The review highlights a striking imbalance in how these preparations are produced. Standardized commercial lysates, particularly the eight-species preparation OM-85, were used in 19 of the 34 studies (55.9 percent), while the remaining 15 studies (44.1 percent) relied on research-grade or laboratory-prepared samples. OM-85 contains thermally or alkaline-lysed material from species including Haemophilus influenzae, Streptococcus pneumoniae, Klebsiella, Staphylococcus, Streptococcus and Moraxella catarrhalis. Research-grade preparations, by contrast, ranged from defined monobacterial lysates of Lactobacillus, Lactococcus, Parabacteroides and Bifidobacterium strains to locally defined polybacterial mixtures such as the UNAM-HIMFG formulation used in urinary tract infection models. The authors identify this heterogeneity in species composition, inactivation method, dose and route as one of the most significant obstacles to reproducibility and to the transition from murine models to clinical studies.

Respiratory infection models remain the most translationally advanced area. In the reviewed studies, oral administration of OM-85 protected mice against influenza virus infection and reduced susceptibility to superinfection with Streptococcus pneumoniae, improving survival and reducing signs of morbidity. Intranasal OM-85 protected against respiratory syncytial virus by inhibiting viral replication and reducing peribronchial and perivascular inflammation. A polyvalent lysate administered as a single intranasal dose protected mice against lethal pneumococcal pneumonia through a MyD88-dependent mechanism that the studies found to be independent of neutrophils, IL-17A or caspase-1 activation. OM-85 also reduced expression of the Ace2 and Tmprss2 receptors in the murine lung in a MyD88/Trif-dependent manner, and maternal administration during gestation improved offspring resistance to neonatal respiratory infections.

Allergic disease models produced consistently supportive anti-inflammatory signals. Oral or intranasal OM-85 reduced airway eosinophilia, serum IgE levels and Th2-associated cytokines including IL-4, IL-5 and IL-13, while expanding regulatory T cells and suppressing the epithelial-derived alarmins IL-33 and TSLP. Mechanistic studies showed that OM-85 modulates the epithelial–IL-33–ILC2 axis and reprograms dendritic cells toward a tolerogenic phenotype, effects that persisted even in germ-free mice, indicating that microbiota-independent mechanisms contribute. Maternal administration during gestation or lactation reduced allergic inflammation in offspring through the IL-33/ILC2 pathway and short-chain fatty acid production. Heat-killed monobacterial lysates of Bifidobacterium longum BBMN68 and Lactococcus lactis showed similar protective effects in allergic airway inflammation models, although differences in sensitization protocols, allergen exposure, dosing regimens and routes limit direct comparisons between studies.

In the gut, four studies of dextran sulfate sodium-induced colitis reported that oral Lactobacillus casei DN-114 001 lysate alleviated disease severity in BALB/c mice but not in SCID mice, indicating that the protective effect depends on an intact immune system. Benefits were associated with restored intestinal barrier integrity through increased tight junction protein expression and reduced permeability. The commercial polybacterial lysate Coliphagein improved clinical and histological scores, increased fecal IgA and reduced pro-inflammatory chemokines. Heat-killed lysates of Lactococcus petauri, Lactococcus lactis L8 and Lacticaseibacillus paracasei 6235 reduced colonic inflammation and TNF-α expression and favorably modulated the gut microbiota, with some studies reporting efficacy comparable to or greater than that of the corresponding live probiotics.

Urinary tract infection models added a non-respiratory dimension. Sublingual administration of the polyvalent UNAM-HIMFG lysate accelerated bacteriuria clearance, prevented chronic bladder inflammation and promoted a balanced response characterized by coordinated TNF-α and IL-10 production. Orally administered multistrain heat-killed postbiotic formulations reduced bladder and renal bacterial colonization, limited tissue inflammation and provided sustained protection against both primary and recurrent infections, maintaining high rates of urinary sterility even after treatment discontinuation. In a single preventive study of experimental autoimmune encephalomyelitis, oral Parabacteroides distasonis lysate delayed disease onset and reduced clinical severity through expansion of regulatory T cells in mesenteric and inguinal lymph nodes and remodeling of the gut microbiota, with no systemic translocation of bacterial antigens detected.

On the mechanistic level, the review describes how lysates engage pattern recognition receptors—TLR2, TLR4, TLR5, TLR9 and the cytoplasmic sensors NOD1 and NOD2—triggering NF-κB signaling, dendritic cell maturation and cytokine secretion. Studies using TLR-, NLRP3- or IL-10-deficient mice provided direct evidence that lysates modulate inflammasome activation, regulatory T cell differentiation and innate immune memory. The direction of immune polarization depended on disease context: Th1 and Th17 responses in infection models, suppression of Th2 responses in allergy models, and Foxp3-expressing regulatory T cell induction in autoimmune and colitis models. The review also emphasizes trained immunity—epigenetic reprogramming of monocytes, macrophages and natural killer cells that produces faster responses to secondary challenges—as a key mechanism behind the prophylactic effects seen in recurrent infection models.

Route of administration emerged as a determining factor in the nature of immune engagement. Oral delivery, the most common route, acts through gut-associated lymphoid tissue, promoting regulatory T cell expansion, secretory IgA production and barrier reinforcement, with the caveat that gastric degradation and species differences in gastrointestinal physiology complicate dose translation. Intranasal and aerosol routes target nasal-associated lymphoid tissue and respiratory epithelium, reducing alarmins such as IL-33 and TSLP in asthma models, but carry risks of local irritation and imprecise dosing. Systemic routes, used less frequently, directly expose the spleen and lymph nodes to bacterial components; in the colorectal cancer model, a Lactobacillus acidophilus lysate combined with anti-CTLA-4 antibodies reduced tumor incidence compared with either treatment alone and shifted the tumor microenvironment toward more effector CD8+ T cells and fewer regulatory T cells and M2 macrophages, though the lysate’s independent antitumor efficacy remains untested.

The authors caution that the evidence base carries substantial limitations. Randomization was not consistently described across the included studies, allocation concealment was rarely reported, and investigator blinding was limited, although the adapted SYRCLE-based assessment did not identify major sources of systematic bias. Most studies used young, healthy, specific pathogen-free mice that do not reflect the immunological history or comorbidities of human patients, and fundamental differences in microbiota composition, immune maturation and endotoxin sensitivity between mice and humans limit predictability. Most studies were also preventive in design, treating animals before disease induction, while therapeutic efficacy after disease onset—the scenario most relevant to clinical practice—remains underexplored, and long-term safety data are sparse.

The review concludes that murine studies provide a multifaceted proof of concept for bacterial lysates as versatile immunomodulatory agents, with the most consistent and translationally advanced evidence in respiratory tract infections, the original clinical indication for these preparations. Promising signals also emerged in allergic asthma, recurrent urinary tract infection, experimental colitis, neuroinflammation and cancer immunotherapy adjuvant settings. Realizing this potential, the authors argue, will require standardized manufacturing and quality-control protocols, a shift toward therapeutic rather than exclusively preventive designs, deeper mechanistic elucidation, and bridging studies in humanized mice or higher-order animal models before the preclinical promise can be converted into safe and effective clinical applications.

Subject of Research: Preclinical evaluation of bacterial lysates as immunomodulators in murine disease models

Article Title: Preclinical Evaluation of Bacterial Lysates in Murine Disease Models: A Systematic Review

Article References: Ancuța, D.-L., & Coman, C. (2026). Preclinical Evaluation of Bacterial Lysates in Murine Disease Models: A Systematic Review. Immunity, Inflammation and Disease, 14(10), Article e70553. https://doi.org/10.1002/iid3.70553

Image Credits: AI Generated

DOI: 10.1002/iid3.70553

Keywords: bacterial lysates, postbiotics, murine models, immunomodulation, OM-85, allergic asthma, respiratory infections, colitis, urinary tract infections, trained immunity, systematic review, preclinical research