Rifampicin-resistant Staphylococcus epidermidis implant-associated infections: the therapeutic potential of ceftaroline and daptomycin combinations.

Rifampicin-resistant (RIF-R) and methicillin-resistant Staphylococcus epidermidis (MRSE) pose a significant challenge in managing implant-associated infections (IAIs). However, the functional consequences of rpoB mutations leading to RIF resistance remain poorly characterized, particularly their impact on bacterial fitness and treatment response. We analyzed clinical RIF-R and RIF-susceptible (RIF-S) MRSE isolates from a single-center IAI cohort. RIF-R strains showed delayed exponential growth and exhibited a reduced growth rate during the exponential phase, while biofilm formation remained largely unchanged. To investigate resistance-associated phenotypes, we induced RIF resistance in three clinical RIF-S strains using subinhibitory exposure in a murine tissue cage model. Sequencing revealed rpoB mutations in cluster I, frequently at codons 481 and 486. One mutant, R611 (Ser486Tyr), mirrored mutations from a clinical isolate and exhibited slower growth, reduced cell density, and markedly increased biofilm production with unchanged susceptibilities to vancomycin or daptomycin compared to its RIF-S parent, S611. In vivo, R611 was more competitive in the planktonic phase, while S611 dominated the adherent fraction. Time-kill assays demonstrated strong bactericidal activity of daptomycin combined with ceftaroline or fosfomycin. In the murine model, ceftaroline monotherapy was partially effective, whereas daptomycin/ceftaroline achieved the highest cure rates even against clinical RIF-R strains at the minimum infective dose, with efficacy decreasing markedly at higher bacterial inocula. These findings reveal fitness trade-offs linked to rpoB-mediated resistance and support ceftaroline-based combinations as a potential treatment strategy for RIF-R MRSE in IAIs, particularly when bacterial burden can be reduced.