In-patient evolution can heterogeneously impact biofilm formation and intraosteoblastic behavior of Staphylococcus aureus in relapsing bone and joint infections

Abstract Background Bone and joint infections caused by Staphylococcus aureus are notoriously difficult to manage and are prone to relapse, often leading to serious clinical complications. Treatment failure is frequently attributed to bacterial persistence mechanisms, notably biofilm formation and intracellular survival within osteoblasts. Methods To elucidate the underlying causes of relapse, we analyzed 25 relapse pairs, each consisting of an isolate collected during the initial infection and a corresponding isolate obtained at relapse from the same patient. We investigated mutation acquisition in relapse strain during host in-patient stay and assessed their phenotypic consequences, focusing on bacterial growth, biofilm formation, internalization into osteoblasts, intracellular persistence, and cytotoxicity. Results Whole-genome comparisons revealed that most relapse strains accumulated mutations during the host in-patient stay, with mutation counts ranging from 0 to 391. These genetic changes variably affected key phenotypes across the relapse strains. Specifically, growth kinetics, biofilm-forming capacity, and intracellular behavior exhibited heterogeneity when compared to initial strains. Integrative analysis of genomic and phenotypic data indicates that mutations in genes involved in virulence regulation and metabolic pathways may contribute to the relapse of S. aureus BJIs. However, no association between specific mutations or phenotypes and the occurrence of relapse in bone and joint infections has been highlighted. Conclusion Despite its limitations, this study represents an essential first step toward large-scale investigations aimed at understanding how bacterial genome alterations and pathophysiological mechanisms contribute to relapse in bone and joint infections.