Photobacterium damselaesubsp.damselae(PDD) is a Gram-negative bacterium that can infect a variety of aquatic organisms and humans. Based on an epidemiological investigation conducted over the past 3 years, PDD is one of the most important pathogens causing septic skin ulcer in deep-sea cage-culturedSebastes schlegeliiin the Huang-Bohai Sea area and present throughout the year with high abundance. To further understand the pathogenicity of this species, the pathogenic properties and genome of PDD strain SSPD1601 were analyzed. The results revealed that PDD strain SSPD1601 is a rod-shaped cell with a single polar flagellum, and the clinical symptoms were replicated during artificial infection. The SSPD1601 genome consists of two chromosomes and two plasmids, totaling 4,252,294 bp with 3,751 coding sequences (CDSs), 196 tRNA genes, and 47 rRNA genes. Common virulence factors including flagellin,Fur,RstB,hcpA, OMPs,htpB-Hsp60,VasK, andvgrGwere found in strain SSPD1601. Furthermore, SSPD1601 is a pPHDD1-negative strain containing the hemolysin genehlyA_chand three putative hemolysins (emrA, yoaF, and VPA0226), which are likely responsible for the pathogenicity of SSPD1601. The phylogenetic analysis revealed SSPD1601 to be most closely related to Phdp Wu-1. In addition, the antibiotic resistance phenotype indicated that SSPD1601 was not sensitive to ceftazidime, pipemidic, streptomycin, cefalexin, bacitracin, cefoperazone sodium, acetylspiramycin, clarithromycin, amikacin, gentamycin, kanamycin, oxacillin, ampicillin, and trimethoprim-sulfamethoxazole, but only the bacitracin resistance genebacAwas detected based on Antibiotic Resistance Genes Database. These results expand our understanding of PDD, setting the stage for further studies of its pathogenesis and disease prevention.

Bacillus spp. could be one of the most suitable substitutes for the control and prevention of aquatic diseases. The occurrence of species population, antimicrobial character, and virulence diversity in Bacillus spp. recovered from the mariculture system in China between 2009 and 2021 were investigated, screening for probiotic Bacillus strains with good biological safety that can inhibit Vibrio parahaemolyticus, V. alginolyticus, V. harveyi, V. owensii, V. campbellii. The results showed that 116 Bacillus isolates were divided into 24 species, and the top three species were B. subtilis (37/116), B. velezensis (28/116), and B. amyloliquefaciens (10/116). Among the 116 Bacillus isolates, 32.8% were effective against V. parahaemolyticus, 30.1% for V. alginolyticus, 60.3% for V. harveyi, 69.8% for V. owensii and 74.1% for V. campbellii. More than 62% of Bacillus isolates were susceptible to florfenicol, doxycycline and tetracycline, etc., and 26/116 Bacillus isolates were found to be multiple-antibiotic-resistant (MAR), with MARI values ranging from 0 to 0.06. Eighteen kinds of antibiotic resistance genes were tested; only tetB, blaTEM, and blaZ were detected. And 9 isolates in 2 Bacillus species were excluded by 6/10 kinds of Bacillus-related toxin gene (hblA, hblC, nheB, nheC, entFM, cykK). Bio-safety testing indicated that three kinds of probiotics were good probiotic candidates to prevent Vibriosis. These results provide comprehensive genetic diversity, potential risks, and probiotic characteristics of Bacillus in the mariculture system in China, and provide basic support for green and healthy development of aquatic industry.

The previous studies have indicated that Photobacterium damselae subsp. damselae (Pdd) was the pathogen of black rockfish (Sebastes schlegeli) with seriously skin ulcer symptom. This study aimed to explore the spleen transcriptome response of S. schlegeli at 24 h (P24 group) and 48 h (P48 group) after Pdd infection and characterize expression profile of immune-related genes. The fish in the control group were injected with equal volume of PBS. Results showed that there were 2,139 (88.42%), 510 (21.08%) and 242 (10%) differentially expressed genes in PBS vs P24, PBS vs P48 and P24 vs P48 respectively. As the time increases, the number of differential genes gradually decreases. GO annotation indicated that the immune-related function category contributed the most in the PBS vs P24 and PBS vs P48 groups, including immune response, immune system process, immune effector process and regulation of immune system process. KEGG enrichment analysis showed that these genes were located on 277, 184 and 164 signalling pathways in the PBS vs P24, PBS vs P48 and P24 vs P48 groups respectively; among them, 30, 26 and 16 pathways were related to immune system, involving 344, 119 and 24 genes. P24 group involved more immune-related signal pathways and genes than P48 group. RT-qPCR validation indicated that the expression patterns of 10 genes at different time points were well agreed with RNA-Seq analysis. This study provided a better understanding of the molecular responses to photobacteriosis in S. schlegeli.

Guanidine disinfectants are cationic polymers recognized for their effective sterilization properties and their ability to prevent bacterial resistance. As a result, they are widely utilized in medical, healthcare, household, and animal husbandry settings. However, the bactericidal effects and mechanisms of guanidine in marine aquaculture systems remain unclear due to the polymeric nature of guanidine ions and the complexity of marine environments. The inhibitory effects and bactericidal mechanisms of polyhexamethylene biguanide (PHMB) on key pathogens and probiotics are examined in this study. It was shown that PHMB had inhibitory effects on Vibrio parahaemolyticus (VP), Photobacterium damselae subsp. damselae (PDD), Bacillus subtilis (BS), Escherichia coli (EPEC), and Staphylococcus aureus (SAU), with minimum inhibitory concentrations (MICs) ranging from 3.91 to 125.0 µg/mL, and minimum bactericidal concentrations (MBCs) from 15.63 to 250.0 µg/mL. A stronger bactericidal effect of PHMB on marine bacteria compared to EPEC and SAU was exhibited. It was shown in ion interference experiments that the addition of calcium ions reduced the bactericidal effectiveness of PHMB against VP and PDD by 87.73% and 53.35%, respectively. At a PHMB concentration of 62.50 µg/mL, minor changes in cell surface potential energy (CSPE) were exhibited by Gram-positive bacteria (SAU and BS), while more significant alterations were shown by Gram-negative pathogens. It was revealed by propidium iodide staining and scanning electron microscopy (SEM) analysis that the bacterial cell membrane was directly disrupted by PHMB. DNA and RNA release analysis further revealed that following PHMB treatment, changes in membrane permeability were exhibited by Gram-negative pathogens, with a significant increase in extracellular DNA content as PHMB concentration increased. No such effect was observed in Gram-positive bacteria. Additional evidence was provided by the findings that PHMB effectively inhibits bacterial pathogens in mariculture systems, with a significantly stronger inhibitory effect on Gram-negative pathogens than on Gram-positive bacteria. These results indicated that PHMB could serve as a new antimicrobial agent in mariculture.

This study focused on Stichopus monotuberculatus and conducted stress experiments at salinity levels of 20‰ and 40‰. Intestinal histological changes and the structural characteristics of the intestinal flora of S. monotuberculatus under salinity stress were analyzed. The results show that acute salinity stress inflicts varying degrees of damage to the intestinal tissues of S. monotuberculatus. Salinity stress enhances the species diversity of intestinal flora in S. monotuberculatus. Eight phyla of bacteria are detected in the intestine of S. monotuberculatus. Dominant phyla include Proteobacteria, Firmicutes, and Actinobacteria. Furthermore, functional prediction reveals that acute salinity stress can significantly modify the abundance of pathways associated with nutrient and energy metabolism mediated by the intestinal flora of S. monotuberculatus. These results indicate that acute salinity stress induces pathological damage to the intestinal tissues of S. monotuberculatus, compromising the microbial habitat and leading to alterations in the intestinal flora composition. Additionally, S. monotuberculatus can mitigate salinity stress by adjusting the composition of its intestinal flora and the corresponding functional pathways.