Ability of cecropin B to penetrate the enterobacterial outer membrane

Abstract

The cationic amphipathic insect peptide cecropin B was almost as active on wild-type enteric bacteria as it was on their lipopolysaccharide and lipid A mutants that have very defective outer membrane. The polymyxin-resistant strains, which elaborate altered, less anionic lipopolysaccharide, were completely susceptible to cecropin B. No synergism was found between cecropin B and hydrophobic antibiotics. Throughout the study, the activity of cecropin B resembled that of quaternary detergents.Cecropins are powerful antimicrobial compounds present in insect hemolymph and are also found in pig intestines (2, 4, 13). They are strongly cationic, amphipathic, a-helical peptides with 35 to 39 residues and are notably active not only against certain gram-positive bacteria but also against gram-negative bacteria. Many other cationic peptides, such as defensins from leukocyte granules and magainins from frog skin, have evoked considerable interest in recent years (5,8,14,16,18,29,30).However, cecropins are in molar terms 10 to 30 times more active than defensins and magainins against Escherichia coli and Pseudomonas aeruginosa (2, 28). Other groups of intensely studied antibacterial peptides are the synthetic fragments of cationic leukocyte proteins CAP37 (azurocidin) and the serine protease cathepsin G (1,19,20). These fragments, too, are rather weakly antibacterial. On a molar basis, they are approximately 2 or 3 orders of magnitude less active than cecropins.Cecropins cause instantaneous lysis of bacterial cells, through disintegration of their cytoplasmic membrane (2-4, 21). They also lyse artificial negatively charged and neutral liposomes, but not erythrocytes or positively charged liposomes containing phosphatidylcholine, stearylamine, and cholesterol. The strongly cationic N-terminal amphipathic helix of cecropins is necessary for effective binding to bacterial membranes. Cecropins form ion channels in artificial membranes (7), and cecropin dimers can be predicted by computer modeling to form channel-containing regular lattice structures on the membrane surface (9). However, such channels probably develop only when cecropin density is high and disintegration of the membrane takes place (7,9). Accordingly, the lethal target of cecropins is the bacterial cytoplasmic membrane. However, to reach it in gram-negative bacteria, cecropins must first penetrate the outermost surface structure, the outer membrane (OM). The OM of E. coli and other enterobacteria is known as an effective permeability barrier to a wide variety of noxious agents (17). The diffusion of hydrophobic compounds through the OM is very restricted because of the lack of glycerophospholipid bilayers, the effective pathway for hydrophobic diffusion. In addition, most of the hydrophilic compounds are excluded; they cannot diffuse through the narrow water-filled porin pores, the channels for simple sugars and other small hydrophilic nutrients (17 Assay for OM permeability-increasing activity. Hydrophobic antibiotics (rifampin, fusidic acid, novobiocin, and er...

Animal antimicrobial peptides: An overview

“…Any active peptide not recognized by peptide transporters must cross the outer membrane by the so-called selfpromoting pathway (a term coined 238 to explain the bactericidal activity of polymixin), which consists of the displacement of divalent cations that keep LPS together, thus allowing passage of external molecules. The phenomenon has been observed for peptides such as indolicidin and analogues, 180 gloverin, 199 ␣-defensins, 240 bactenecin, 241 cecropin B, 242 or cecropin A-melittin hybrids. 174,243 Such studies have focused on either the microbicidal activity of the peptides, or on permeabilization to (otherwise excluded) fluorescent probes, hydrophobic antibiotics, or substrates for periplasmic enzymes.…”

Mutations That Impact the Enteropathogenic Escherichia coli Cpx Envelope Stress Response Attenuate Virulence in Galleria mellonella

“…To investigate the sensitivity of the tested bacterial strains to AMPs, the MIC of cecropin B (Sigma) was evaluated as previously described (70). Experiments were conducted in triplicate, and the lowest concentration of cecropin B that reduced the bacterial growth by Ն95% was interpreted as the MIC (70).…”

The Two Murein Lipoproteins ofSalmonella entericaSerovar Typhimurium Contribute to the Virulence of the Organism

“…To determine the effect of antibiotics, the WT and lpp double-knockout mutant of serovar Typhimurium were spread on LB agar plates, and filter paper disks containing 10 to 100 g of rifampin were placed on the agar plates. The plates were incubated overnight at 37°C, and the zone of inhibition of bacterial growth was measured (78). A 50% increase in sensitivity of the culture to rifampin was considered positive.…”