As intensive aquaculture expands globally to meet protein demands, high-density stocking frequently triggers opportunistic bacterial infections, compelling farms to rely heavily on chemical therapeutics and antibiotics. The widespread use of these pharmacological interventions has accelerated antimicrobial resistance, contaminated aquatic ecosystems, and disturbed host microbiota, intensifying the search for non-antibiotic preventive alternatives. In an experimental investigation published in Frontiers in Microbiology, researchers evaluated a fish-derived lactic acid bacterium, Loigolactobacillus coryniformis strain LJR-37, isolated from healthy crucian carp (Carassius auratus), to assess its capacity to bolster performance and pathogen defenses in farmed tench (Tinca tinca). The study paired in vitro physiological characterization and whole-genome sequencing with a 28-day feeding trial followed by an experimental Aeromonas hydrophila challenge. Dietary administration of the bacterium enhanced feed efficiency, preserved intestinal villus architecture, elevated systemic immune parameters, and improved survival following bacterial exposure, while restructuring the gut microbial community and its predicted metabolic potential.

The investigation specifically tested the host-derived probiotic hypothesis by recovering an indigenous candidate from a cyprinid fish and administering it to tench, a member of the same taxonomic order, Cypriniformes. Because the colonization behavior and functional efficacy of probiotic strains often exhibit strict phylogenetic and host specificity, isolating candidate bacteria from closely related host lineages provides a biologically congruent model rather than extrapolating across distant taxa. The bacterial strain LJR-37 formed smooth, milky-white colonies of Gram-positive rods, exhibiting 16S ribosomal RNA gene homology with Loigolactobacillus coryniformis. High-throughput hybrid genome sequencing combining PacBio, Oxford Nanopore, and Illumina platforms produced a single circular chromosome spanning 2,888,804 base pairs with an average GC content of 43.04 percent and no detectable plasmids. Pairwise average nucleotide identity analyses yielded values between 97.55 and 98.80 percent against reference Loigolactobacillus coryniformis strains, decisively exceeding the standard 95 to 96 percent prokaryotic species boundary and confirming the definitive taxonomic identity of the isolate.

Before introducing the candidate bacterium into live animals, researchers conducted detailed biosafety and physiological tolerance evaluations in vitro. Safety screenings established that LJR-37 exhibited gamma-hemolysis with no hemolytic halo on sheep blood agar, contrasting sharply with the positive control pathogen Aeromonas hydrophila, which displayed clear beta-hemolytic activity. Genome-wide alignments against the Virulence Factor Database and the Comprehensive Antibiotic Resistance Database identified no acquired resistance determinants or known toxin genes above an eighty percent sequence identity threshold, with hits representing wild-type essential housekeeping genes. In simulated gastrointestinal stress assays, LJR-37 survived exposure to acidic broth at pH 3.0 with a 40.6 percent survival rate after eight hours, while maintaining 19 percent viability in media containing 0.1 percent bile salts. Genomic annotation confirmed that the bacterium encodes multiple stress-protective pathways, including the dltABCD operon for lipoteichoic acid modification, molecular chaperones dnaK and clpL, universal stress protein uspA, and osmoprotectant transport systems, alongside strong biofilm formation.

The antibacterial capabilities of the strain were thoroughly profiled against Aeromonas hydrophila ATCC 7966, a widespread Gram-negative aquatic pathogen causing motile septicemia and enteritis. Cell-free supernatant harvested from LJR-37 cultures during mid-log and stationary growth phases generated marked inhibition zones, reaching a peak diameter of 20.46 millimeters after twenty-four hours. Neutralization of the supernatant to pH 6.5 and enzymatic treatment with catalase failed to abolish antibacterial activity, demonstrating that pathogen suppression did not arise merely from lactic acid accumulation or hydrogen peroxide generation. Stability testing revealed that concentrated cell-free supernatant retained more than fifty percent of its antagonistic activity after heating at 90 degrees Celsius for one hour and approximately seventy percent at 100 degrees Celsius, while also withstanding ultraviolet-C radiation for up to eight hours and maintaining function across pH 4 to 9. However, incubation with proteolytic enzymes caused substantial activity losses, providing preliminary evidence that the active antimicrobial metabolite possesses a heat-stable, proteinaceous bacteriocin-like chemical nature.

To clarify the mechanical basis of this antibacterial activity, investigators examined pathogen morphology and membrane physiology following exposure to the concentrated bacterial supernatant. Fluorescent double staining with SYTO-9 and propidium iodide revealed an abrupt shift from intact green-fluorescing cells to red-fluorescing envelopes in Aeromonas hydrophila after two hours, verifying that the cell-free supernatant compromised inner membrane permeability. Ultrastructural examination under scanning electron microscopy documented severe morphological deformities, transforming smooth, rod-shaped bacterial bodies into collapsed structures marked by deep surface wrinkles, irregular lesions, and distinct pores. Concurrently, biochemical quantification of intracellular adenosine triphosphate demonstrated a rapid leakage of energetic contents from the challenged pathogen cells. Relative to untreated controls, intracellular ATP levels fell by 16.7 percent within thirty minutes of exposure, dropped by 40.2 percent after one hour, and declined by 80.3 percent at two hours. These combined observations confirmed that the supernatant exerts its bactericidal effect through targeted permeabilization and rupture of the cytoplasmic membrane.

Building on these in vitro findings, the researchers launched an in vivo animal feeding trial using 200 juvenile tench with an initial mean body weight of approximately fifteen grams. Fish were acclimated for seven days, divided across six independent tanks representing two experimental groups, and maintained on a fixed daily ration equivalent to four percent of body weight divided into two equal meals. The experimental diet was supplemented daily with freshly cultured LJR-37 cells bound with two percent sodium carboxymethyl cellulose to deliver 1.18 times 10 to the tenth colony-forming units per gram of feed. Over the 28-day feeding trial, fish consuming the supplemented regimen exhibited significantly higher weight gain rates of 57.5 percent compared to 39.3 percent in control animals, while specific growth rates reached 1.62 percent per day versus 1.19 percent per day. Concurrently, the feed conversion ratio dropped from 3.17 to 2.16, reflecting enhanced metabolic conversion of the fixed feed allotment.

Histological evaluations of the tench digestive tract indicated that probiotic administration reinforced mucosal integrity and enhanced intestinal digestive capacity before and after pathogenic exposure. Cross-sectional morphometric measurements of the distal intestine demonstrated that fish in the probiotic group possessed significantly longer mucosal villi prior to infection, averaging 369.9 micrometers compared to 334.5 micrometers in controls, alongside greater initial villus width. Following bacterial challenge, both groups experienced intestinal tissue damage, but structural degradation was substantially mitigated in supplemented animals, which retained significantly greater villus height (341.8 micrometers versus 281.0 micrometers) and maintained a stable villus height-to-crypt depth ratio, whereas controls experienced marked villus atrophy. Furthermore, goblet cell counts displayed a significant diet-by-challenge interaction, showing a greater compensatory increase in probiotic-fed tench. Complementing these structural benefits, intestinal enzymatic assays revealed that luminal trypsin activity rose from 23.26 milligrams per milliliter in controls to 41.60 milligrams per milliliter in the LJR-37 group, demonstrating increased proteolytic capacity.

Assessment of humoral immune indicators and circulating antioxidant markers confirmed that dietary supplementation bolstered physiological defenses prior to infection and regulated inflammatory cascades during subsequent disease challenge. Probiotic-fed tench exhibited significantly elevated baseline serum immunoglobulin M levels of 2.28 nanograms per milliliter compared to 1.64 nanograms per milliliter in controls, paired with significantly reduced tumor necrosis factor-alpha concentrations. Antioxidant capacity was likewise reinforced, as superoxide dismutase activity increased significantly from 3.06 to 4.18 nanograms per milliliter, while malondialdehyde concentrations dropped from 1.04 to 0.38 nanomoles per milliliter, indicating suppressed lipid peroxidation. When fish were subsequently subjected to an intraperitoneal challenge with Aeromonas hydrophila at a median lethal dose of 2.0 times 10 to the seventh colony-forming units per fish, cumulative seven-day mortality was 44 percent in the LJR-37 group compared to 63 percent in controls, reflecting a statistically significant relative protection level of 30.2 percent alongside elevated anti-inflammatory interleukin-10.

To resolve the ecological changes accompanying these physiological improvements, the authors performed shotgun metagenomic sequencing on distal intestinal contents collected prior to pathogen infection. Bioinformatic analysis generated 764,007 non-redundant gut microbial genes, revealing that LJR-37 administration significantly lowered microbial alpha diversity, as evidenced by marked reductions in both Shannon and Chao indices. Principal component analysis of Bray-Curtis distances demonstrated distinct, tight clustering of probiotic-fed samples apart from the more dispersed control community. Taxonomic profiling established that dietary supplementation drove a substantial expansion of the phylum Bacillota and the genus Loigolactobacillus, which escalated from 46.53 percent relative abundance in control fish to 87.74 percent in the treated cohort. Concurrently, the relative abundances of opportunistic aquatic pathogens fell dramatically, with Aeromonas decreasing from 9.93 percent in controls to 0.29 percent in supplemented fish, and Acinetobacter declining from 4.18 percent to 0.13 percent, reflecting competitive exclusion and dominant niche colonization by lactic acid bacteria.

Functional profiling of the metagenomic catalog through the Kyoto Encyclopedia of Genes and Genomes and Carbohydrate-Active Enzymes databases indicated a broad shift toward microbial anabolic pathways and carbohydrate utilization. The microbiota of supplemented fish exhibited enriched representation of genes encoding glycoside hydrolases, glycosyltransferases, aminoacyl-tRNA synthetases, and branched-chain amino acid biosynthesis, alongside diminished representation of pathways dedicated to fatty acid degradation and aromatic compound catabolism. However, the authors emphasized critical scientific qualifications and methodological limitations that preclude drawing direct causal conclusions. The observed connections between gut microbial reorganization, immune enhancement, and disease resistance remain associative, particularly because high background levels of Loigolactobacillus were already present in control fish and absolute pathogen densities were not quantified by quantitative polymerase chain reaction. Furthermore, the feeding trial tested only a single inclusion level in tench, the putative bacteriocin remained unpurified, and metagenomic comparisons rested on three tank units per group, underscoring the need for dose-response validation and metabolomic tracing.

Subject of Research: Evaluation of fish-derived Loigolactobacillus coryniformis LJR-37 on growth, disease resistance, and gut microbiota in tench

Article Title: Fish-derived Loigolactobacillus coryniformis LJR-37 enhances growth performance and disease resistance in tench in association with altered gut microbial ecology and predicted metabolic potential

Article References: Lin, J., Liu, S., Tang, D., Zhu, G., He, Y., Xu, N., Wang, K., Sha, W., Yin, B., & Dong, W. (2026). Fish-derived Loigolactobacillus coryniformis LJR-37 enhances growth performance and disease resistance in tench in association with altered gut microbial ecology and predicted metabolic potential. Frontiers in Microbiology, 17, Article 1961620. https://doi.org/10.3389/fmicb.2026.1961620

Image Credits: AI Generated

DOI: 10.3389/fmicb.2026.1961620

Keywords: Loigolactobacillus coryniformis, tench, Tinca tinca, probiotics, Aeromonas hydrophila, aquaculture, gut microbiota, metagenomics, Fish-derived, Loigolactobacillus, coryniformis, LJR-37