A new study published in Applied Microbiology and Biotechnology offers a creative solution to both the production problem and the targeting problem at once. A team led by Laura Colomina-Alfaro and Antonella Bandiera at the University of Trieste, working with colleagues at Empa, the Swiss Federal Laboratories for Materials Science and Technology, King’s College London and the University of Birmingham, has engineered a fusion polypeptide called H1FK16. The construct genetically fuses FK16, a bioactive fragment derived from LL-37, to a human elastin-like polypeptide carrier known as HELP. The work, published as an open-access article in September 2026, demonstrates that this fusion system can serve as a versatile platform for biocompatible materials that selectively target pathogenic microorganisms through enzymatically triggered release of their antimicrobial payload.

The logic behind the carrier is elegant. Traditional chemical synthesis of antimicrobial peptides relies on solid-phase peptide chemistry, which becomes prohibitively costly at scale and generates significant waste. Recombinant expression, in which bacteria are genetically instructed to manufacture the desired protein, offers a cost-effective and scalable alternative. However, antimicrobial peptides are toxic to the host bacteria used to produce them, and they often fold poorly or aggregate, leading to disappointing yields. The HELP carrier sidesteps these obstacles in several ways simultaneously. Because the antimicrobial domain is tethered to a large, inert carrier protein, it cannot damage the producing cells. The carrier also enhances the solubility of the attached peptide, keeping it in a form that the bacterial protein-making machinery can handle.

Perhaps the most remarkable property of elastin-like polypeptides is their thermoresponsive phase-transition behavior. Below a characteristic transition temperature, the polypeptide remains dissolved in solution; above it, it reversibly aggregates out of the liquid. This means researchers can purify the fusion protein simply by warming and cooling the bacterial lysate in a cycle, spinning down the aggregated protein and redissolving it, without expensive chromatography columns. In previous studies, this approach effectively mitigated host toxicity, enhanced solubility and enabled simple purification of antimicrobial domains. In the new work, the production yield of H1FK16 was comparable to that of the carrier alone, averaging 180 milligrams of fusion protein per liter of bacterial culture, a figure that bodes well for industrial scalability.

FK16 itself is a sixteen-residue fragment of LL-37 that retains the peptide’s essential antimicrobial activity. The design team made a crucial decision when assembling the gene: they positioned an upstream glutamic acid residue at the junction between the carrier and the antimicrobial domain. This single amino acid acts as a molecular tripwire. Glutamyl endopeptidase, a proteolytic enzyme secreted by Staphylococcus aureus as part of its virulence arsenal, cleaves proteins specifically after glutamate residues. In other words, the very pathogen the material is designed to fight carries the key to its own destruction. When S. aureus is present and secreting its virulence-associated protease, the enzyme snips the fusion protein at the engineered site, liberating free FK16 exactly where it is needed.

This enzymatically triggered release transforms the fusion protein from a simple antibiotic into a smart, pathogen-responsive material. In its intact form, H1FK16 is non-cytotoxic, meaning it can coexist harmlessly with human cells and tissues. Only when a disease-causing bacterium betrays its presence by secreting glutamyl endopeptidase does the antimicrobial warhead detach and become active. Such triggered targeting addresses one of the central dilemmas of antimicrobial therapy: how to deploy a potent membrane-disrupting agent without harming beneficial microbes or host tissue. The strategy effectively lets the pathogen’s own biology decide when and where the drug should be unleashed.

The researchers put the released FK16 through rigorous functional testing. The liberated peptide retained its antimicrobial activity against both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli, exhibiting a minimum inhibitory concentration of 2.5 micromolar, a value comparable to figures reported in the literature for chemically synthesized versions of the peptide. This is a critical validation: it shows that recombinant production, fusion to a large carrier and enzymatic cleavage do not compromise the peptide’s ability to kill bacteria. The recombinant product is functionally indistinguishable from its chemically made counterpart, but potentially far cheaper to manufacture at scale.

Beyond killing planktonic, free-floating bacteria, the team also examined how H1FK16 treatment affected biofilms, the stubborn, matrix-encased bacterial communities that coat medical implants, chronic wounds and catheters, and that resist conventional antibiotics with alarming tenacity. Their experiments showed that treatment with the fusion polypeptide reduced biofilm biomass, one of the three key findings highlighted by the authors alongside the recombinant production of FK16 and the enzyme-triggered cleavage process. Biofilm eradication is a notoriously difficult goal, and any platform that can both prevent and disrupt these protective communities could have significant clinical value in the management of device-related and chronic infections.

The implications extend beyond a single peptide. Because the HELP carrier is genetically encoded, the same platform could in principle accommodate other antimicrobial domains, simply by swapping the fused sequence. The glutamic acid trigger could likewise be exchanged for cleavage sites recognized by other pathogen-specific proteases, creating a family of materials that each respond to a different microbial signature. Such materials could be incorporated into wound dressings, implant coatings or tissue-engineered scaffolds, lying dormant until a pathogen’s own enzymes activate them. The biocompatibility of the human-derived elastin-like carrier further supports applications in direct contact with tissue, and the study’s cytotoxicity findings suggest the intact fusion is safe for surrounding cells.

The research was supported by the Horizon Europe project STOP under grant agreement number 101057961, the NextGenerationEU project iNEST, and the Swiss State Secretariat for Education, Research and Innovation. It represents a convergence of materials science, microbiology and synthetic biology, and it arrives at a moment when the clinical pipeline for new antibiotics is dangerously thin. Challenges certainly remain before H1FK16 or its descendants reach patients: the stability of the fusion protein in complex biological fluids, the efficiency of enzymatic release in vivo, and the immunological consequences of long-term exposure all require further study. But the core demonstration stands. A human antimicrobial peptide can be produced cheaply by bacteria, rendered harmless by fusion to a thermoresponsive carrier, and reactivated on demand by the proteolytic machinery of the very pathogens it is meant to destroy. In the escalating arms race between medicine and resistant microbes, weapons that wait patiently for the enemy to disarm them first may prove to be among the most ingenious yet.

Subject of Research: Recombinant production of a cathelicidin-derived antimicrobial fusion polypeptide with pathogen-triggered peptide release

Article Title: Evaluation of a cathelicidin derivative fusion polypeptide for pathogen-targeted antimicrobial approach

Article References: Colomina-Alfaro, L., Lee, M., Zhang, S., Sist, P., Urbani, R., Shaalan, A., Di Silvio, L., Stamboulis, A., Ren, Q., & Bandiera, A. (2026). Evaluation of a cathelicidin derivative fusion polypeptide for pathogen-targeted antimicrobial approach. Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-026-14037-z

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14037-z

Keywords: antimicrobial peptides, LL-37, cathelicidin, FK16, elastin-like polypeptide, recombinant expression, Staphylococcus aureus, glutamyl endopeptidase, biofilm, antimicrobial resistance, fusion protein, triggered drug release

The research team collected RSV-positive upper respiratory tract samples from patients who tested positive between October 2023 and March 2025, spanning two consecutive seasons that straddled the deployment of nirsevimab. Whole viral genomes were recovered using an amplicon-based sequencing protocol carried out on Illumina platforms, with assemblies generated through a consensus-building pipeline known as vcf-consensus-builder. This approach allowed the investigators to capture near-complete genomes from a substantial fraction of positive samples, providing the resolution needed to place each virus within a fine-grained phylogenetic framework and to examine amino acid variation in the proteins most relevant to prevention and treatment.

In total, the study retrieved 302 RSV genome sequences, with 129 derived from the 2023–2024 season and 173 from the 2024–2025 season. The clinical composition of the cohort reflects the burden of RSV disease in the region: most infections occurred in infants aged one year or younger, who accounted for 56.0 percent of cases, and males made up 57.3 percent of the sampled population. Lower respiratory tract infections were documented in 72.7 percent of cases, 67.2 percent of patients required hospitalization, and 15.0 percent were admitted to an intensive care unit. Notably, the dataset included 17 nirsevimab breakthrough infections, cases in which infants who had received the monoclonal antibody nonetheless developed symptomatic RSV disease.

Phylogenetic analysis using maximum-likelihood methods implemented in IQ-TREE, with a GTR plus F, invariant sites, and gamma model of substitution, assigned the circulating viruses to established clades within the two major RSV subgroups. The most frequently detected clades were A.D.3, representing 26.8 percent of sequences, and B.D.E.1, representing 20.2 percent, followed by A.D.1.2 at 17.5 percent and A.D.5.2 at 14.2 percent. This distribution confirms that multiple lineages of both RSV-A and RSV-B co-circulated in Lombardy throughout the study period, with no single clade achieving complete dominance across the two seasons.

Beyond clade assignment, the researchers applied Bayesian coalescent methods under a constant population size prior and a strict molecular clock to define what they termed large phylogenetic clusters, or LPCs. These were groups of more than ten sequences supported by a posterior probability of one, indicating tightly linked transmission chains or rapidly expanding lineages. Four such clusters were identified, and two of them proved particularly striking: a cluster of 56 A.D.3 strains and another of 17 A.D.1.2 strains were detected exclusively in the 2024–2025 season, the first season of routine nirsevimab use. The emergence of season-specific clusters suggests that new viral lineages established themselves in the population after the prophylaxis program began.

A central concern of the study was the monitoring of mutations associated with resistance to the preventive and therapeutic agents now deployed against RSV. The fusion protein, or F, is the target of the monoclonal antibodies palivizumab, nirsevimab, and clesrovimab, while the nucleoprotein, or N, is the target of the small-molecule inhibitor zelicapavir. Resistance-associated mutations were identified in two samples from the 2023–2024 season and two from the 2024–2025 season. These included K272E in an A.D.5.3 strain and N262Y in an A.D.3 strain, both associated with reduced susceptibility to palivizumab, and N208D in a B.D.E.1 strain as well as the combination of K68I and L204S in a B.D.E.1 nirsevimab breakthrough infection, mutations linked to nirsevimab resistance.

Among the 17 nirsevimab breakthrough infections, the investigators also detected the S213R substitution at antigenic site Ø of the F protein in an A.D.1.2 sample. Site Ø is the principal binding site for nirsevimab, and changes at this region can diminish antibody binding. By contrast, no mutations conferring resistance to clesrovimab, a newer monoclonal antibody that binds a different region of F, were detected during the study. However, four substitutions were observed at the clesrovimab binding site: K433R in a B.D.4.1.1 strain, C439R in an A.D.1.2 strain, N437S in an A.D.5.2 strain, and N444S in an A.D.3 strain, with three of the four appearing during the 2024–2025 season. These variants were not classified as resistance mutations, but their presence at a pharmacologically critical epitope warrants continued attention.

The nucleoprotein analysis likewise revealed signals relevant to antiviral therapy. The I129M substitution in the N protein, which is associated with resistance to zelicapavir, was detected in a B.D.E.1 sample from the 2024–2025 season at an intrapatient relative abundance of 0.05, indicating that the variant existed as a minor species within the quasispecies of that infection rather than as the dominant viral population. Such low-frequency variants are typically invisible to conventional diagnostic assays but can be resolved by deep sequencing, and their detection underscores the value of high-resolution genomic surveillance in identifying resistance determinants before they become established at the population level.

Multivariable analysis comparing the two seasons revealed several statistically significant differences associated with the 2024–2025 period. That season was characterized by an increased number of sequences belonging to large phylogenetic clusters, greater amino acid variability at antigenic site Ø of the F protein, and higher Ct values on diagnostic testing, which correspond to lower viral loads in respiratory samples. Hospital intensive care unit admissions were also fewer in the second season. While the observational design of the study precludes drawing direct causal links between nirsevimab deployment and these shifts, the concurrence of prophylaxis introduction with reduced disease severity, altered viral population structure, and expanded F protein variability provides a compelling rationale for sustained monitoring.

The overall message from the Lombardy surveillance program is one of cautious reassurance combined with vigilance. Nirsevimab breakthrough infections remained rare relative to the scale of the prophylaxis campaign, and bona fide resistance mutations were detected in only a handful of samples across two seasons. At the same time, the virus is demonstrably diversifying, with season-specific phylogenetic clusters and accumulating variation at antibody binding sites on the F protein. As monoclonal antibodies and vaccines become mainstays of RSV prevention worldwide, the Lombardy experience illustrates how whole-genome sequencing, phylogenetic clustering, and mutation-level analysis can be woven into routine public health practice, ensuring that shifts in viral evolution are detected early enough to inform and adjust prophylaxis strategies before they compromise protection of the most vulnerable patients.

Subject of Research: Genomic evolution and monoclonal antibody resistance of respiratory syncytial virus in Lombardy before and after nirsevimab introduction

Article Title: Genomic characterization of circulating respiratory syncytial virus strains in Lombardy before and after the introduction of nirsevimab prophylaxis

Article References: Uceda Renteria, S., Galli, C., Ferrari, G., Cossu, M. V., Lombardi, A., Tagliabue, C., Galli, G., Parisi, A., Pellegrinelli, L., Romano, G., Bono, P., Pitrolo, A. M. G., Salpini, R., Salvini, F., Svicher, V., Bosis, S., Callegaro, A., Gori, A., Baldanti, F., … Alteri, C. (2026). Genomic characterization of circulating respiratory syncytial virus strains in Lombardy before and after the introduction of nirsevimab prophylaxis. Genome Medicine. https://doi.org/10.1186/s13073-026-01768-x

Image Credits: AI Generated

DOI: 10.1186/s13073-026-01768-x

Keywords: respiratory syncytial virus, nirsevimab, genomic surveillance, viral evolution, monoclonal antibodies, fusion protein, phylogenetics, whole-genome sequencing, antiviral resistance, breakthrough infections, Lombardy, public health