High extracellular polymeric substance production and biofilm-forming capacity of <i>Ralstonia pickettii</i> isolates from ISS potable water

The potable water dispenser (PWD) system plays a critical role as a source of drinking water for astronauts on the International Space Station (ISS). In this study, we examined the bioburden in the potable water produced by the PWD. The amount of extracellular polymeric substances (EPSs) in the PWD water was approximately 19 or 55 times greater than the bacterial count, and the EPS biomass accounted for approximately 24% or 86% of the bacterial biomass. <i>Ralstonia pickettii</i> consistently comprised approximately 70% or 80% of the bacteria for 3 years. Under simulated microgravity conditions, the isolated <i>R. pickettii</i> strains exhibited higher cell and EPS concentrations and higher total volume concentrations (average volume multiplied by concentration) of cell and EPS than under 1G conditions, whereas the average cell volume was smaller and the average EPS volume was larger. The ISS isolates showed higher EPS production and biofilm-formation abilities than terrestrial strains under nutrient-rich conditions and possessed high biofilm-formation ability comparable to those of terrestrial strains under nutrient-poor conditions. The ability of <i>R. pickettii</i> to produce EPS may play a crucial role in its adaptation to the water environment on the ISS.IMPORTANCEIn space habitation environments, the use of recycled water is indispensable, and ensuring its microbiological safety is essential. In this study, we elucidated the microbiological characteristics of water from the potable water dispenser (PWD) on the International Space Station (ISS). Our findings revealed that bacteria of the <i>Ralstonia pickettii</i> are the predominant species in PWD water and that extracellular polymeric substances (EPSs) constitute a large proportion of the biomass. Furthermore, the isolated <i>R. pickettii</i> was shown to possess high EPS production ability and strong biofilm-forming capacity. Since EPS plays a crucial role in biofilm formation, these abilities may be important factors enabling <i>R. pickettii</i> to adapt to the water environment of the ISS.