Adaptive silver resistance in Cupriavidus metallidurans

Assessing and controlling microbial contamination during long-term manned spaceflight is of paramount importance as contamination can cause problems for the astronaut’s health and the infrastructure of the spacecraft. Cupriavidus and Ralstonia genera were identified and isolated during numerous monitoring campaigns from different space-related environments. This study aimed to gather more insights in the ability of these strains to thrive in these environments. All isolates acquired moderate to high tolerance against several stressors and can grow in oligotrophic conditions, enabling them to persist in the ISS and related environments in spite of the implemented disinfection and sterilization procedures. Silver resistance mechanisms were studied, as silver is used to sanitize water sources in space. All isolates tolerated silver concentrations higher than those measured in the drinking water aboard the ISS, and survived a 23-month exposure to silver in drinking water. Rapid evolution towards increased silver resistance was observed in C. metallidurans strains.The identified silver efflux pumps did not participate in this response. By contrast, the agrS gene coding for the histidine kinase was affected by mutation, resulting in the increased expression of the response regulator AgrR. AgrR cross-activated the expression of mmmQ, resulting in the increased silver resistance.