Serratia marcescens-Induced Pericarditis Leading to Rapid-Onset Cardiac Tamponade in a Patient With Underlying Malignancy

<i>Serratia marcescens</i> is a Gram-negative bacterium that mostly causes urinary tract infections, respiratory infections, and catheter-related bloodstream infections. It rarely causes pericarditis, especially leading to tamponade, which is exceptionally rare. We describe a case that had a history of intravenous drug usage and presented with dyspnea and chest pain. Small cell lung carcinoma and a newly developed moderate pericardial effusion with echocardiographic indications of imminent tamponade were discovered during imaging. Pericardial fluid cultures identified Serratia marcescens. The patient was treated successfully with pericardial fluid drainage followed by antibiotics. This case emphasizes the importance of considering unusual organisms like <i>S. marcescens</i> when treating pericardial infections, particularly in patients who are at high risk. Early identification and focused treatment are crucial for the management of complications like cardiac tamponade.