Concurrent MRSA and MSSA Bacteremia in Critical Care: Diagnostic and Treatment Challenges

Introduction: Bacteremia in critically ill patients carries high morbidity and mortality. While methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible Staphylococcus aureus (MSSA) usually manifest as distinct entities requiring tailored management, concurrent MRSA/MSSA bacteremia is exceptionally rare. This underreported phenomenon poses diagnostic challenges, antimicrobial selection dilemmas, and therapeutic failure risks. We report a case of concurrent MRSA/MSSA bacteremia in a patient in the intensive care unit.


Description: A 45-year-old female with asthma, hyperlipidemia, and type 2 diabetes mellitus presented with respiratory failure requiring intubation. Initial labs revealed respiratory acidosis, leukocytosis, and hyperglycemia. Influenza B was detected on day 2 and treated with oseltamivir. By day 4, recurrent fever and rising leukocytosis led to blood cultures and empiric cefepime and vancomycin. Rapid diagnostic testing via polymerase chain reaction (PCR) detected concurrent MRSA/MSSA bacteremia, confirmed by final cultures. Therapy was adjusted to cefazolin for MSSA and vancomycin for MRSA. Serial cultures indicated fluctuating bacteremia: MSSA on day 6 and MRSA on day 10. Sputum cultures mirrored this shift, with MSSA on days 4 and 6 and MRSA on days 8, 9, 10, and 14. Chest imaging showed cavitary pneumonia. Despite appropriate antimicrobial therapy, the patient developed septic shock on day 8, requiring vasopressor therapy. Antibiotics were broadened to cefepime, nafcillin, and vancomycin. On this day, the patient suffered a cardiac arrest due to tension pneumothorax, requiring chest tubes. The patient expired on day 14 from a cardiac arrest due to hemothorax from necrotizing pneumonia.


Discussion: This case highlights a rare instance of concurrent MRSA/MSSA bacteremia, confirmed via PCR and cultures. Dual therapy was necessary, but fluctuating cultures and deterioration complicated management. Culture shift from MSSA to MRSA suggests selective antibiotic pressure or dynamic colonization patterns, complicating antibiotic choice. Providers should remain alert to the possibility of mixed infections and tailor therapy accordingly. Early recognition, source control, and antimicrobial optimization are essential in optimizing outcomes in such high-risk cases.