Antimicrobial Peptides for Detection and Diagnostic Assays - Designing Receptors for the Next Generation of Biosensors - page 88

While redistribution of lipids and membrane thinning can result at low peptide
concentrations in the carpet model, capable of causing significant global physio-
logical disturbance [ 23 ], many cationic AMPs will form pores through the carpet
mechanism if a high peptide concentration is applied. The human cathelicidin,
LL-37, in addition to the toroidal pore, and cecropin peptides have been suggested
as using the carpet mechanism of antimicrobial activity through surface adsorbance
and electrostatic interactions with membrane phospholipids and cationic regions of
the peptide [ 18 ].

3 Broad-Based Sensing Using AMPs

AMP-based detection represents a new paradigm in sensing — namely, the ability
to screen a sample for the presence of many different microbes without target-
specific reagents, and to provide broad classification information on the species
detected. The concept for such a detection strategy is simple: a series of AMPs with
overlapping, but not identical, binding specificities binds to and detects a broad
spectrum of microbial targets. Based on the pattern of which AMPs bind the target,
the user can glean information regarding what type of microbe is present, although
an unequivocal identification is generally not possible (Fig. 1 ). By using this
strategy, target-specific reagents (e.g., antibodies, PCR primers) do not need to be
developed or be available on-site, and no knowledge of what might (or might not)
be present is needed. In this manner, unexpected or unknown microbial species can
be detected and sufficient information provided by the binding patterns to allow
more targeted follow-on testing for species identification.
Affinity-based biosensors often utilize a sandwich-type format, using a surface-
immobilized recognition molecule to capture the target from solution, and a
“tracer” component used to quantify the target bound to the surface. We will
describe integration of AMPs as both solution-phase and surface-immobilized
components in detection assays. Although the successful combination of AMP-
tracer and AMP-capture reagents in detection assays has not been reported to date,
we believe that optimization of both components separately will lead to a system
utilizing no affinity reagents other than AMPs for biological recognition.

3.1 Solution-Phase Detection Using AMPs

Use of AMPs as solution-phase “tracers” should be considered as distinct from
their application as antimicrobial or decontaminating agents. While the latter
applications rely on their microbiocidal properties, the former use requires cell
recognition and binding without killing. Furthermore, AMP tracers require modifi-
cation with a label that can be measured by an appropriate detector. Most impor-
tantly, this tag must not interfere with the AMP's recognition and binding

characteristics.