As shown in Figure 6,celastrol at sub-inhibitory concentrations dosedependently inhibited (p < 0.05) the proteolytic activity of S. maltophilia strains (ATCC 13637 and GNU2233). Notably, celastrol at 5 μg/ml significantly (p < 0.05) reduced the diameters of the halo zones of the ATCC 13637 strain by more than 49.1%, compared to the DMSO-treated control.In addition, S. maltophilia GNU2233 treated with celastrol at 20 μg/ml showed a decrease in the proteolytic activity by approximately 13.62%. These results showed that celastrol at sub-inhibitory concentrations inhibited protease secretion by S. maltophilia strains.