Novel Leech Antimicrobial Peptides — Hirunipins

Antimicrobial resistance (AMR) is among the top global health threats identified by the WHO. The clinical antibiotic pipeline has stagnated, with few novel mechanism classes entering development. Antimicrobial peptides (AMPs) represent a promising alternative: they employ membrane-disruption mechanisms fundamentally different from conventional antibiotics, making cross-resistance less likely.

The medicinal leech secretome — with 434+ identified proteins — has been largely unexplored for antimicrobial activity until recently. Kumar et al. (2025) changed that with the first systematic characterization of hirunipins.[R1]

The researchers isolated a family of alpha-helical antimicrobial peptides from Hirudo medicinalis salivary gland transcriptomes and confirmed their expression through proteomic analysis. These peptides — named hirunipins — showed structural features including amphipathic alpha-helical topology, cationic charge, and membrane-active conformation.[R1]

Hirunipins kill bacteria through a three-step membrane-disruption mechanism visualized via optical diffraction tomography (ODT):

A particularly significant finding: hirunipins demonstrated the ability to penetrate and disrupt established bacterial biofilms. Biofilms are a major clinical challenge — they protect bacteria from both antibiotics and immune cells, contributing to chronic and device-associated infections. Conventional antibiotics typically require 100–1000x higher concentrations to affect biofilm-embedded bacteria.

Combination testing using fractional inhibitory concentration index (FICI) methodology showed synergistic or additive interactions between hirunipins and multiple conventional antibiotic classes:

Beyond direct antimicrobial activity, hirunipins showed immunomodulatory properties in cell-based assays. Treatment of LPS-stimulated macrophages resulted in dose-dependent reductions in pro-inflammatory cytokines:

The hirunipins study demonstrates preclinical potential of the medicinal leech secretome as a viable bioprospecting platform for novel antimicrobial agents. With the WHO reporting that only 13 new antibiotics were approved between 2017 and 2023, sources of structurally novel antimicrobial compounds are critically needed. The leech offers advantages: its AMPs have been evolutionarily refined over 400+ million years to interact with bacterial membranes, and 434+ salivary proteins remain largely uncharacterized for antimicrobial activity.

For hirudotherapy specifically, these findings add to the scientific understanding of what happens at the leech application site — beyond anticoagulation and anti-inflammation, there is a localized antimicrobial microenvironment created by the leech’s own defense molecules.