mRNA Vaccines and Host-Directed Therapeutics Remain Unproven Against Drug-Resistant ESKAPE Bacteria, Review Finds

A new narrative review concludes that although messenger RNA platforms have transformed viral vaccinology, their application to multidrug-resistant ESKAPE pathogens remains conceptually attractive but empirically unsupported. The review, published open access on 17 September 2026 in Discover Bacteria, examines two mRNA modalities: prophylactic vaccines encoding bacterial antigens and host-directed therapeutics encoding antibacterial effectors such as antibodies, antimicrobial peptides, and immunomodulators. The authors, Emmanuel Okoro Enyi and Joy Chimuanya Nnanna, find that while vaccine candidates against Staphylococcus aureus and Pseudomonas aeruginosa have reached Phase 1/2 clinical trials, no mRNA-encoded therapeutic has entered clinical testing for bacterial infection, and no licensed bacterial mRNA vaccine exists anywhere in the pipeline.

The urgency motivating this assessment is substantial. The review cites an estimated 1.14 million deaths worldwide and 4.74 million antimicrobial resistance infections in 2021 attributable to the broader resistance burden. The WHO Global AMR and Use Surveillance System 2025 report documents carbapenem resistance in Klebsiella pneumoniae rising approximately 15 percent per year globally from 2018 to 2023, with ESKAPE pathogens accounting for more than 60 percent of reported multidrug-resistant bloodstream infections across 104 countries. The ESKAPE group—Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species—persists through horizontal gene transfer, target-site mutations, efflux pump overexpression, and biofilm formation, mechanisms that collectively blunt antibiotic efficacy. The 2024 WHO Bacterial Priority Pathogens List designates carbapenem-resistant K. pneumoniae, A. baumannii, and P. aeruginosa, plus methicillin-resistant S. aureus, as critical or high priority.

The review distinguishes three mRNA strategies to avoid conceptual ambiguity. Prophylactic mRNA vaccines express ESKAPE surface antigens to prime protective immunity before exposure; candidates encoding S. aureus clumping factor A, manganese transport protein C, and capsular polysaccharide antigens are in Phase 1/2 trials, while P. aeruginosa outer membrane protein F/I fusion mRNA remains in preclinical development. Therapeutic mRNA vaccines express bacterial antigens during established infection to rescue dysfunctional adaptive immunity, with preclinical data showing reduced bacterial burden in models of S. aureus and K. pneumoniae. Host-directed mRNA therapeutics encode antibacterial effectors that act without adaptive immune activation, using the host cell as a factory for proteins such as DNase I or interferon gamma. Crucially, none of these approaches involve direct bacterial uptake of mRNA, because prokaryotes lack eukaryotic translation machinery; all rely on transfection of host cells.

A central biological constraint shapes the entire field. Because bacteria lack receptor-mediated endocytosis and do not transport exogenous mRNA across the peptidoglycan cell wall or plasma membrane, direct mRNA targeting of bacteria is not biologically feasible under any current delivery paradigm. Both mRNA strategies therefore operate through host-cell intermediaries and inherit infection-site barriers irrelevant to intramuscular viral vaccines. Purulent fluid contains nucleases and proteases that degrade lipid nanoparticles before host-cell uptake. Biofilm extracellular polymeric substance forms a physical diffusion barrier preventing carrier access to underlying epithelium. Intracellular reservoirs, such as K. pneumoniae within macrophages, reside in phagocytes that are poorly transfected by lipid nanoparticles optimized for hepatocytes or alveolar epithelium.

The authors frame these obstacles as three resistance archetypes that any candidate must address. Physical exclusion—reduced porin expression plus RND tripartite efflux pumps in P. aeruginosa and A. baumannii—demands mucosal or inhaled mRNA-lipid nanoparticles for direct epithelial transfection. Genetic fluidity—mobile plasmids carrying blaNDM-1, mecA, and erm genes spreading resistance across species within months—requires multivalent coverage of conserved targets or host-directed pathways. Immune evasion—biofilm matrices and pore-forming toxins like S. aureus alpha-toxin and P. aeruginosa ExoU—necessitates Th17-biased or anti-toxin responses. No mRNA-ESKAPE candidate has Phase 2 data or meets all three criteria to date, and published preclinical work clusters on single, species-specific outer membrane proteins rather than conserved mobile resistance determinants.

The translational discontinuity between animal models and clinical reality is stark. Preclinical studies report striking results under controlled conditions: mRNA and self-amplifying RNA vaccines encoding Yersinia pestis F1 and LcrV antigens achieved full protection in murine pneumonic plague models, and mRNA encoding peptibodies delivered via anti-inflammatory lipid nanoparticles to the lung eradicated multidrug-resistant bacteria in pneumonia models, outperforming a comparator antibiotic. Yet the lone inhaled clinical candidate relevant to lung delivery, MRT5005—an inhaled codon-optimized CFTR mRNA-lipid nanoparticle—completed a randomized, double-blind, placebo-controlled Phase 1/2 in 42 adults with cystic fibrosis and was generally safe through 28 days but produced no consistent improvement in FEV1, indicating failure to restore functional chloride transport despite strong animal expression data. The review treats this as evidence that delivery to diseased human lungs does not recapitulate rodent results.

Delivery chemistry itself presents a mismatch. Current lipid nanoparticles, including ionizable lipids such as DLin-MC3-DMA, SM-102, and ALC-0315, were optimized for hepatic uptake and systemic protein expression, not infected lung. After intravenous dosing, more than 90 percent of lipid nanoparticles accumulate in the liver within seven hours, leaving minimal exposure for infections in lung, kidney, or wound beds. No lipid nanoparticle has shown preferential uptake in infected versus healthy tissue. In rats, intratracheal doses of 500 micrograms per kilogram caused significant acute lung injury, with mitigation requiring co-delivery of anti-inflammatory hIL-10 mRNA—evidence that the injury is driven by the carrier itself rather than the payload. Exosomes fare no better, with less than 1 percent of injected dose reaching target tissue, while cationic polymers like PEI cause membrane-disruptive necrotic cytotoxicity that is a non-starter in sepsis.

Stability and manufacturing compound the problem. Commercial mRNA-lipid nanoparticle vaccines impose rigid cold-chain requirements because phosphodiester backbone hydrolysis is the dominant instability mechanism for mRNA, and no current lipid system eliminates it. Excursions to non-frozen conditions cause loss of biological activity even when particle size and encapsulation appear unchanged. No approved lyophilized mRNA-lipid nanoparticle product exists. For inhaled products, room-temperature stability would be required for bedside nebulization and hospital stocking—a specification no current formulation meets. The regulatory environment has similarly not kept pace: existing guidance frameworks either do not apply to RNA therapeutics or lack widely accepted definitions, no agency-accepted animal model of biofilm pneumonia exists for IND-enabling toxicology, and no lung-specific no-observed-adverse-effect level thresholds are defined for intratracheal lipid nanoparticle exposure.

The review’s authors resist projecting broad clinical impact. They estimate 5 to 8 years before a first-in-human trial of an inhaled mRNA-lipid nanoparticle vaccine or host-directed biologic for bacterial pneumonia, assuming investigational new drug-enabling studies begin by 2028, with a realistic best-case approval of 2033 to 2035 for a host-directed mRNA or small molecule against multidrug-resistant A. baumannii pneumonia, contingent on solving delivery first. They position mRNA as a potential complement to, not replacement for, antibiotics within the next decade, with near-term feasibility lying in prophylaxis for high-risk patients or as adjuncts to shorten antibiotic duration. Their proposed research priorities include head-to-head benchmarking of mRNA and host-directed carriers in infection-mimetic models such as cystic fibrosis sputum and biofilm co-cultures, testing adjuvant and immunomodulator safety in septic hosts rather than healthy mucosa, and validating formulation stability under cold-chain-break conditions. As a narrative rather than systematic review, it applied no formal risk-of-bias assessment, and the authors note that most ESKAPE-specific mRNA work remains preclinical or computational. Until delivery, stability, and safety barriers are resolved with pathogen-specific data, they conclude, statements about the platform’s promise for these bacteria remain speculative.

Subject of Research: mRNA vaccines and host-directed therapeutics for multidrug-resistant ESKAPE pathogens

Article Title: A review of mRNA vaccines and host-directed therapeutics for multidrug-resistant ESKAPE pathogens

Article References: Enyi, E. O., & Nnanna, J. C. (2026). A review of mRNA vaccines and host-directed therapeutics for multidrug-resistant ESKAPE pathogens. Discover Bacteria, 3(1), Article 23. https://doi.org/10.1007/s44351-026-00058-6

Image Credits: AI Generated

DOI: 10.1007/s44351-026-00058-6

Keywords: mRNA vaccines, ESKAPE pathogens, antimicrobial resistance, host-directed therapeutics, lipid nanoparticles, biofilm, Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, inhaled delivery, WHO priority pathogens