First characterization of Staphylococcus felis in diabetic foot osteomyelitis: from intracellular persistence to phage treatment

ABSTRACT Staphylococcus felis is a coagulase-negative Staphylococcus (CoNS) primarily associated with the feline microbiota and only rarely reported in human disease. Here, we report its implication in diabetic foot osteomyelitis, and provide the first comprehensive characterization of its pathogenic potential. Two isolates (NSF001 and NSF002), recovered 5 months apart from bone biopsies of the same patient, were analyzed for growth kinetics, biofilm formation, and intracellular persistence in macrophages and osteoblasts. Both isolates proliferated efficiently, produced robust biofilm, and persisted within host cells, most markedly in osteoblasts. In a zebrafish embryo infection model, both isolates caused significant mortality, confirming their pathogenic potential in vivo . Whole-genome sequencing revealed conserved virulence determinants, a narrow resistome, and strain-specific genomic variations affecting genes involved in virulence regulation, phage defense, and iron acquisition. The lytic phage SAVM02, previously characterized for activity against other Staphylococcus species, effectively inhibited S. felis growth in vitro and conferred protection in vivo against lethal infection. Notably, the two sequential isolates differed in their in vivo virulence and phage susceptibility, paralleling these within-host microevolutionary changes and illustrating bacterial adaptation during chronic infection. Altogether, this study establishes S. felis as a CoNS capable of intracellular persistence, biofilm formation, and in vivo virulence in chronic human infection. Our findings also highlight the therapeutic potential of lytic phages against virulent CoNS species and support further investigation of phage therapy for chronic staphylococcal infections. IMPORTANCE Coagulase-negative staphylococci (CoNS) are increasingly recognized as genuine agents of chronic infection, yet the pathogenic capacity of most individual species remains undefined. Staphylococcus felis , a commensal of cats only exceptionally reported in humans, had never been implicated in a chronic human infection. Here, we describe two sequential S. felis isolates recovered from bone biopsies of a patient with diabetic foot osteomyelitis and show that this species combines biofilm formation, intracellular persistence in macrophages and osteoblasts, and lethality in a zebrafish embryo model. Whole-genome comparison of the two isolates uncovered microevolutionary changes, most notably in iron-acquisition and genome-defense loci, that paralleled differences in virulence and phage susceptibility. These findings extend the list of CoNS capable of causing invasive human disease and provide a rationale for lytic phage therapy against emerging, difficult-to-treat staphylococcal pathogens.