Interaction of human defensins with Escherichia coli. Mechanism of bactericidal activity.

Abstract

Defensins are smalL cysteine-rich antimicrobial peptides that are abundant in human, rabbit, and guinea pig neutrophils (PMN). Three defensins (human neutrophil peptide defensin constitute between 30 and 50% of the total protein in azurophil granules of human PMN. We examined the mechanism of HNP-mediated bactericidal activity against Escherichia coli ML-35 (i-, y-, z+) and its pBR322-transformed derivative, E. coli ML-35p. Under conditions that supported bactericidal activity, HNP-1 sequentially permeabilized the outer membrane (OM) and inner membrane (IM) of E. coli. Coincident with these events, bacterial synthesis of DNA, RNA, and protein ceased and the colony count fell. Although these events were closely coupled under standard assay conditions, OM permeabilization was partially dissociated from IM permeabilization when experiments were performed with E. coli that had been plasmolyzed by mannitol. Under such conditions, the rate and extent of bacterial death more closely paralleled loss of IM integrity than OM permeabilization. Electron microscopy of E. coli that had been killed by defensins revealed the presence of striking electron-dense deposits in the periplasmic space and affixed to the OM.Overall, these studies show that HNP-mediated bactericidal activity against E. coli ML-35 is associated with sequential permeabilization of the OM and IM, and that inner membrane permeabilization appears to be the lethal event.

Large-scale synthesis and functional elements for the antimicrobial activity of defensins

“…Defensins have been reported to exhibit powerful antibacterial, antifungal and antiviral activity against a wide variety of microorganisms [17][18][19]. The antibiotic activity of defensins has generally been attributed to their effects on microbial membranes [19,20].…”

“…The antibiotic activity of defensins has generally been attributed to their effects on microbial membranes [19,20]. They have been reported to interact with lipopolysaccharides in Gram-negative bacteria, polysaccharides (teichoic acid) in Grampositive bacteria, and phospholipids [17].…”

The antibacterial peptide seminal plasmin alters permeability of the inner membrane of E. coli

“…Thus SPLN and defensins do not share a common structural motif. However, our present work as well as studies on defensins [9] indicate that the antibacterial activity of these peptides stems from their ability to permeabilize the bacterial inner membrane. Hence, important pre-requisites for peptide antibacterial agent would be: (i) entry into the bacterial inner membrane by either making use of the colicin pathway or by perturbation of the outer membrane by virtue of being cationic; (ii) mild perturbation of bacterial membrane due to the presence of amphipathic structures which could be either helices of P-sheets.…”

“…It is well established that influx of ONPG through the bacterial inner membrane occurs through lac permease and, in the absece of the protein transporter, no influx of ONPG is possible. Enhanced activity of /I-galactosidase in the presence of antibacterial agents would thus reflect the permeabilization of the bacterial membrane to ONPG [9]. Hence, SPLN clearly permeabilizes the bac;erial inner membrane, providing additional pathways for ONPG influx.…”

Identification of a second membrane‐active 13‐residue peptide segment in the antimicrobial protein, bovine seminalplasmin

“…contribute to host defense mechanisms in higher mammals [2], and also prevent infection in frogs, particularly on the skin surface [3]. Although some of these peptides, like cecropins and defensins, have significant potency, as well as a broad antimicrobial spectrum, antiviral and antifungal properties [1, 4,5], they are relatively long, composed of -37 residues.…”