Plasminogen activator inhibitor 1 (PAI-1) is a serine protease inhibitor (serpin) and a key molecule that regulates fibrinolysis by inactivating human plasminogen activators. Here we show that two important human pathogens, the plague bacterium Yersinia pestis and the enteropathogen Salmonella enterica serovar Typhimurium, inactivate PAI-1 by cleaving the R346-M347 bait peptide bond in the reactive center loop. No cleavage of PAI-1 was detected with Yersinia pseudotuberculosis, an oral/fecal pathogen from which Y. pestis has evolved, or with Escherichia coli. The cleavage and inactivation of PAI-1 were mediated by the outer membrane proteases plasminogen activator Pla of Y. pestis and PgtE protease of S. enterica, which belong to the omptin family of transmembrane endopeptidases identified in Gram-negative bacteria. Cleavage of PAI-1 was also detected with the omptins Epo of Erwinia pyrifoliae and Kop of Klebsiella pneumoniae, which both belong to the same omptin subfamily as Pla and PgtE, whereas no cleavage of PAI-1 was detected with omptins of Shigella flexneri or E. coli or the Yersinia chromosomal omptins, which belong to other omptin subfamilies. The results reveal a novel serpinolytic mechanism by which enterobacterial species expressing omptins of the Pla subfamily bypass normal control of host proteolysis.Plasminogen activator inhibitor 1 (PAI-1) is a key regulator of the mammalian fibrinolytic/plasminogen system (29, 37). The fibrinolytic system comprises the serine protease zymogen plasminogen, urokinase-type plasminogen activator (uPA), tissue-type plasminogen activator (tPA), PAI-1, and plasmin inhibitor ␣ 2 -antiplasmin (␣ 2 AP) (for a review, see reference 52). Plasminogen is converted to plasmin, which is a broad-spectrum serine protease that dissolves fibrin in blood clots, degrades laminin of basement membranes, and activates matrix metalloproteinases that degrade collagens and gelatins in tissue barriers. Herewith, plasmin controls physiological processes such as fibrinolysis/coagulation, cell migration and invasion, and tumor metastasis (29, 37). PAI-1 maintains normal hemostasis by inhibiting the function of the plasminogen activators tPA and uPA, which are serine proteases and highly specific for cleavage of the plasminogen molecule. tPA binds to fibrin and is associated with plasmin-mediated breakdown of fibrin clots, whereas uPA has low affinity for fibrin and associates with cell surface proteolysis, cellular migration, and damage of tissue barriers (52).The mammalian fibrinolytic and coagulation systems are targeted by invasive bacterial pathogens during infection (reviewed in references 6, 11, 34, and 61). In bacterial sepsis, increased production of fibrin clots at a damaged endothelium results from enhanced thrombin-catalyzed fibrin generation and from an increased serum level of PAI-1. Coagulation can protect the host by activating immune systems or by physically restraining the bacteria (6,15,25,41). On the other hand, several invasive bacterial pathogens enhance fibrinolysis eithe...