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Elizabethkingia miricola is a multidrug-resistant pathogen that can cause life-threatening infections in immunocompromised humans and outbreaks in amphibians. However, the specific virulence factors of this microorganism have not been described. In this study, we identified the polysaccharide biosynthesis protein-encoding gene capD , which is located in the conserved region of the Wzy-dependent capsule synthesis gene cluster in the E. miricola strain FL160902, and investigated its role in the pathogenesis of E. miricola . Our results revealed that the capD deletion strain (Δ capD ) lost its typical encapsulated structure, with a 45% reduction in cell wall thickness. CapD affects wza expression in the capsule polysaccharide synthesis pathway. Furthermore, the survival rates were significantly reduced in Δ capD in response to complement-mediated killing, desiccation stress, and macrophage phagocytosis, whereas biofilm formation, surface hydrophobicity, and adherence to both endothelial and epithelial cells were increased. Additionally, the deletion of capD sharply attenuated the virulence of E. miricola in a frog infection model. Complementation of the capD gene restored the biological properties and virulence to wild-type levels. Overall, these findings suggest that CapD contributes to polysaccharide synthesis and plays a crucial role in the pathogenesis of E. miricola .

Elizabethkingia miricola is an emerging nosocomial pathogen responsible for meningitis, sepsis, urinary tract infection, pneumonia, and joint infection in humans. These pathogens were also reported to be causal agents for meningitis-like disease in cultured frogs, which displayed high infectivity, mortality, and significant loss. In July 2023, 10 outbreaks of infectious meningitis-like disease in bullfrogs occurred in Tangshan area. To determine the causal agent, 70 diseased frogs from 10 farms were collected for etiological identification. Gram-negative bacilli were isolated from the brain and liver of sick bullfrogs and identified as members of E. miricola by biochemical characterization and 16S rRNA sequencing analysis. A total of 42 strains of E. miricola were isolated and further determined as the etiological agent by reproducing neurological symptoms and deaths in an artificial infection test. A representative isolate, HBTS-1, was picked up for the pathogenicity test, and the data showed that this stain was highly pathogenic to bullfrogs with an LD50 of 3.7 × 105 CFU. Notably, the isolate also showed high pathogenicity to 5-day-old suckling mice, with an LD50 of 3.1 × 106 CFU, indicating its potential threat to mammals. Moreover, all the 42 E. miricola isolates showed resistance to multiple antibotics without an apparent inhibition zone observed in the test, making the choice of antimicrobial therapy challenging. These novel findings prioritized E. miricola as an important zoonotic agent, which may provide a reference for human medicine.

We retrospectively reviewed Elizabethkingia spp. culture and susceptibility results from 86 veterinary diagnostic laboratory results from US dogs and cats. We noted 26 E. menigoseptica, 1 E. miricola, and 59 unspeciated Elizabethkingia isolates from 9 US states (2-22 isolates per state). Elizabethkingia infections in animals might increase risks to humans.

The genus Rahnella has been isolated from human, fish, and water environments. We recently reported on the isolation and genomic identification of a novel pathogen R. aquatilis strain KCL-5 from crucian carp Carassius auratus. To investigate the evolution of bacterial virulence and resistance properties of R. aquatilis, comparative genomics analyses were performed for genus Rahnella strains including R. aquatilis, R. variigena, R. bruchi, and R. victoriana. This analysis provides up-to-date information on genus Rahnella genomic diversity, including comparative analyses of virulence and resistance, synteny, single-nucleotide polymorphisms, average nucleotide identity, core-genes, gene families, and genomic islands. The sister species to R. aquatilis is R. victoriana, and closer R. victoriana than with other Rahnella sp. Multiple genes encoding functions that likely contribute to antimicrobial resistance and pathogenic factors were identified by comparative genome analysis, including multidrug resistance efflux pump, adherence, invasion, and secretion systems. To our knowledge, this is the first report to provide a more detailed insight into the comparative genomic characteristics of Rahnella spp., contributing to the understanding of its diversity and evolution, as well as concerning the virulence of R. aquatilis.

Tilapia is one of the most important farmed fish in the world and the most cultivated in Brazil. The increase of this farming favors the appearance of diseases, including bacterial diseases. Therefore, the aim of this study was to evaluate the bactericidal activity of copaiba oil, Copaifera duckei , against Streptococcus agalactiae and Flavobacterium columnare and the dietary effect of copaiba oil on zootechnical performance, hematological, biochemical, immunological, and histological analysis before and after an intraperitoneal infection (body cavity) with S. agalactiae in Nile tilapia. For this, fish were randomly distributed into 15 fiber tanks in five treatments (0, 0.25, 0.50, 0.75, and 1.0%) and fed with a commercial diet supplemented with copaiba oil for 30 days. After this period, the fish were randomly redistributed for the experimental challenge with S. agalactiae into six treatments (T0, T1, T2, T3, T4, and T5), the fish were anesthetized, and blood samples were collected to assess hematological, biochemical, immunological, and histological parameters. Copaiba oil showed bactericidal activity against Streptococcus spp . and Flavobacterium spp. in vitro. In addition, concentrations of 0.75 and 1.0% of copaiba oil have an anti-inflammatory effect and improve hematological and immunological parameters, increasing leukocyte numbers, albumin, and serum lytic activity. Furthermore, there is an increase in the intestinal villus length and tissue damage in groups at concentrations of 0.75 and 1.0% of copaiba oil. In conclusion, copaiba oil presented bactericidal activity against Streptococcus spp . and Flavobacterium spp. in vitro, and oral supplementation at concentrations of 0.75 and 1.0% compared to the control group enhanced non-specific immune parameters and digestibility in Nile Tilapia.

Ecological changes affect pathogen epidemiology and evolution and may trigger the emergence of novel diseases. Aquaculture radically alters the ecology of fish and their pathogens. Here we show an increase in the occurrence of the bacterial fish disease Flavobacterium columnare in salmon fingerlings at a fish farm in northern Finland over 23 years. We hypothesize that this emergence was owing to evolutionary changes in bacterial virulence. We base this argument on several observations. First, the emergence was associated with increased severity of symptoms. Second, F. columnare strains vary in virulence, with more lethal strains inducing more severe symptoms prior to death. Third, more virulent strains have greater infectivity, higher tissue-degrading capacity and higher growth rates. Fourth, pathogen strains co-occur, so that strains compete. Fifth, F. columnare can transmit efficiently from dead fish, and maintain infectivity in sterilized water for months, strongly reducing the fitness cost of host death likely experienced by the pathogen in nature. Moreover, this saprophytic infectiousness means that chemotherapy strongly select for strains that rapidly kill their hosts: dead fish remain infectious; treated fish do not. Finally, high stocking densities of homogeneous subsets of fish greatly enhance transmission opportunities. We suggest that fish farms provide an environment that promotes the circulation of more virulent strains of F. columnare. This effect is intensified by the recent increases in summer water temperature. More generally, we predict that intensive fish farming will lead to the evolution of more virulent pathogens.

Background Bacterial cold-water disease (BCWD), caused by Flavobacterium psychrophilum , remains a major challenge for rainbow trout ( Oncorhynchus mykiss ) aquaculture, due to the absence of effective vaccines and increasing concerns over antibiotic use. Genetic selection for disease resistance offers a sustainable alternative. In this study, we investigated the genetic architecture of BCWD resistance in two French commercial rainbow trout populations using a standardized waterborne infection model and high-density SNP genotyping. Results Survival following experimental infection varied significantly between populations, with population B showing higher resistance (71.3% vs. 50.7% of survival at 29 days post-infection). Genome-wide association studies (GWAS) were performed using a Bayesian sparse linear mixed model (BSLMM), separately in each population and in a combined dataset. Eleven quantitative trait loci (QTLs) were identified across the analyses, with limited overlap between populations, highlighting the complexity and partial divergence of resistance architectures. Several candidate genes located within QTL regions were involved in immune signalling, inflammation, macrophages/neutrophils biology, and soluble factors important for antibacterial defences. Notably, two QTLs contained genes from the complement system (e.g., C3, Cfb), highlighting their central role in resistance to F. psychrophilum . Conclusions Our findings underscore the polygenic nature of BCWD resistance, the influence of host genetic background, and provide valuable targets for selection for BCWD resistance in rainbow trout breeding programs.

Although increased disease severity driven by intensive farming practices is problematic in food production, the role of evolutionary change in disease is not well understood in these environments. Experiments on parasite evolution are traditionally conducted using laboratory models, often unrelated to economically important systems. We compared how the virulence, growth and competitive ability of a globally important fish pathogen, Flavobacterium columnare, change under intensive aquaculture. We characterized bacterial isolates from disease outbreaks at fish farms during 2003–2010, and compared F. columnare populations in inlet water and outlet water of a fish farm during the 2010 outbreak. Our data suggest that the farming environment may select for bacterial strains that have high virulence at both long and short time scales, and it seems that these strains have also evolved increased ability for interference competition. Our results are consistent with the suggestion that selection pressures at fish farms can cause rapid changes in pathogen populations, which are likely to have long-lasting evolutionary effects on pathogen virulence. A better understanding of these evolutionary effects will be vital in prevention and control of disease outbreaks to secure food production.

Background Bergeyella porcorum is a newly identified bacterium that has an ambiguous relationship with pneumonia in pigs. However, few studies have adequately characterized this species. Results In this study, we analyzed the morphological, physiological, and genomic characteristics of the newly identified B. porcorum sp. nov. strain QD2021 isolated from pigs. The complete genome sequence of the B. porcorum QD2021 strain consists of a single circular chromosome (2,271,736 bp, 38.51% G + C content), which encodes 2,578 genes. One plasmid with a size of 70,040 bp was detected. A total of 121 scattered repeat sequences, 319 tandem repeat sequences, 4 genomic islands, 5 prophages, 3 CRISPR sequences, and 51 ncRNAs were predicted. The coding genes of the B. porcorum genome were successfully annotated across eight databases (NR, GO, KEGG, COG, TCDB, Pfam, Swiss-Prot and CAZy) and four pathogenicity-related databases (PHI, CARD, VFDB and ARDB). In addition, a comparative genome analysis was performed to explore the evolutionary relationships of B. porcorum QD2021. Conclusions To our knowledge, this is the first study to provide fundamental phenotypic and whole-genome sequences for B. porcorum . Our results extensively expand the current knowledge and could serve as a valuable genomic resource for future research on B. porcorum .

Rainbow trout fry syndrome (RTFS) is a disease caused by the Gram-negative bacterium Flavobacterium psychrophilum, responsible for significant economic losses in salmonid aquaculture worldwide. The diversity of F. psychrophilum isolates and the inherent difficulties in vaccinating juvenile fish has hampered the development of a vaccine for RTFS. Disease episodes tend to occur between 10–14 °C with necrotic lesions often seen on the skin surrounding the dorsal fin and tail. At present no commercial vaccines are available for RTFS in the UK, leaving antibiotics as the only course of action to control disease outbreaks. The current work was performed as a pilot study to assess the efficacy of a polyvalent, whole cell vaccine containing formalin-inactivated F. psychrophilum , to induce protective immunity in rainbow trout fry. Duplicate groups of 30 trout (5 g) were immersed in 1 L of the vaccine for 30 s. Samples were taken 4 h, day 2 and 7 post-vaccination (pv) of skin mucus, tissues for histology and gene expression analysis; serum and histology samples were taken 6 weeks pv. A booster vaccination was given at 315 degree days (dd) also by immersion. Challenge was by immersion with a heterologous isolate of F. psychrophilum 630 dd post primary vaccination. The vaccine provided significant protection to the trout fry with a RPS of 84% ( p  < 0.0001). Detection of increased numbers of IgT positive cells in systemic organs, up-regulation of IgT expression in hind-gut and an increase in total IgT in serum was observed in vaccinated fish; however a functional role of IgT in the observed protection remains to be demonstrated.