Rosy discoloration has affected the stone steps in the main spiral staircase of the Sully-sur-Loire castle donjon for many years now. This study monitors the development of the coloration and the environmental climatic conditions in the staircase to understand the conditions favoring the presence of this rosy discoloration. High-throughput sequencing was performed on healthy stone and on pink pigmented stone to identify the agent responsible for this discoloration. The results suggest that the rosy discoloration is the mark of a former degradation process which is now inactive, and that bacteria were the main agent of the pink patina formation. Nitiliruptor was the main genus identified in the pink parts of the staircase. This is the first time that this bacterial genus has been linked to a cultural heritage biodeterioration process. There was no evidence that this bacterial genus was responsible for the development of the pink patina, but it could have replaced it as a subsequent evolution of the process.