To determine the specificity of the microbial proteases in triggering the respiratory proteins to produce bactericidal ROS, we compared the level of bacterial clearance of protease-producing and non-producing strains of typical Gram-negative and Gram-positive bacteria. For Gram-negative bacteria, we chose Pseudomonas aeruginosa strain PAO-Iglewski which produces PAE (the major extracellular protease virulence factor), and the PAE-knockout mutant PAO-B1A1 which does not produce PAE (1). For Gram-positive bacteria, we cultured Staphylococcus aureus PC1839 strain which produces active V8-protease, and AK3 strain which is a V8 protease-mutant (2). The genes controlling PAE in P. aeruginosa and the V8-protease in S. aureus are under the control of quorum-sensing signals. Therefore the culture conditions which facilitate the production of active extracellular proteases are utilized.