Antimicrobial Peptide Reagents: Families, Formats and the MIC Traps

The logic of antimicrobial peptide research departs from the receptor pharmacology that runs through most of this catalogue. Being generally cationic and amphipathic, antimicrobial peptides (AMPs) are reported to work physically in the first instance — inserting into anionic bacterial membranes and making them leak — rather than by occupying a defined receptor. Assembled on our antimicrobial topic page are the peptide standards, antisera and kits for immunoassay that this literature relies on, covering human host-defence peptides through to AMPs from insects and amphibians. What follows maps the families and the formats, and covers the handling that AMP assays specifically demand.

What the antimicrobial topic covers

A family earns a place here when its parent peptide has recorded activity against bacteria, fungi or viruses, or when it forms part of innate immune defence. Under that heading fall the two classical human families — cathelicidins and defensins — together with tissue-restricted AMPs, peptides that withhold iron, invertebrate and amphibian peptides used as mechanistic models, and the receptor agonists connecting AMPs to inflammatory signalling. Since many of these peptides double as immunomodulators, the topic runs into research on immunity more broadly. Everything listed is for in-vitro and approved animal research only; none of it is an anti-infective product.

Human host-defence families

• LL-37 — humans have exactly one cathelicidin, and this is it: the last 37 residues of hCAP18. No AMP in the catalogue is ordered more often, whether for membrane activity, neutralisation of LPS, chemotaxis through FPR2 or signalling in wound repair. We also stock scrambled and truncated variants to serve as mechanistic controls.
• Defensins — the alpha group (HNP-1 through HNP-4, HD-5, HD-6) and the beta group (hBD-1 through hBD-4), told apart by how their disulfides connect. Those three bridges are structural necessities, which makes oxidised and linearised preparations genuinely distinct reagents.
• Dermcidin — secreted in eccrine sweat and anionic rather than cationic, it is an illuminating exception to the rule and turns up regularly as a comparator in mechanistic studies.
• LEAP peptides — the liver-expressed antimicrobial peptides, hepcidin (LEAP-1) among them, which straddle host defence and iron regulation and are measured in a very large number of phenotyping studies.
• Lipocalin-2 (NGAL) — by sequestering siderophores it denies bacteria iron, making it a reagent for nutritional immunity rather than for membrane lysis.
• Intelectin — binds microbial glycans as a lectin, and is ordered for studies of pattern recognition.
• Antisecretory factor — investigated in models of inflammatory bowel disease and of enteric fluid secretion.

Model and comparator AMP families

• CRAMP — what the mouse has in place of LL-37. Murine cathelicidin biology demands CRAMP and not the human peptide: their sequences diverge considerably and activity does not carry across.
• Apidaecin — proline-rich and of insect origin, it acts on the ribosome inside the cell instead of lysing the membrane, which makes it the go-to comparator for separating lytic from non-lytic mechanisms.
• Bombinakinin — skin peptides from Bombina species, a genus that yields AMPs and bradykinin-related peptides in abundance.
• AMP-IBP5 — derived from IGF-binding protein 5, it illustrates antimicrobial activity arising out of a fragment of a protein with no immune role.

Companions from innate-immune signalling

• fMLF (fMLP) and WKYMVm — agonists at the formyl peptide receptors, serving as controls for neutrophil chemotaxis and as the benchmark ligands whenever an AMP is tested for action via FPR1 or FPR2.
• Peptidoglycan-related reagents — components of the bacterial cell wall, applied as stimuli for pattern-recognition receptors alongside AMP experiments.
• Ac2-26 — this annexin A1 peptide agonises FPR2 in a pro-resolving direction and is often run with LL-37, the two converging on one receptor.
• Interleukin and CXCL families — a number of chemokines are directly antimicrobial, and cytokine measurements are the customary downstream readout when AMP immunomodulation is studied.

Formats available under this topic

Peptide standards made synthetically form the heart of the topic and are bought in larger amounts than elsewhere, since assays for minimum inhibitory concentration get through material — fills in milligrams are routine rather than exceptional. For mechanism, tagged peptides matter: fluorescently marked AMPs allow microscopy of where they sit on bacterial membranes, and biotin-tagged versions allow binding partners to be pulled down. Polyclonal antisera underpin detection of endogenous AMP expression in tissue, which is how most host-defence papers show induction. Kits based on ELISA put figures on the human cathelicidin, hepcidin, lipocalin-2 and the beta-defensins in plasma, sputum and culture supernatant. Screening for structure–activity relationships runs on peptide libraries, which is the principal route to designed AMP analogues.

Choosing within a family

Species comes first here, and more rigidly than in other topics. CRAMP cannot stand in for LL-37, and deploying the human peptide in a mouse model is an acknowledged way to produce uninterpretable data. Oxidation state is second: defensins need their disulfide bridges paired correctly, and a catalogue entry ought to say whether the peptide ships oxidised and folded or linear. Counter-ion is third, and it is the trap peculiar to AMPs: TFA carried over from synthesis is antimicrobial and cytotoxic in its own right at AMP assay concentrations, so acetate-exchanged material is standard for MIC and viability work. Purity and content come fourth: a peptide quoted at 95% purity can be well under 95% peptide by mass once salt and water are accounted for, which moves every MIC figure you publish. Peptide content against gross weight is explained in the COA guide.

Handling notes

Because cationic peptides bind tightly to polystyrene and to plastic tips, losses in the low micromolar range are big enough to shift an MIC by a dilution or more — work in polypropylene, use low-binding tips, or include a carrier such as 0.01% acetic acid with BSA wherever the assay allows. The medium counts too: conventional cation-adjusted Mueller-Hinton broth holds AMP activity down compared with low-salt media, which is precisely why AMP papers describe their medium so precisely. Otherwise the usual rules apply — lyophilised vials sealed at −20 °C, aliquots used once, freeze–thaw kept to a minimum, and an eye on how well hydrophobic sequences dissolve (peptide solubility, storage guide, aliquoting guide).

Where to go next

Browse the complete antimicrobial topic, the immune system hub, or the skin hub, where most human AMP expression gets studied. Among adjacent guides, the guide to research peptide types helps with reading specifications, while the cancer section shares the chemokine families.