The patient presented with increasing fatigue and dyspnoea. The patient had medical history of rheumatoid arthritis for which she had been taking methotrexate for the past 15 years and etanercept for the past 6 years. Initial diagnosis was cardiac failure but further investigation by echocardiogram revealed a large pericardial effusion. Empirical piperacillin-tazobactam was started due to moderately raised inflammatory markers. Four hundred millilitre of frank pus was aspirated from the pericardial sac and antimicrobial treatment was changed to meropenem. Gram positive cocci were seen in the initial Gram stain, but conventional cultures remained negative. However, 16S ribosomal RNA gene sequencing of the pus sample detected the presence of Parvimonas micra genome. Reaccumulation of the effusion required further drainage where again P micra was detected by 16S ribosomal RNA gene sequencing. Two weeks of meropenem was completed followed by treatment with benzylpenicillin and metronidazole.