Co-adaptation of Streptococcus mutans to simulated microgravity and silver nitrate

Abstract

To sustain life on extended space missions, it is essential to maintain clean potable water. NASA currently uses iodine as the primary biocide in the potable water dispenser on the International Space Station and has recently proposed a potential switch to silver-based antimicrobials. Streptococcus mutans is the primary etiological agent of dental caries, part of the normal oral flora, and would endure direct exposure to water from the potable water dispenser. In our previous work, we examined the 100-day adaptive response of Streptococcus mutans to simulated microgravity (sMG). Here, we examined the evolutionary co-adaptation of S. mutans under sMG and silver nitrate (AgNO3) to evaluate the consequences of using silver as a primary biocide in space and the impact on the evolution of microbes from the oral microbiome. To do this, we adapted four populations of S. mutans under sMG and co-adapted four populations in simulated microgravity and silver nitrate using high-aspect ratio vessels for 100 days. Genomic analysis at multiple time points showed that S. mutans in sMG evolved variants consistent with our previous findings (SMU_1307c and SMU_399) while also acquiring novel mutations in the glutathione reductase gorA. The co-adapted populations showed mutations specific for the environment in ciaH/R, PBP1a, trkA, and trkB. We also assessed virulence phenotypes, and while simulated microgravity increased antibiotic susceptibility, sucrose-dependent adhesion, and, in some populations, acid tolerance, co-adaptation to silver nitrate reversed these effects. Overall, these data show that the use of silver as a biocide in simulated microgravity can evolve strains with novel genotypic and phenotypic traits that could alter virulence.

Considerations for oral and dental tissues in holistic care during long-haul space flights

“…There have been cases of dental pain related to caries progression during space flights reported with no success of identifying the origin of the problem ( Gontcharov et al, 2005 ; Menon, 2012 ). It is understandable that progression of the caries would be related to the increase in anaerobic bacteria as described above and the elevated Streptococcus mutants count ( Orsini et al, 2017 ) as well as the variations/evolution of S. mutants species that alters their survival and pathogenic properties ( Fernander et al, 2022 ; Fernander et al, 2023 ). However, the origin of the carious lesion could have been an area of pre-existing demineralization or arrested caries.…”

“…To minimize infection, extensive microbial screening is conducted ( Fernander et al, 2022 ) The implementation of prevention strategies and prior screenings significantly reduce oral issues rates of occurrence ( Lloro et al, 2019 ). However, the crew’s microflora may develop unique evolutionary traits due to their exposure ( Fernander et al, 2022 ; Fernander et al, 2023 ). NASA has developed tight requirements for cosmonaut choosing, continuation, as well as prior to flight oral examinations for particular space flights, plus a rigid clinical regimen.…”