Cecropins induce the hyperosmotic stress response in Escherichia coli

Cecropin A and B, below or near their minimum inhibitory concentrations in viable Escherichia coli, interfered with the rapid NaCl-induced hyperosmoti c shrinkage of the cytoplasmic volume (plasmolysis), and also activated the promoter of the hyperosmotic stress gene osmY. The same promoter was also expressed by hyperosmolar NaCl or sucrose, two of the most commonly used an timicrobial food preservatives. Stress responses were monitored during the logarithmic growth phase of E. coli strains that contain specific promoters fused to a luxCDABE operon on a plasmid. The luminescence assay, developed to monitor the transcriptional response to stresses, is based on the premi se that organisms often respond and adapt to sublethal environmental advers ities by increased expression of stress proteins to restore homeostasis. Th e luminescence response from these fusion strains to a specific stress occu rs as the transcription at the promoter site is activated. Cecropins induce d luminescence response only from the osmY-luxCDABE fusion, but not the cor responding stress promoter activation associated with macromolecular or oxi dative damage, or leakage of the cytoplasmic content including the proton g radient. The inhibitory effect of cecropins on plasmolysis is interpreted t o suggest that the primary locus of action of these antimicrobial peptides in the periplasmic space is on the coupling between the inner and outer mem brane. (C) 1998 Elsevier Science B.V. All rights reserved.