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Microbial Membrane-Permeating Peptides and Their Applications 383
Although cationic peptide uptake into bacteria is viewed differently by many
researchers, it is accepted that electrostatic attraction is often an important first step
in uptake. Consistent with this, it is generally observed that increased salt concentrations
shield the interaction and reduce peptide potency. Also, resistance can arise
from cell barrier modifications that limit permeation. For example, resistant Salmonella
cells contain modified LPS molecules with reduced membrane negative charge
density. 33 Thus, an initial electrostatic attraction appears to be needed for cellpermeating
peptides to enter cells. However, some variations and exceptions exist
within this rule. For example, certain cationic peptides are more salt tolerant and
some antimicrobial peptides are not cationic. 34
Based on this general view of peptide permeation, numerous synthetic cationic
peptides in the range of 10–20 amino acids have been made with variations in their
amphipathic structure. The peptides have been subjected to a variety of biochemical
analysis with artificial membranes in order to elucidate the mechanism of action, as
reviewed. 28-30,35 However, given uncertainties surrounding the mechanism of action
of cell-permeating peptides and the possibility for several different mechanisms, the
design process remains uncertain.
18.3.1.2 Receptor-Mediated Transport of Peptides and Endocytosis
A variety of peptide sequences are known to enter microbial cells via receptormediated
translocation or permeases; these mechanisms certainly offer opportunities
to deliver foreign substances into microbial cells. Furthermore, receptor-mediated
delivery is a very attractive approach for cellular delivery because of the potential
for cell type specificity. The best known receptor-mediated transport mechanism in
microorganisms involves the oligopeptide permeases, which import di- and trioligopeptides.
There are multiple oligopeptide permeases in microbial cells, and overlapping
substrate specificity between species. For more information on receptormediated
uptake we direct readers to two excellent reviews. 36,37
Endocytosis also offers a range of attractive possibilities for cell delivery; however,
it is very complex and the intracellular fate of internalized vesicles is difficult
to predict. Furthermore, most substances that undergo endocytosis are not released
into the cytoplasm or able to reach the nucleus. Nevertheless, endocytosis is used
by many bacterial toxins to reach the intracellular targets in eukaryotic hosts, so
there are possibilities for practical applications. 38,39 We direct interested readers to
an excellent recent review by Sandvig and van Deurs. 40
18.3.2 CELL-KILLING MECHANISMS
18.3.22.1 Membrane Leakage
Membrane leakage is usually referred to as the main cytotoxic activity of antimicrobial
peptides. However, it is not clear which events actually lead to microbial
death by most antimicrobial peptides. There is no doubt that antimicrobial peptides
interact with membranes, but several different hypotheses explain how this could
lead to cell death. These include physical disturbance of membrane integrity, fatal