Clinical Ramifications of Bacterial Aggregation in Pleural Fluid.

Background: Bacterial aggregation has been well described to occur in synovial fluid, but it is unknown if bacteria form aggregates in body fluids beyond the synovial fluid. Consequently, this translational study evaluated the ability to form bacterial aggregates in different pleural fluids. Methods: Four of the most common causes of thoracic empyema-Streptococcus mitis, Streptococcus pneumoniae, Staphylococcus aureus, and Pseudomonas aeruginosa-were used here. The different pleural fluids included one transudative and two exudative pleural fluids. Twenty-four-well microwell plates were used to form the aggregates with the aid of an incubating shaker at different dynamic conditions (120 RPM, 30 RPM, and static). The aggregates were then visualized with SEM and evaluated for antibiotic resistance and the ability of tissue plasminogen activator (TPA) to dissolve the aggregates. Statistical comparisons were made between the different groups. Results: Bacterial aggregates formed at high shaking speeds in all pleural fluid types, but no aggregates were seen in TSB. When a low shaking speed (30 RPM) was used, only exudative pleural fluid with a high protein content formed aggregates. No aggregates formed under static conditions. Furthermore, there was a statistical difference in the CFU/mL of bacteria present after antibiotics were administered compared to bacteria with no antibiotics (p < 0.005) and when TPA plus antibiotics were administered compared to antibiotics alone (p < 0.005). Conclusions: This study shows that bacteria can form aggregates in pleural fluid and at dynamic conditions similar to those seen in vivo with thoracic empyema. Importantly, this study provides a pathophysiological underpinning for the reason why antibiotics alone have a limited utility in treating empyema.