First report of bacteremia caused by CO2-dependent methicillin-resistant Staphylococcus epidermidis in a patient with symptomatic multiple myeloma.

We report the first bacteremia case caused by CO₂-dependent methicillin-resistant Staphylococcus epidermidis (MRSE). Blood cultures from a symptomatic multiple myeloma patient yielded Gram-positive cocci. Molecular testing (BioFire FilmArray Blood Culture Identification 2 Panel) identified S. epidermidis with positive mecA/C. However, subculturing failed under ambient air, yielding colonies only under 5% CO₂. Using the MicroScan WalkAway 40 plus system (Pos Combo 1 J panel) in ambient air misidentified the isolate as Micrococcus sp. and produced indeterminate antimicrobial susceptibility testing (AST) results due to insufficient growth. Conversely, testing under 5% CO₂ correctly identified MRSE, matching molecular findings; final species identification was confirmed using the MALDI Biotyper. Furthermore, the same CO₂-dependent isolate was also recovered from a surgical site fluid accumulation following posterior spinal fusion. This case highlights the importance of considering CO₂ dependence for accurate identification and AST of S. epidermidis, especially when initial cultures or automated systems yield inconclusive or erroneous results.