[Objective] To screen and identify an endophytic bacterial strain with plant growth-promoting effects on rice from the stem of rice grown in the soda saline-alkaline soil in Daqing City, Heilongjiang Province and explore its plant growth-promoting effects and related genes, with a view to enriching and utilizing the endophytic microbial resources of rice. [Methods] An endophytic bacterial strain was isolated from rice stems via the dilution coating method, and the strain was identified by morphological observation, physiological and biochemical tests, and 16S rRNA gene sequencing. The mechanisms underlying the plant growth-promoting effects of the strain were deciphered by Illumina sequencing of the whole genome. antiSMASH was used to predict the synthetic gene clusters of secondary metabolites. The plant growth-promoting performance of the strain was evaluated by promotion performance, rice seed germination, and seedling cultivation tests. [Results] An isolated endophytic bacterial strain of rice was designated as J-7, and the strain was identified as Acinetobacter baumannii by whole genome sequencing and ANI analysis. The strain was a mesophilic bacterium with an OD₆₀₀ value of 0.679 at pH 10.5, showcasing alkaline tolerance. The strain had an OD₆₀₀ value of 0.293 in the presence of 1.0 mol/L NaCl, exhibiting salt tolerance. The strain had the ability to solubilize inorganic and organic phosphorus, with the amounts of inorganic and organic phosphorus solubilized being (179.54±1.21) mg/L and (65.57±1.07) mg/L, respectively. In addition, the strain had the ability to produce siderophores and utilize ferric ammonium salts. The strain genome (national microbiology data center accession number: NMDC60154488) was 3 666 630 bp in length, with the G+C content of 39% and a total of 3 432 genes (including 8 potential synthetic gene clusters of secondary metabolites). The rice seed germination test results showed that the application of 1×10⁷ CFU/mL suspension of the strain significantly increased the radicle length and stem base width by 13.02% and 17.68%, respectively, compared with the control group. The rice seedling cultivation test results showed that the application of 2×10⁹ CFU/mL suspension of the strain significantly increased the plant height and root length by 32.69% and 36.55%, respectively, compared with the control group. [Conclusion] Strain J-7 has a good growth-promoting effect on rice, showing application potential in agriculture as a microbial strain resource. Whole genome sequencing provides a theoretical basis for in-depth study of the plant growth-promoting mechanism of A. baumannii.

Yeast β-glucan has been widely used as a dietary supplement for its multiple biological activities, including immune stimulation. Previous studies in our laboratory have shown that yeast chs3Δ spores exposed in the glucan layer can activate immune effects more effectively than trophoblasts. However, chs3Δ spores are formed within the ascospores and are encapsulated by the ascospore cell wall, which greatly limits their direct application. [Objective] To explore whether the lysates of chs3Δ asci have the potential to be used as novel immune-stimulating dietary supplements. [Methods] We prepared chs3Δ asci lysates by ultrasonic freeze drying, enzymatic freeze drying, and enzymatic-assisted ultrasonic freeze drying and then explored their immune effects in depth. [Results] The chs3Δ asci lysate prepared by ultrasonic freeze drying induced the highest level of inflammatory cytokines compared with the vegetative cell lysate and chs3Δ spores, and the asci lysate achieved immunostimulatory responses through activation of Dectin-1 receptor. Further studies showed that the chs3Δ asci lysate also exhibited immunostimulatory activity and trained immunity-inducing ability in mice. [Conclusion] The chs3Δ asci lysate has great potential as a highly effective immunostimulatory dietary supplement.

Photobacterium damselae subsp. damselae (PDD), a pathogenic bacterium widely found in seawater, can infect a variety of economic fish and cause huge economic losses to the global aquaculture industry. The flagellar gene flgK encodes the flagellar hook protein FlgK, which is essential for the normal formation of bacterial flagella. [Objective] To systematically analyze the influencing mechanism of flgK on the virulence of PDD. [Methods] The flgK-deleted mutant of PDD (ΔflgK-PDD) was constructed by homologous recombination mediated by a high-efficiency suicide plasmid, and the mutation was confirmed by gene sequencing. The biological characteristics, virulence gene expression, and pathogenicity were compared between ΔflgK-PDD and the wild-type strain (WT-PDD). [Results] There was no significant difference in the growth ability, hemolytic activity or phospholipase activity between ΔflgK-PDD and WT-PDD. However, the motility and biofilm formation of ΔflgK-PDD were significantly lower than those of WT-PDD. Transmission electron microscopy showed that ΔflgK-PDD failed to form a flagellar structure. The artificial infection experiments showed that the LD₅₀ of ΔflgK-PDD in Sebastes schlegelii was 557% that of WT-PDD, and the pathogenicity was significantly reduced. Real-time quantitative PCR results showed that compared with WT-PDD, ΔflgK-PDD demonstrated significantly down-regulated expression of the flagellar-related genes fliK and flgL, the type II secretion system (T2SS)-related genes gspC and gspD, and the virulence gene hlyA_pl. The expression levels of flagellar-related gene fliH, T2SS-related gene gspE, outer membrane-related genes ompP, lapB, and flhB were significantly up-regulated, and those of the remaining genes did not change significantly. [Conclusion] The mutation of flgK can lead to the failure of ΔflgK-PDD to form a complete flagellar structure and significantly change the relative expression levels of flagellar-related genes, thereby reducing the motility and colonization ability and ultimately weakening the pathogenicity of PDD.

Pseudomonas plecoglossicida is the pathogen of visceral white spot disease in large yellow croaker (Larimichthys crocea). At present, the pathogenic mechanism of P. plecoglossicida has been partially understood, and some effective vaccines have been screened, whereas no commercial vaccine has been developed. [Objective] To deepen the understanding about the pathogenic mechanism and develop efficient attenuated live vaccines of P. plecoglossicida. [Methods] We targeted luxR and the RNA polymerase σ factor gene rpoE associated with quorum sensing to construct deletion mutants of P. plecoglossicida NB2011 by double homologous recombination. In addition, the strain ΔT6SS1ΔluxR with deletion of both the type VI secretion system 1 (T6SS1) gene and luxR was constructed. The biological characteristics and virulence of the mutants were analyzed. The relative percent of survival of fish after vaccination with the double deletion mutant was investigated. [Results] We successfully constructedΔluxR, ΔT6SS1ΔluxR, and ΔrpoE. Compared with the wild type, none of the three mutants showed significant changes in the growth rate, swarming ability or swimming ability, while ΔluxR and ΔT6SS1ΔluxR showed significant decreases in biofilm formation. The internalization, adsorption, and intracellular proliferation of the mutants in mouse macrophages were observed. All the three mutants could survive in J774A.1 macrophages and showed lower proliferation capacities than the wild type. Compared with the wild type, the two single mutants showed significantly reduced virulence to goldfish (Carassius auratus) after intraperitoneal injection at a concentration of 1.0×10⁷ cells/mL. The double deletion mutant ΔT6SS1ΔluxR showed no virulence when challenging goldfish at the same dose, with the LD₅₀>10⁸ cells/mL. In the immune protection experiment, the goldfish was vaccinated with ΔT6SS1ΔluxR, and the fish was artificially challenged with the wild type 28 days later. The relative percent of survival reached 78.60%, which indicated effective protection. [Conclusion] In this study, three gene deletion mutants were successfully constructed, and the virulence of the mutants was significantly decreased. Among them, ΔT6SS1ΔluxR is expected to be a candidate strain for the development of the attenuated live vaccine against P. plecoglossicida infection.

[Objective] In view of the low decomposition rate of rice straw in black soil fields of cold regions, it is crucial to isolate lignin-degrading bacteria adaptive to low temperatures to enhance the straw degradation efficiency. [Methods] Soil samples were collected in winter, and the bacterial strains capable of degrading lignin were isolated by the streak-plate method with sodium lignosulfonate as a sole carbon source. The degradation conditions was carried out through optimized by single factor experiment sand response surface methodology. [Results] Pseudomonas psychrophila BYAU-6 was isolated, exhibiting strong lignin-degrading activity in low-temperature environments (5-15 ℃). The culture conditions for strain screening were as follows: sodium lignosulfonate addition amount of 0.5 g/L, a peptone-to-yeast powder mass ratio of 5:1, initial pH 7.0, and a liquid loading volume of 80%. The optimal culture conditions for lignin degradation were determined as follows: sodium lignosulfonate addition amount of 0.3 g/L, a peptone-to-yeast powder mass ratio of 3.2:2.8, initial pH 5.3, and a liquid loading volume of 80%. Under these conditions, the lignin degradation rate increased from 12.33% to 15.78%, representing an increase of 21.9%. The results of the pot experiment showed that the control group (without inoculation) achieved a straw degradation rate of 27.0%, while the inoculation with strain BYAU-6 achieved a straw degradation rate of 37.5%, an increase of 38.89% compared with the control (P<0.05). [Conclusion] This study provides novel microbial resources for straw degradation in cold regions and valuable data for future research on lignin-degrading strains under low-temperature conditions.

[Objective] To investigate the in vitro uric acid degradation performance and physiological and biochemical characteristics of Limosilactobacillus fermentum H3260 isolated from human feces and examine the effects of this strain on the serum uric acid level and gut microbiota in the mouse model of hyperuricemia, providing scientific evidence for the development of functional food for the prevention and treatment of hyperuricemia. [Methods] HPLC and the uric acid production assay were employed to determine the abilities of the target strain to degrade uric acid, adenosine, and nucleosides and to inhibit xanthine oxidase. The probiotic characteristics of the strain were evaluated by antimicrobial sensitivity tests and in vitro tolerance tests. The uric acid-lowering effect of L. fermentum H3260 was verified by in vivo experiments. [Results] L. fermentum H3260 was screened out. The strain exhibited degradation rates of (86.84±0.03)% for uric acid, (60.84±2.21)% for adenine, and (100.00±0.00)% for nucleosides, along with an inhibition rate of 22.48% for xanthine oxidase. The strain was sensitive to seven common antibiotics, including erythromycin, ceftriaxone, penicillin G, and chloramphenicol. After treatment in 0.3% bile salt for 2.5 h, the bacterial count remained above 1.00×10⁶ CFU/mL. Animal experiments showed that the strain significantly reduced uric acid, creatinine, and blood urea nitrogen in hyperuricemic mice and regulate the gut microbiota to alleviate hyperuricemia. [Conclusion] We successfully screened out a strain L. fermentum H3260 capable of efficiently degrading uric acid, adenosine, and nucleosides. The strain exhibited good physiological and biochemical characteristics in vitro and significantly improved hyperuricemia-related indicators and regulated the gut microbiota in vivo, showing potential as an elite strain for the prevention and treatment of hyperuricemia.

[Objective] Adipic acid is a key monomer for plastics such as nylon 66 and poly (butylene adipate-co-terephthalate) (PBAT), with a vast market potential. This study aims to explore the optimal expression levels of genes in the biosynthetic pathway of adipic acid. [Methods] We regulated the expression levels of genes in the adipic acid synthesis pathway by randomly combining gradient-strength constitutive promoters. The high-throughput screening based on an adipic acid biosensor was conducted to select the strain with the optimal combination. Subsequently, the fermentation media, carbon sources, metal ions, and precursor substance addition amounts were optimized. [Results] After screening, the optimal strain Escherichia coli MG1655 Δ8-D47 was obtained, with an adipic acid yield of 431.32 mg/L. After fermentation condition optimization, the yield of adipic acid in a shake flask reached 550.34 mg/L, which represented a 134% increase compared with that of the control strain Z1. [Conclusion] Metabolic pathway imbalance in microbial synthesis of adipic acid is the main factor limiting the increase in yield.

[Objective] We employed the wild-type strain of Listeria monocytogenes, the lipoprotein gene pplA- deleted strain, and the complementary strain to investigate the role of PplA in the infection of L. monocytogenes. [Methods] We compared the hemolytic capacity, cell adhesion and invasion, intracellular proliferation, cell-to-cell migration, mouse organ colonization, transcription levels of virulence factors in mouse organs, and transcription levels of quorum sensing-related genes among wild-type, pplA-deleted, and complementary strains to explore the role of PplA in the infection of L. monocytogenes. [Results] After the deletion of pplA, L. monocytogenes showed no significant change in intracellular proliferation or cell-to-cell migration. However, its hemolytic capacity, cell adhesion and invasion, mouse organ colonization, and transcription levels of virulence factors such as plcB, hly, and prfA in mouse organs were significantly reduced. Moreover, the transcription levels of quorum sensing-related genes agrA, agrB, agrC, and luxS were altered in the pplA-deleted strain. [Conclusion] Thelipoprotein PplA is involved in the virulence regulation and affects the pathogenicity of L. monocytogenes.

[Objective] Black soil acidification may exacerbate the soil degradation processes and reduce microbial functions, thus threatening the crucial role of the northeast region in guaranteeing the food security of China. Unraveling the impacts of soil acidification on the soil microbial community and its underlying mechanisms can help clarify the relationship between soil organic carbon (SOC) stabilization and soil acidification. [Methods] Soil samples with different acidification degrees were collected from the corn belts of black soil regions. The changes of living microbial groups in the soil samples with different pH were investigated by the phospholipid fatty acid (PLFA) analysis. Additionally, the relationship between changes in the soil physicochemical properties and microbial community composition was analyzed. [Results] A threshold effect of black soil acidification on SOC was identified in the corn belts. Moderate acidification did not cause significant changes in SOC. However, when pH dropped below a certain threshold (6.75), further acidification resulted in a significant loss of SOC. The cation buffering effect in soil changed significantly with different acidification degrees. Calcium ion was primarily responsible for buffering black soil acidification, while when the pH fell below 6.00, both calcium and magnesium ions buffered the acidification. Soil acidification imposed noticeable stress on soil microorganism growth. Different microbial groups exhibited an S-shaped response pattern, with PLFA content initially decreasing, remaining stable within the range of pH 5.25-6.25, and subsequently declining as acidification progressed. However, different microbial groups exhibited varying sensitivities to soil acidification. Gram-negative bacteria were the most sensitive, followed by Gram-positive bacteria and arbuscular mycorrhizal fungi. Fungi, particularly arbuscular mycorrhizal fungi, may play a crucial role in stabilizing SOC during soil acidification. [Conclusion] Soil acidification significantly alters the structure of the living microbial community, primarily through changes in cation exchange capacity and substrate availability, which further affect SOC accumulation. These findings provide scientific support for developing management strategies to alleviate black soil degradation and acidification.

[Objective] To clarify the spatial distribution characteristics of soil organic carbon (SOC) age and microbial diversity, explore the relationship of microbial diversity and network complexity with SOC age, and quantitatively assess the relative contributions of microbial diversity, network complexity, climate, vegetation, and soil properties to SOC age. [Methods] Using global soil radiocarbon (Δ¹⁴C) data and environmental variable data, we constructed nine machine learning models for predicting SOC age and selected the best-performing model. Based on global soil microbial 16S rRNA gene data and environmental variable data, microbial network analysis, multiple regression analysis, random forest models, and structural equation modeling were employed to analyze the correlation between SOC age and soil microorganisms and identify the main driving factors of SOC age. [Results] Soil microbial richness decreased with the rise in absolute latitude (P<0.001), being higher near the equator and lower at higher latitudes. Among the nine machine learning models constructed, the rule regression model showed the best prediction performance (R²=0.77, RMSE=0.84). Soil microbial richness and Shannon index were negatively correlated with absolute latitude and SOC age (P<0.001). The global soils were classified into young (44-171 a), middle-aged (172-321 a), and old (322-5 035 a) soil groups, and the network densities followed a trend of young soil group (0.400)>middle-aged soil group (0.285)>old soil group (0.125). Multiple regression analysis, random forest models, and structural equation modeling all showed that microbial network complexity explained the largest portion of SOC age variation (34%), far surpassing vegetation (10%) and climate (6%). [Conclusion] Global soil SOC age has significantly negative correlations with soil microbial diversity and network complexity. The soil with old SOC has lower microbial diversity and simpler microbial network structure. Microbial network complexity is a key factor influencing SOC age, and its impact is significantly greater than that of vegetation and climate. These results provide new insights into the driving mechanisms of SOC age and suggest that future models of SOC dynamics should fully consider the role of microbial interaction network.