Research article

The periplasmic serine protease inhibitor ecotin protects bacteria against neutrophil elastase

Christopher T. EGGERS, Iain A. MURRAY, Valerie A. DELMAR, Anthony G. DAY, Charles S. CRAIK


Ecotin is a dimeric periplasmic protein from Escherichia coli that has been shown to inhibit potently many trypsin-fold serine proteases of widely varying substrate specificity. To help elucidate the physiological function of ecotin, we examined the family of ecotin orthologues, which are present in a subset of Gram-negative bacteria. Phylogenetic analysis suggested that ecotin has an exogenous target, possibly neutrophil elastase. Recombinant protein was expressed and purified from E. coli, Yersinia pestis and Pseudomonas aeruginosa, all species that encounter the mammalian immune system, and also from the plant pathogen Pantoea citrea. Notably, the Pa. citrea variant inhibits neutrophil elastase 1000-fold less potently than the other orthologues. All four orthologues are dimeric proteins that potently inhibit (<10 pM) the pancreatic digestive proteases trypsin and chymotrypsin, while showing more variable inhibition (5 pM to 24 µM) of the blood proteases Factor Xa, thrombin and urokinase-type plasminogen activator. To test whether ecotin does, in fact, protect bacteria from neutrophil elastase, an ecotin-deficient strain was generated in E. coli. This strain is significantly more sensitive in cell-killing assays to human neutrophil elastase, which causes increased permeability of the outer membrane that persists even during renewed bacterial growth. Ecotin affects primarily the ability of E. coli to recover and grow following treatment with neutrophil elastase, rather than the actual rate of killing. This suggests that an important part of the antimicrobial mechanism of neutrophil elastase may be a periplasmic bacteriostatic effect of protease that has translocated across the damaged outer membrane.

  • ecotin
  • Gram-negative bacteria
  • neutrophil elastase
  • outer membrane permeability
  • serine protease inhibitor


  • 1 Present address: Krebs Institute of Biomolecular Sciences, Department of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield S10 2TN, U.K.

  • Abbreviations used: BPI, bactericidal/permeability-increasing protein; CatG, cathepsin G; CFU, colony-forming unit; DOE JGI, Department of Energy Joint Genome Institute; NE, neutrophil elastase; Omp, outer membrane protein; TSA, tryptic soya agar; TSB, tryptic soya broth; uPA, urokinase-type plasminogen activator; WT, wild-type.