Serratia marcescens - caused arthritis with negative and positive birefrengent crystals.

We encountered an unusual case of arthritis caused by Serratia marcescens, with both positive and negative birefringent crystals in the same inflammatory synovial fluid. This combination of events is most likely to occur in men over 40 years old who have a predisposing illness or are receiving immunosuppressive drugs. This case shows the need to consider multiple pathological processes occurring in the same joint.