Approximately 90 percent of P aeruginosa strains produce extracellular protease. Three separate proteases have been purified that differ in pH optimum, isoelectric point, and substrate specificity. Although all are capable of digesting casein, one of them, protease II, also digests elastin. When injected into the skin of animals, purified P aeruginosa proteases induce formation of hemorrhagic lesions, which become necrotic within 24 hours. These proteases also cause rapid tissue destruction when injected into the cornea of animal eyes or into rabbit lungs; they also probably contribute to the tissue destruction that accompanies P aeruginosa eye or lung infections and may aid bacteria in tissue invasion. Their effects, however, appear to be localized, and they are not highly toxic to animals (LD50 = approximately 200 μg/mouse) (Table 27-1).