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378 Cell-Penetrating Peptides: Processes and Applications
18.1 INTRODUCTION
This chapter reviews peptides that are able to permeate microbial cell membranes
and enter cells. The existence of microbial-permeating peptides is surprising because
peptides are generally considered too large to pass microbial cell barriers, which are
typically more stringent than mammalian cells. Nevertheless, there are many clear
examples of peptides able to permeate microbial cells without damage to host cells.
Indeed, most of our knowledge of cell-permeating peptides and their potential
practical applications stems from the discovery that the innate immune response in
animals, plants, and insects involves the release of antimicrobial peptides.
Antimicrobial peptides have been shown to kill microbes with impressive potency
and specificity and many research groups have sought to develop them as therapeutic
agents for infectious disease treatment. In related applications, antimicrobial peptides
could provide pathogen resistance to agricultural crops or be used in post harvest food
preservation. 1 In all such cases, the objective is to develop peptides to augment the
host’s own defense mechanisms against pathogenic microorganisms.
The need is increasing for more prudent antibiotic usage and new strategies to
fight antibiotic-resistant strains. 2 Despite early warnings during the 1940s, drug
resistance has increased into a serious problem that can extend to “stand-by” antibiotics
and even the newly introduced oxazolidinones. 3,4 To overcome this problem,
antimicrobial peptides or their derivatives offer some exciting possibilities and “antibiotic
peptides” may prove to be less susceptible to resistance mechanisms. 5 Effective
antimicrobial peptides brought to market would lower our reliance on conventional
antibiotics and help to overcome resistant strains.
A second possible application is to use permeating peptides as carriers for delivery
of foreign substances into microorganisms. 6 The idea seems reasonable, given that many
antimicrobial peptides appear to enter cells and act on intracellular targets. 1,7,8 Therefore,
it should be possible to use permeating peptides to deliver foreign substances that
otherwise would enter microbes only poorly. This application has the same objective
as earlier efforts to use receptor-mediated oligopeptide transport mechanisms for antimicrobial
delivery, but the cellular uptake mechanism is different. 9-11 The microbialpermeating
peptides described here enter cells independent of classical receptor-mediated
uptake, as is also the case for the eukaryotic cell-permeating peptides described
in earlier chapters in this book (see Chapters 1 to 7, 16, and 17).
In this chapter, we first briefly describe microbial membranes and how they
differ from mammalian cell membranes. Second, the chapter describes the origin
and structures of antimicrobial peptides and their ability to permeate microbial cells.
The chapter then focuses on practical applications for cell-permeating peptides in
antimicrobial development and cellular delivery. Finally, we describe several
research strategies to characterize the uptake of microbial cell-permeating peptides.
18.2 BACKGROUND
18.2.1 MICROBIAL AND MAMMALIAN CELL BARRIERS
The cell membrane is formed by a phospholipid bilayer that is generally impermeable
to fluids, ions, and most foreign substances. This would seem to isolate the cell from