Clinical features and medical–surgical management overweight the inoculum effect and genotypic characteristics of MSSA in infective endocarditis

Abstract Objectives MSSA remains the leading cause of infective endocarditis (IE) and is responsible for significant mortality. While clinical factors tied to mortality are well documented, possible contributing strain-specific characteristics have not been extensively explored. This study investigates MSSA phenotypic and genotypic characteristics and medical–surgical data related to Day-90 mortality in IE. Methods We included all patients enrolled in a monocentric prospective cohort (2016–23), with definite or probable MSSA IE. Cefazolin and oxacillin MICs and inoculum effects were determined by broth microdilution. Genotyping analysis and BlaZ typing were obtained from WGS. Phenotypic and genotypic characteristics of strains and clinical risk factors were confronted with Day-90 mortality. Results Eighty-eight patients with MSSA IE were included. The most frequent clinical presentations were left-sided native valve IE (25/88), left-sided prosthetic valve IE (12/88) and right-sided IE (19/88). Day-90 mortality rate was 39% (34/88). Most patients were treated with antistaphylococcal penicillin as a primary antibiotic (60/88). The main MSSA clonal complexes identified were CC398 (17/88), CC30 (13/88) and CC5 (13/88). Cefazolin inoculum effect was observed in 18/88 strains, and oxacillin inoculum effect in 13/88. Overall, 15/88 isolates exhibited an inoculum effect on primary antibiotic therapy. Factors independently associated with improved outcomes included cardiac surgery [hazard ratio (HR) 0.34, 95% CI (0.13–0.89)] and source control [HR 0.21, 95% CI (0.03–0.53)]. Neither genetic background, blaZ carriage, nor in vitro inoculum effect to the primary antibiotherapy was associated with Day-90 mortality. Conclusions This cohort of MSSA IE did not find any microbiological factors correlated with Day-90 mortality. Clinical features and infection management appear to be the main factors in the prognosis of MSSA IE.