596. Distinct, Segregated Daptomycin-Susceptible and Daptomycin-Nonsusceptible Staphylococcus aureus Populations Associated with Tricuspid-Valve Infective Endocarditis

BACKGROUND: Loss of daptomycin susceptibility in Staphylococcus aureus is often associated with sequestered foci of infection, driven by selection pressure from both administered antibiotics and host defense peptides. Susceptibility testing of the organism cultured from blood is assumed to parallel that of the infectious foci, such as heart valves. We studied a case of tricuspid valve endocarditis where one leaflet yielded exclusively daptomycin-nonsusceptible S. aureus and another leaflet yielded purely daptomycin-susceptible S. aureus. We examined the responses of the two populations to different anti-staphylococcal therapies to identify regimens effective against both isolates. METHODS: Both isolates were whole-genome-sequenced using Illumina technologies. The presence of heterogeneous daptomycin-resistant subpopulations was assessed by dilution plating and population analysis profiling. One compartment pharmacokinetic/pharmacodynamic modeling was used to simulate different potential antistaphylococcal pharmacotherapies against each isolate. Hemolysin activity was evaluated as a surrogate for accessory gene regulator function. RESULTS: The daptomycin-susceptible isolate did not demonstrate heteroresistance while the daptomycin-resistant population was uniformly daptomycin nonsusceptible. The daptomycin non-susceptible isolate demonstrated regrowth by 72 hours of simulated treatment with vancomycin (2 g Q12H) or daptomycin (10 mg/kg daily). Adding cefazolin (2 g Q8H) to vancomycin or daptomycin prevented regrowth at 72 hours. The daptomycin-resistant isolate was deficient in hemolysin production suggesting agr dysfunction. Comparative sequencing identified daptomycin-resistant isolate mutations in mprF, purR and agrA. CONCLUSION: This case underscores the complex dynamics of the emergence of S. aureus resistance to daptomycin in vivo. Our pharmacokinetic modeling supports combination therapy in the treatment of endovascular MRSA infection. Reduced hemolytic activity supports the hypothesis that agr modulation is associated with persistent infection and/or treatment failure. Ongoing studies will identify features of distinct bacterial populations that promote ecological succession during infection at a sequestered anatomical site. DISCLOSURES: All authors: No reported disclosures.