaut Fred Haise to become sick during the Apollo 13 mission. We examined biofilm formation in artificial urine media (AUM) containing either 5 or 50 mM phosphate because water purification and recycling is essential for the ISS and other space travel endeavors. Phosphate has been previously reported to modulate microbial virulence. Direct measurements of biomass showed that P.aeruginosa PA14 formed 3 to 5-fold more biofilm during spaceflight compared to normal gravity, regardless of phosphate concentration. This result was confirmed using confocal laser scanning microscopy (CLSM) and image analysis with the COMSTAT software package. In contrast with the biofilm results, we found that AUM with 5 mM phosphate showed increased planktonic biomass during spaceflight, while there was no difference between spaceflight and ground controls for AUM with 50 mM phosphate. We also examined the role of swimming motility in biofilm formation during spaceflight. Motility is an important community behavior and is important for P. aeruginosa biofilm formation. Further, a link between motility and microbial response to spaceflight has been postulated in the literature. Unlike wild-type P. aeruginosa, two swimming motility mutants, ΔflgK and ΔmotABCD, did not show an increase in biofilm biomass during spaceflight. These results suggest that swimming motility may play an important role in increasing biofilm biomass during spaceflight.  In conclusion, the community-level behavior of P. aeruginosa PA14 was significantly modulated during spaceflight. This work has set the stage for our ongoing experiment designed to elucidate the role of microgravity on cell-cell communication and polymicrobial biofilms formation.