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Atlantic bluefin tuna (ABFT; Thunnus thynnus ) is among the most valuable commodities in Mediterranean mariculture, and recent increases in seawater temperatures have coincided with the re-emergence of bacterial diseases causing catastrophic losses. During the summer 2025 mortality event, we investigated stranded and moribund ABFT using bacteriological isolation and identification, high-throughput 16S amplicon profiling of tissue-associated bacterial communities, whole-genome sequencing to resolve a complete genome of the etiologic agent, transmission electron microscopy, and gross and histopathological examinations. Across multiple organs, the metagenomic profiles were overwhelmingly dominated by Lactococcus garvieae , supporting a primary systemic bacterial etiology. The isolate displayed a capsulated phenotype, and genome analysis identified a capsule-associated gene cluster consistent with a capsulated lineage. Capsule expression was further confirmed ultrastructurally by transmission electron microscopy. Pathology indicated fulminant septicemia with prominent hemorrhagic lesions and severe cardioperitoneal involvement, including fibrinous epicarditis with abundant Gram-positive cocci, alongside marked hepatic and splenic pathology. Collectively, these data document, for the first time in two decades, the detection of a capsulated L. garvieae serotype or lineage associated with ABFT mass mortality. Rapid etiologic confirmation, mitigation of temperature-related and husbandry-associated stress, and targeted prevention strategies (including vaccination and biosecurity) are recommended to reduce recurrence in warming coastal waters.

The qualified presumption of safety (QPS) approach was developed to provide a regularly updated generic pre‐evaluation of the safety of biological agents, intended for addition to food or feed, to support the work of EFSA's Scientific Panels. The QPS approach is based on an assessment of published data for each agent, with respect to its taxonomic identity, the body of relevant knowledge, safety concerns and occurrence of antimicrobial resistance. Safety concerns identified for a taxonomic unit (TU) are, where possible, confirmed at the species/strain or product level and reflected by ‘qualifications’. In the period covered by this statement, no new information was found that would change the status of previously recommended QPS TUs. Schizochytrium limacinum, which is a synonym for Aurantiochytrium limacinum, was added to the QPS list. Of the 78 microorganisms notified to EFSA between October 2020 and March 2021, 71 were excluded; 16 filamentous fungi, 1 Dyella spp., 1 Enterococcus faecium, 7 Escherichia coli, 1 Streptomyces spp., 1 Schizochytrium spp. and 44 TUs that had been previously evaluated. Seven TUs were evaluated: Corynebacterium stationis and Kodamaea ohmeri were re‐assessed because an update was requested for the current mandate. Anoxybacillus caldiproteolyticus, Bacillus paralicheniformis, Enterobacter hormaechei, Eremothecium ashbyi and Lactococcus garvieae were assessed for the first time. The following TUs were not recommended for QPS status: A. caldiproteolyticus due to the lack of a body of knowledge in relation to its use in the food or feed chain, E. hormaechei, L. garvieae and K. ohmeri due to their pathogenic potential, E. ashbyi and C. stationis due to a lack of body of knowledge on their occurrence in the food and feed chain and to their pathogenic potential. B. paralicheniformis was recommended for the QPS status with the qualification ‘absence of toxigenic activity’ and ‘absence of genetic information to synthesize bacitracin’.

Lactococcus garvieae is known as main agent of the bacterial diseases, Lactococcosis in trout farms. The present study was aimed to study the metabolic bases of the temperature-dependent pathogenicity of the L. garvieae using 1H NMR spectroscopy. The bacteria were grown at different temperatures, including 10, 14, 18 and 22˚C and then the metabolites extracted, identified and quantified. The results of PLS-DA analysis clearly separated the experimental treatments. The main metabolites responsible for this separation were acetate, acetoacetate, creatine phosphate, succinylacetone and trehalose. Furthermore, the result of the analysis of variance indicated also significant differences in metabolome content between temperature treatments. The bacteria exposed to higher temperatures showed more concentration of acetate and acetoacetate compared to those grown at 10°C. The concentrations of trehalose were higher in the bacteria grown at 14 and 18°C compared to other temperature treatments. The higher levels of succinylacetone were found in the bacteria exposed to the temperature less than 14°C compared to those grown at 18 and 22°C. The creatine phosphate concentrations increased with temperature, however, a significant decline occurred at 22°C. The levels of isoeugenol, methionine and betaine significantly declined with increase of temperature from 10 to 22°C. Also, the concentration of N-Acetylglutamine significantly raised as the temperature increased from 10 to 22°C. In conclusion, the temperature altered the metabolome of L. garvie, which this may be linked to the pathogenicity. The temperature probably affects fermentation, homeostasis, energetic condition and metabolism of amino acids in L. garvieae.

Probiotics represent an alternative to chemotherapy and vaccination to control fish diseases, including lactococcosis caused by Lactococcus garvieae. The aims of this study were (i) to determine the in vitro probiotic properties of three bacteriocinogenic Lactococcus lactis subsp. cremoris of aquatic origin, (ii) to evaluate in vivo the ability of L. cremoris WA2-67 to protect rainbow trout (Oncorhynchus mykiss, Walbaum) against infection by L. garvieae, and (iii) to demonstrate the role of nisin Z (NisZ) production as an anti-infective mechanism. The three L. cremoris strains survived in freshwater at 18 °C for 7 days, withstood exposure to pH 3.0 and 10 % (v/v) rainbow trout bile, and showed different cell surface hydrophobicity (37.93–58.52 %). The wild-type NisZ-producer L. cremoris WA2-67 and its non-bacteriocinogenic mutant L. cremoris WA2-67 ∆nisZ were administered orally (10⁶ CFU/g) to rainbow trout for 21 days and, subsequently, fish were challenged with L. garvieae CLG4 by the cohabitation method. The fish fed with the bacteriocinogenic strain L. cremoris WA2-67 reduced significantly (p < 0.01) the mortality (20 %) compared to the fish treated with its non-bacteriocinogenic knockout isogenic mutant (50 %) and the control (72.5 %). We demonstrated the effectiveness of L. cremoris WA2-67 to protect rainbow trout against infection with the invasive pathogen L. garvieae and the relevance of NisZ production as an anti-infective mechanism. This is the first report demonstrating the effective in vivo role of LAB bacteriocin (NisZ) production as a mechanism to protect fish against bacterial infection. Our results suggest that the wild-type NisZ-producer strain L. cremoris WA2-67 could be used in fish farming to prevent lactococcosis in rainbow trout.

Lactococcosis caused by Lactococcus garvieae is an emerging threat to warmwater aquaculture, yet evidence integrating field outbreaks with robust molecular confirmation and controlled virulence testing remains limited for Thailand’s cage-cultured tilapia. From May to October 2025, acute mortality events were investigated in cage-cultured Nile tilapia (Oreochromis niloticus) in a reservoir in Ubon Ratchathani Province, Thailand. Suspected outbreaks were defined by abrupt daily mortality exceeding 5% accompanied by septicemia-like clinical signs. Water quality during sampling covered the following ranges: temperature 28.6–31.9 °C, pH 6.5–7.0, salinity 0.02–0.03 ppt, electrical conductivity 0.036–0.046 mS/cm, TDS 22.20–26.50 mg/L, total alkalinity 17.0–34.0 mg/L as CaCO3, total hardness 12.0–60.0 mg/L as CaCO3, dissolved oxygen 6.5–7.0 mg/L, and NH3 were below the limit of detection. Full-length 16S rRNA tissue profiling revealed strong tissue partitioning: blood microbiomes were consistently dominated by Lactococcus and L. garvieae at the species level, whereas gills showed higher richness and mixed communities with multiple opportunistic taxa. Culture isolation was more reliable from blood than gills, yielding 16 Gram-positive, catalase-negative isolates (AAHM-LG2501–AAHM-LG2516) that clustered within the L. garvieae clade in near full-length 16S rRNA phylogenetic analysis and were separated from closely related Lactococcus lineages. A representative blood isolate (AAHM-LG2501) showed dose-dependent virulence in controlled challenges, with an LD50 of ~1.05 × 105 CFU/fish by intraperitoneal injection and an LC50 of ~1.20 × 106 CFU/mL by immersion. Histopathology supported systemic dissemination, with injection producing more consistent multi-organ lesions than immersion, particularly in head kidney, liver, and spleen, while gills exhibited route-associated epithelial and vascular alterations. Together, these findings confirm L. garvieae as a major etiological agent of septicemic outbreaks in cage-cultured tilapia in Thailand and support a practical surveillance framework prioritizing blood sampling, molecular confirmation, and risk-based monitoring to guide biosecurity and vaccine-oriented prevention.

Lactococcus garvieae has recently been identified and listed as one of the causative agents of hyperacute hemorrhagic sepsis in fish. In intensive recirculating aquaculture systems where there are high fish densities and minimal water changes, not only will it be conducive to the growth of bacteria, but Cryptocaryon irritans as a marine protozoan fish parasite is also prone to appear. This study reports the disease status of Trachinotus ovatus in an aquaculture area in Yangjiang City, Guangdong Province. Through the diagnosis of clinical symptoms of the diseased fish, identification of specific primers, 16s rRNA sequences phylogenetic tree analysis, physiological and biochemical identification, and observation of histopathological sections, the result of the experiment is that the mass death of T . ovatus is caused by a mixture of L . garvieae and C . irritants infections. Subsequently, regression infection experiments were performed to verify Koch’s law. It was confirmed that the pathogen had strong virulence to T . ovatus . This is the first time that the co-infection of L . garvieae and C . irritans to T . ovatus was found in South China. The research results of this experiment have certain enlightenment significance for the epidemic trend of fish diseases in relevant sea areas.

Mannose phosphotransferase system (Man-PTS) serves as a receptor for several bacteriocins in sensitive bacterial cells, namely subclass IIa bacteriocins (pediocin-like; pediocins) and subclass IId ones - lactococcin A (LcnA), lactococcin B (LcnB) and garvicin Q (GarQ). Here, to identify the receptor for three other narrow-spectrum subclass IId bacteriocins - garvicins A, B and C (GarA-C) Lactococcus garvieae mutants resistant to bacteriocins were generated and sequenced to look for mutations responsible for resistance. Spontaneous mutants had their whole genome sequenced while in mutants obtained by integration of pGhost9::IS S1 regions flanking the integration site were sequenced. For both types of mutants mutations were found in genes encoding Man-PTS components IIC and IID indicating that Man-PTS likely serves as the receptor for these bacteriocins as well. This was subsequently confirmed by deletion of the man-PTS operon in the bacteriocin-sensitive L . garvieae IBB3403, which resulted in resistant cells, and by heterologous expression of appropriate man-PTS genes in the resistant Lactococcus lactis strains, which resulted in sensitive cells. GarA, GarB, GarC and other Man-PTS-targeting bacteriocins differ in the amino acid sequence and activity spectrum, suggesting that they interact with the receptor through distinct binding patterns. Comparative analyses and genetic studies identified a previously unrecognized extracellular loop of Man-PTS subunit IID (γ+) implicated in the L . garvieae sensitivity to the bacteriocins studied here. Additionally, individual amino acids localized mostly in the sugar channel-forming transmembrane parts of subunit IIC or in the extracellular parts of IID likely involved in the interaction with each bacteriocin were specified. Finally, template-based 3D models of Man-PTS subunits IIC and IID were built to allow a deeper insight into the Man-PTS structure and functioning.

Changes occurring in freshwater ecosystems seem to be fundamental in the development of all microorganisms, including those pathogenic to fish. This has been especially evident in recent years during which dynamic variations in bacterial fish pathology have been observed. Gram-negative bacteria commonly known to be pathogenic to fish, like spp., spp., spp., and are replaced by other species, which until now have not been known to be virulent or even conditionally pathogenic to fish. Nowadays, among these other species spp., , , and are the most frequently isolated from fish exhibiting clinical signs of disease. Two Gram-positive bacteria have become pathogens of particular importance in fish pathology in Poland: and . In addition, infections caused by the Gram-positive bacterium have appeared in recent years. This bacterium has not been known until now to be pathogenic to fish. Therefore, this infection could be called an emergent disease.

Lactococcus muris was published in Cell Host & Microbe in November 2022 and validated in validation list no. 213 in October 2023. Lactococcus intestinalis was published in Antonie van Leeuwenhoek in May 2023 and validated in validation list no. 216 in March 2024. Both of them were isolated from the mouse gut. Previous studies indicated that L. muris DSM 109779 T had the highest 16S rRNA gene sequence identity to Lactococcus garvieae (96.7%), and that L. intestinalis M2458 T had the highest 16S rRNA gene sequence similarities to Lactococcus formosensis NBRC 109475 T (97.6%) and L. garviae NBRC 100934 T (97.1%). However, the taxonomic relationship between L. muris and L. intestinalis has not yet been investigated and therefore is the focus of the present study. In the present study, the relationship between L. muris and L. intestinalis was evaluated. The type strains of L. muris and L. intestinalis shared 100% 16S rRNA gene sequence similarity, 100% pheS sequence similarity, 100% rpoA sequence similarity, 99.9% average nucleotide identity (ANI) value and 98.5% digital DNA-DNA hybridization (dDDH) value, indicating that they represented the same species. On the basis of the results present here, we propose L. intestinalis Sun et al . 2024 as a later heterotypic synonym of L. muris Afrizal et al. 2023.

ObjectiveThis study was aimed to assess the effect of a novel postbiotic on bacterial community composition and structure within the intestinal ecosystem of rainbow trout (Oncorhynchus mykiss), as well as evaluate its capacity to protect rainbow trout from Lactococcus garvieae infection.ResultsAfter 30 days of dietary postbiotic supplementation, high-throughput 16S rRNA gene sequencing revealed that bacterial community composition, diversity and richness were significantly higher in treated fish than in control fish. The proportion of sequences affiliated to the phylum Tenericutes, and to a lesser extent, the phyla Spirochaetes and Bacteroidetes was increased in fish fed a postbiotic-enriched diet compared to control fish, whereas the abundance of Fusobacteria was higher in control fish. Moreover, the treated fish showed significantly (p < 0.05) improved protection against L. garvieae compared to control fish.ConclusionsThese findings suggest that dietary postbiotic supplementation may represent an environmentally friendly strategy for preventing and controlling diseases in aquaculture.