Outer membrane protein FrpA, the siderophore piscibactin receptor of Photobacterium damselae subsp. piscicida, as a subunit vaccine against photobacteriosis in sole (Solea senegalensis)

Circular PCR as an efficient and precise umbrella of methods for the generation of circular dsDNA with staggered nicks: Mechanism and types

Here, we introduce the highly versatile circular polymerase chain reaction (CiPCR) technique, propose a mechanism of action, and describe a number of examples demonstrating the versatility of this technique. CiPCR takes place between two fragments of dsDNA with two homologous regions, as long as one of the fragments carries said regions at its 3'- and 5'-ends. Upon hybridization, elongation by a polymerase occurs from all 3'-ends continuously until a 5'-end is reached, leading to stable circular dsDNA with staggered nicks. When both dsDNA fragments carry the homology at their 3'- and 5'-ends (Type I CiPCR), all four 3'-ends effectively prime amplification of the intervening region and CiPCR products can function as template during the reaction. In contrast, when only one of the two dsDNA fragments carries the homologous regions at its 3'- and 5'-ends and the other carries such regions internally (Type II CiPCR), only two 3'-ends can be amplified and CiPCR products possess no template activity. We demonstrate the applicability of both CiPCR types via well-illustrated experimental examples. CiPCR is well adapted to the quick resolution of most of the molecular cloning challenges faced by the biology/biomedicine laboratory, including the generation of insertions, deletions, and mutations.

Characterization of Photobacterium damselae subsp. damselae isolated from diseased Asian seabass (Lates calcarifer) and the preliminary development of a formalin-killed cell vaccine

Asian seabass (Lates calcarifer) is an economically important fish species that is widely cultivated in Thailand. However, aquaculture of Asian seabass is limited by infectious diseases. One of the most serious diseases is photobacteriosis, caused by Photobacterium damselae. Vaccination is recognized as an efficient disease prevention and pathogen control method for strengthening the aquaculture industry. To promote vaccine development, the characterization of pathogenic bacteria and their pathogenesis is required. In this study, isolates of P. damselae were obtained from commercial aquaculture farms in Thailand during 2019-2021. Analyses of 16S rRNA and the urease subunit alpha genes identified the isolates as P. damselae subsp. damselae (Phdd). Antibiotic susceptibility analyses showed that all Phdd isolates were resistant to amoxicillin (10 μg). Haemolysis and phospholipase activities were used to categorize P. damselae into three groups based on their biological activities. The pathogenicity of four candidates (SK136, PD001, PD002 and T11L) was tested in Asian seabass. Isolate SK136 showed the highest virulence, with a lethal dose (LD50) of 1.47 × 105 CFU/fish, whereas isolate PD001 did not show any virulence. Genotypic characterization, based on multi-locus sequence typing analysis, demonstrated that all candidates were novel strains with new sequence types (64, 65, 66 and 67). Preliminary vaccination using formalin-killed cells (FKCs) protected Asian seabass from artificial challenges. Taken together, these results provide fundamental knowledge for vaccine development against Phdd infection in Asian seabass.

Anti-Bacterial and Immunostimulatory Properties of Ulvan-Loaded Chitosan Nanoparticles for Use in Aquaculture

Alternative prophylactic strategies to limit farm animal infection are needed in order to avoid the use of antibiotics. Anti-bacterial and immunostimulatory properties of bioactive compounds are of great interest in aquaculture. Marine derived polysaccharides, such as chitosan and ulvan, together with nanotechnology, have become the focus of attention in the scientific community due to their wide range of biological properties. In this work, chitosan and ulvan-loaded chitosan nanoparticles (referred as CS-TPP NPs and CS-UL-TPP NPs, respectively), obtained by the ionotropic gelation method, had round shape, and the mean sizes were 137.00 ± 5.44 and 325.50 ± 4.95 nm, respectively. No study about the anti-bacterial activity of both types of NPs against Photobacterium damselae subsp. piscicida, an important fish pathogen, has been reported so far. Furthermore, the potential immunostimulatory effects of CS-UL-TPP NPs after oral administration in fish have not yet been evaluated. The percentage of bacterial inhibition against P. damselae subsp. piscicida was determined through in vitro assays, and it was significantly higher in CS-UL-TPP NPs than in CS-TPP NPs at concentrations below 0.03 mg mL-1. The effects on the immune system of CS-TPP and CS-UL-TPP NPs were evaluated in Solea senegalensis juveniles at 30 days after oral administration. Lysozyme activity as well as gene expression levels of il1b, il6, hamp1, tf and c3 was significantly higher in CS-UL-TPP NP-treated groups than in the controls, and no significant differences were observed in CS-TPP NP-treated groups. Thus, ulvan extracted from the macroalgae Ulva ohnoi could improve anti-bacterial and immunostimulant properties of CS-TPP NPs thereby making them suitable to be used as vaccine adjuvant or as immunostimulant.

Lc-pPG-612-OmpU-CTB: A promising oral vaccine for protecting Carassius auratus against Vibrio mimicus infection

Vibrio mimicus (V. mimicus) is known to cause severe bacterial diseases with high mortality rates in fish, resulting in significant economic losses in the global aquaculture industry. Therefore, the objective of this study was to develop a safe and effective vaccine for protecting Carassius auratus (C. auratus) against V. mimicus infection. Recombinant Lactobacillus casei (L. casei) strains, Lc-pPG-612-OmpU and Lc-pPG-612-OmpU-CTB (surface-displayed), were constructed using a L. casei strain (ATCC 393) as an antigen delivery carrier and the cholera toxin B subunit (CTB) as an adjuvant. The two recombinant strains of L. casei were administered to C. auratus via oral immunization, and the protective efficacy of the oral vaccines was assessed. The results demonstrated that oral immunization with the two strains significantly increased the levels of nonspecific immune indicators in C. auratus, including alkaline phosphatase (AKP), lysozyme (LYS), acid phosphatase (ACP), complement 3 (C3), complement 4 (C4), lectin, and superoxide dismutase (SOD). Moreover, the experiment groups exhibited significant increases in specific immunoglobulin M (IgM) antibodies against OmpU, as well as the transcription of immune-related genes (ie., IL-1β, TNF-α, IL-10, and TGF-β), when compared to the control groups. Following infection of C. auratus with V. mimicus, the mortality rate of the recombinant L. casei-treated fish was observed to be lower compared to the control group. This finding suggests that recombinant L. casei demonstrates effective protection against V. mimicus infection in C. auratus. Furthermore, the addition of the immune adjuvant CTB was found to induce a more robust adaptive and innate immune response in C. auratus, resulting in reduced mortality after infection with V. mimicus.

Transforming the untransformable with knockout minicircles: High-efficiency transformation and vector-free allelic exchange knockout in the fish pathogen Photobacterium damselae

Gene inactivation studies are critical in pathogenic bacteria, where insights into species biology can guide the development of vaccines and treatments. Allelic exchange via homologous recombination is a generic method of targeted gene editing in bacteria. However, generally applicable protocols are lacking, and suboptimal approaches are often used for nonstandard but epidemiologically important species. Photobacterium damselae subsp. piscicida (Pdp) is a primary pathogen of fish in aquaculture and has been considered hard to transform since the mid-1990s. Consequently, conjugative transfer of RK2/RP4 suicide vectors from Escherichia coli S17-1/SM10 donor strains, a system prone to off-target mutagenesis, was used to deliver the allelic exchange DNA in previous studies. Here we have achieved efficient electrotransformation in Pdp using a salt-free highly concentrated sucrose solution, which performs as a hypertonic wash buffer, cryoprotectant, and electroporation buffer. High-efficiency transformation has enabled vector-free mutagenesis for which we have employed circular minimalistic constructs (knockout minicircles) containing only allelic exchange essentials that were generated by Gibson assembly. Preparation of competent cells using sucrose and electroporation/integration of minicircles had virtually no detectable off-target promutagenic effect. In contrast, a downstream sacB selection apparently induced several large deletions via mobilization of transposable elements. Electroporation of minicircles into sucrose-treated cells is a versatile broadly applicable approach that may facilitate allelic exchange in a wide range of microbial species. The method permitted inactivation of a primary virulence factor unique to Pdp, apoptogenic toxin AIP56, demonstrating the efficacy of minicircles for difficult KO targets located on the high copy number of small plasmids.

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

FrpA is the outer membrane piscibactin transporter in Vibrio anguillarum: structural elements in synthetic piscibactin analogues required for transport

Piscibactin (Pcb) is a labile siderophore widespread among Vibrionaceae. Its production is a major virulence factor of some fish pathogens such as Photobacterium damselae subsp. piscicida and Vibrio anguillarum. Although FrpA was previously suggested as the putative outer membrane transporter (OMT) for ferri-piscibactin, its role in piscibactin uptake was never demonstrated. In this work, we generated mutants of V. anguillarum defective in FrpA and analyzed their ability to use piscibactin as iron source. The results showed that inactivation of frpA completely disables piscibactin utilization, and the original phenotype could be restored by gene complementation, confirming that FrpA is the OMT that mediates ferri-Pcb uptake. Additionally, the ability of several Pcb thiazole analogues, with different configurations at positions 9, 10, and 13, to be internalized through FrpA, was evaluated measuring their ability to promote growth under iron deficiency of several indicator strains. The results showed that while those analogues with a thiazole ring maintain almost the same activity as Pcb, the maintenance of the hydroxyl group present in natural piscibactin configuration at position C-13 is crucial for Fe³⁺ chelation and, in consequence, for the recognition of the ferri-siderophore by the cognate OMT. All these findings allowed us to propose a Pcb analogue as a good candidate to vectorize antimicrobial compounds, through the Trojan horse strategy, to develop novel compounds against bacterial fish diseases.

The moonlighting protein fructose 1,6-bisphosphate aldolase as a potential vaccine candidate against Photobacterium damselae subsp. piscicida in Asian sea bass (Lates calcarifer)

Vaccination is the most effective, safe, and environmentally friendly method to prevent the outbreak of Photobacterium damselae subsp. piscicida (Phdp), a dangerous pathogen in aquaculture worldwide. Here, recombinant proteins of catalase, superoxide dismutase, isocitrate dehydrogenase, fructose 1,6-bisphosphate aldolase (Fba), and a mixture of all four proteins were investigated for their immunoprotective effects against photobacteriosis in Asian sea bass (Lates calcarifer). After immunization, experimental fish showed an increase in specific antibody levels and lysozyme activities, especially the Fba group. After a lethal challenge with Phdp strain AOD105021, the Fba group achieved the highest relative percentage of survival rate (70.21%) and a significantly lower bacterial load in the spleens than other groups 3 days after infection. The results suggest that Fba is a good candidate for subunit vaccine development against photobacteriosis in fish.

Evaluation of an oral DNA nanovaccine against photobacteriosis in Solea senegalensis

Infectious diseases are one of the main causes of social and economical losses in world aquaculture. Senegalese sole (Solea senegalensis) is an important species for aquaculture in southern Europe, whose production is affected by the appearance of bacterial diseases such as photobacteriosis, a septicemia caused by Photobacterium damselae subsp. piscicida (Phdp). The aim of this study was to obtain an oral DNA nanovaccine and to evaluate its efficacy against Phdp in S. senegalensis juveniles. For this purpose, the amplified product corresponding to the protein inosine-5'-monophophate dehydrogenase (IMPDH) from Phdp, was cloned into the expression vector pcDNA™6.2/C-EmGFP-GW obtaining the DNA vaccine named as pPDPimpdh. The correct transcription and protein expression was verified at 48 h post tansfection in HEK293 cells. Chitosan nanoparticles (CS-TPP NPs) were prepared by ionotropic gelation and their features were appropriate for use as oral delivery system. Therefore, pPDPimpdh was protected with chitosan CS-TPP NPs throughout complex coacervation method giving as a result a DNA nanovaccine referred as CS-TPP+pPDPimpdh NPs. Sole juveniles were vaccinated orally with CS-TPP NPs, pPDPimpdh and CS-TPP+pPDPimpdh NPs followed by a challenge with Phdp at 30 days post vaccination (dpv). The relative percentage survival (RPS) for pPDPimpdh vaccinated groups was 6.25%, probably due to its degradation in the digestive tract. RPS value obtained for CS-TPP NPs and CS-TPP+pPDPimpdh NPs was 40% and antibodies were observed in both cases. However, a delay in mortality was observed in sole juveniles vaccinated orally with CS-TPP+pPDPimpdh NPs. In fact, an upregulation of tf, mhcII, cd8a and igm in the posterior gut and c3, hamp1, tf and cd4 in spleen was observed in juveniles vaccinated with CS-TPP+pPDPimpdh NPs. After challenge, a modulation of cd8a and cd4 expression levels in the posterior gut and c3, tf, lyg, cd4, igm and igt expression levels in spleen was observed. Moreover, the concentration of lysozyme in skin mucus significantly increased in fish vaccinated orally with CS-TPP+pPDPimpdh NPs at 11 dpc. These data indicate that oral vaccination with CS-TPP+pPDPimpdh NPs could be acting through the non-specific immune responses as well as the specific humoral and cell mediated immunity and provide the first step toward a development of an oral DNA nanovaccine against Phdp in sole.

Protective efficacy of four heat-shock proteins as recombinant vaccines against photobacteriosis in Asian seabass (Lates calcarifer)

Photobacterium damselae subsp. piscicida (Phdp) is the causative agent of photobacteriosis in marine fish and is responsible for huge losses to marine aquaculture worldwide. Efforts have been made to develop a vaccine against this disease. Heat-shock proteins (HSPs) are a family of proteins that are ubiquitous in cellular life. Bacteria produce elevated levels of HSPs as a survival strategy when exposed to stressful environments in a host during infection. This group of proteins are also important antigens that can induce both humoral and cellular immune responses. In this study, four HSPs of Phdp, HSP90, HSP33, HSP70, and DnaJ, were selected for cloning and recombinant expression. Western blotting with rabbit anti-Phdp helped identify rHSP70 and rHSP33 as immunogenic proteins. Asian seabass (Lates calcarifer) immunised with rHSP90, rHSP33, rHSP70, and rDnaJ showed 48.28%, 62.07%, 51.72%, and 31.03% relative percent survival, respectively, after being challenged with Phdp strain AOD105021. High expression levels of immune-related genes and high antibody titres were observed in the rHSP33 group, and the sera of this group also exhibited a high level of bactericidal activity against Phdp. Collectively, our results suggest that HSP33 is a potential candidate for vaccine development against Phdp infection.

Iron uptake mechanisms as key virulence factors in bacterial fish pathogens

This review summarizes the current knowledge about iron uptake systems in bacterial fish pathogens and their involvement in the infective process. Like most animal pathogens, fish pathogens have evolved sophisticated iron uptake mechanisms some of which are key virulence factors for colonization of the host. Among these systems, siderophore production and heme uptake systems are the best studied in fish pathogenic bacteria. Siderophores like anguibactin or piscibactin, have been described in Vibrio and Photobacterium pathogens as key virulence factors to cause disease in fish. In many other bacterial fish pathogens production of siderophores was demonstrated but the compounds were not yet chemically characterized and their role in virulence was not determined. The role of heme uptake in virulence was not yet clearly elucidated in fish pathogens although there exist evidence that these systems are expressed in fish tissues during infection. The relationship of other systems, like Fe(II) transporters or the use of citrate as iron carrier, with virulence is also unclear. Future trends of research on all these iron uptake mechanisms in bacterial fish pathogens are also discussed.