Development of a SYBR green I real-time PCR assay for specific identification of the fish pathogen Aeromonas salmonicida subspecies salmonicida

Major antigenic differences in Aeromonas salmonicida isolates correlate with the emergence of a new strain causing furunculosis in Chilean salmon farms

Aeromonas salmonicida is the etiological agent of furunculosis, a septicemic disease with high mortality rates affecting salmonids and other teleost species worldwide. Reviewing molecular diagnostic protocols for routine diagnostics, we realized that the amplification of the vapA target gene failed in some cases of furunculosis. Therefore, we hypothesized that the emergence of a new strain may be involved in recent outbreaks. In this work, we demonstrate that the vapA locus is absent in the new strain, which explains why it lacks the major membrane component VapA protein, a critical virulence factor. In addition, we found that the vapA-absent strain differs from its counterparts in outer membrane protein and lipopolysaccharide profiles, suggesting profound changes at the membrane structure level and in antigenic properties. These features along with sequence analysis information allowed us to infer that a complex genomic rearrangement, probably an indel encompassing the entire vapA locus, gave rise to this membrane phenotype. Although the causes for pathogen evolution and emergence were not fully elucidated, our results strongly suggest that the vapA-absent strain is responsible for a raising proportion of recent furunculosis cases, and that it may be related to a less virulent disease and a low serological response upon vaccination with the A. salmonicida antigen formulation currently used in Chile.

A novel portable label-free electrochemical immunosensor for ultrasensitive detection of Aeromonas salmonicida in aquaculture seawater

Infectious diseases caused by Aeromonas salmonicida (A. salmonicida) have a huge impact and produce significant losses in aquaculture and fish farming. Fish pathogen early detection is a critical step for the rapid identification and prevention of these problems. This work presents a novel portable label-free ultrasensitive electrochemical immunosensor for A. salmonicida detection in seawater. It consists of a fluidic integrated electrochemical-cell-chip (ECC) with independent chambers enclosing three electrochemical cells (ECs). Anti-A. salmonicida (AbSalm) antibodies were covalently attached to the gold surface of the microfabricated electrodes and were used for the sensitive detection of A. salmonicida. The antibody-antigen immunoreaction was studied by enzyme-linked immunosorbent assay (ELISA), and the surface functionalization was characterized by using quartz crystal microbalance (QCM), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). The performance of the developed immunosensor, in terms of sensitivity, repeatability, and specificity, was also studied. The linear working range varied between 1 and 10⁷ CFU mL^-1, with a limit of detection (LOD) as low as 1 CFU mL^-1. The suitability of the immunosensor for real sample detection was successfully demonstrated via recovery studies performed in spiked seawater samples. The proposed technology supports the use of low-cost and portable instrumentation that concedes the ultrasensitive, simple, and fast quantification of the A. salmonicida. To the best of our knowledge, this is the first portable sensing system for the detection of A. salmonicida in seawater samples, which provides a promising online monitoring platform for the detection of this bacterium in aquaculture facilities.

Whole-genome association study searching for QTL for Aeromonas salmonicida resistance in rainbow trout

Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis, has extensive negative effects on wild and farmed salmonids worldwide. Vaccination induces some protection under certain conditions but disease outbreaks occur even in vaccinated fish. Therefore, alternative disease control approaches are required to ensure the sustainable expansion of rainbow trout aquaculture. Selective breeding can be applied to enhance host resistance to pathogens. The present work used genome-wide association study (GWAS) to identify quantitative trait loci (QTL) associated with A. salmonicida resistance in rainbow trout. A total 798 rainbow trout exposed to A. salmonicida by bath challenge revealed 614 susceptible and 138 resistant fish. Genotyping was conducted using the 57 K single nucleotide polymorphism (SNP) array and the GWAS was performed for survival and time to death phenotypes. We identified a QTL on chromosome 16 and located positional candidate genes in the proximity of the most significant SNPs. In addition, samples from exposed fish were examined for expression of 24 immune-relevant genes indicating a systematic immune response to the infection. The present work demonstrated that resistance to A. salmonicida is moderately heritable with oligogenic architecture. These result will be useful for the future breeding programs for improving the natural resistance of rainbow trout against furunculosis.

Rapid diagnosis of Ralstonia solanacearum infection sweet potato in China by loop-mediated isothermal amplification

Bacterial wilt of sweet potato is caused by Ralstonia solanacearum, which is distributed in southern China and causes significant economic losses each year. The pathogen is soil- and rhizome-borne, and thus its rapid detection may prevent the occurrence and spread of the disease. R. solanacearum has been listed as a quarantine disease in China. With the advent of molecular biology, many novel tools have been explored for the rapid identification of plant pathogens. In this study, a strain-specific detection method was developed for this specific pathogen that infects sweet potato using loop-mediated isothermal amplification (LAMP). A set of new LAMP-specific primers was designed from the orf428 gene, which can specifically detect the R. solanacearum bacterium that infect sweet potato. The LAMP reaction consisted of 8.0 mmol·L^-1Mg²⁺, 1.4 mmol·L^-1 dNTPs, and 0.32U μL^-1 Bst 2.0 DNA polymerase and was performed at 65 °C for 1 h. The amplification products were detected by visualizing a mixture of color changes using SYBR Green I dye and assessing ladder-like bands by electrophoresis. Our method has specificity, i.e., it only detected R. solanacearum in sweet potato, and it has high sensitivity, with a detection limit of 100 fg·μL^-1 genomic DNA and 10³ CFU·mL^-1 of bacterial fluid. In addition, R. solanacearum could be directly detected in infected sweet potato tissues without the need for DNA extraction. The LAMP method established in this study is a highly specific, sensitive, and rapid tool for the detection of bacterial wilt in sweet potato caused by R. solanacearum.

Detection and characterization of Aeromonas salmonicida subsp. salmonicida infection in crucian carp Carassius auratus

Aeromonas salmonicida is one of the most important pathogens in salmonids and non-salmonids species. Nevertheless, very little was reported in cyprinids about A. salmonicida infection. Hence, a pathogenic A. salmonicida subsp. salmonicida, namely isolate GCA-518, was isolated from diseased crucian carp Carassius auratus. Its optimal growth conditions were at 28 °C, pH 7.0 and 1.5% NaCl. Furthermore, the quantitative real-time PCR (qPCR) targeting serine protease (aspA) gene was established for rapid detection of the lowest limit of 5.6 × 10² copies per reaction. The pathogenicity was confirmed in crucian carp by intraperitoneal infection. Histopathologic examination displayed multifocal necrosis and infiltration of inflammatory cells in gill, liver, kidney and intestine. This is the first report on typical A. salmonicida infection in cultured crucian carp.

Comparative genomics of Streptococcus parauberis: new target for molecular identification of serotype III

This paper describes the predicted structure for the cps loci involved in capsule biosynthesis for Streptococcus parauberis serotypes III, IV, and V. Based on the specific serotype regions I, II, and III, a multiplex PCR protocol (mPCR) was designed to differentiate the main serotypes causing fish diseases. A real-time PCR method (qPCR) is also described to identify S. parauberis of serotype III in bacterial cultures and fish tissues. In silico and in vitro analyses revealed that both methods have a 100% specificity. The mPCR assay was optimized for the detection of S. parauberis strains of subtypes Ia (amplicon size 213 bp), subtypes Ib and Ic (both amplicon size 303 bp), serotype II (amplicon size 403 bp), and serotype III (amplicon size 130 bp) from bacterial cultures. The qPCR assay was optimized for the identification and quantification of S. parauberis serotype III strains in bacterial cultures and fish tissues. This assay achieved a sensitivity of 2.67 × 10² gene copies (equivalent to 3.8 × 10^-9 ng/μl) using pure bacterial cultures of S. parauberis serotype III and 1.76 × 10² gene copies in fish tissues experimentally and naturally infected with S. parauberis of the serotype III. The specificity and sensitivity of the protocols described in this study suggest that these methods could be used for diagnostic and/or epidemiological purposes in clinical diagnostic laboratories. KEY POINTS: • Structure of loci cps for S. parauberis of serotypes III, IV and V was described. • mPCR to differentiate S. parauberis serotypes causing disease in fish was optimized. • qPCR assay to quantify strains of S. parauberis serotype III in fish tissues.

Kuma Bamboo Grass (Sasa veitchii) Extracts Exhibit Protective Effects Against Atypical Aeromonas salmonicida Infection in Goldfish (Carassius auratus)

Atypical Aeromonas salmonicida ( i.e. subsp. achromogenes and subsp. masoucida) are one of the major opportunistic pathogens that cause ulcer diseases in a variety of fishes, in which this pathogen has become a worldwide economic threat in sectors that handle of particular high-priced ornamental fishes like varicolored carp and goldfish due to appearance damages. Here we reported that the kuma bamboo grass (Sasa veitchii) extracts (KBGE) that contained a variety of fatty acids, exhibited antibacterial activity against nine Aeromonas strains including 5 atypical A. salmonicida strains. Experimental challenges with four atypical A. salmonicida strains revealed that supplementation with 375 to 750 μg/ml of the KBGE restored the survival of goldfish in coincidence of inhibition of both bacterial replication and superoxide dismutase (SOD) activity upon infection, compared with those of untreated control. Together, our data demonstrating the antibacterial effects of the plant extracts proposes its possible implication for prevention of Aeromonas infection in the ornamental fish.

High-throughput identification and quantification of Vagococcus salmoninarum by SYBR Green I-based real-time PCR combined with melting curve analysis

This work describes a primer pair and a high-throughput SYBR Green I-based real-time PCR protocol combined with melting curve analysis for identification and quantification of Vagococcus salmoninarum in bacterial cultures and infected fish tissues. The 16S rRNA gene was selected for the design of the primer pair (SalF and SalR). The sensitivity and specificity of this primer pair were compared with other previously designed for conventional PCR. Although both primer pairs showed 100% specificity using pure bacterial cultures or DNA extracted from bacteria or fish tissues, the primer pairs designed in this study showed the highest sensitivity with a detection limit of 0.034 × 10⁰ amplicon copies per assay (equivalent to 2 × 10^-11  ng/µl, C_q value of 30.49 ± 1.71). The developed qPCR protocol allowed the detection of V. salmoninarum in non-lethal and lethal fish samples with detection levels of 0.17 × 10⁰ gene copies in tissues artificially infected and 0.02 × 10⁰ in tissues of fish experimentally infected with V. salmoninarum. The high sensitivity of the developed method suggests that it could be considered as a useful tool for diagnosis of vagococcosis and the detection of V. salmoninarum in asymptomatic or carrier fish.

Vaccine-Induced Protection Against Furunculosis Involves Pre-emptive Priming of Humoral Immunity in Arctic Charr

With respect to salmonid aquaculture, one of the most important bacterial pathogens due to high mortality and antibiotic usage is the causative agent of typical furunculosis, Aeromonas salmonicida spp. salmonicida (Asal). In Atlantic salmon, Salmo salar, the host response during infections with Asal is well-documented, with furunculosis outbreaks resulting in significant mortality in commercial settings. However, less is known about the host-pathogen interactions in the emerging aquaculture species, Arctic charr Salvelinus alpinus. Furthermore, there is no data on the efficacy or response of this species after vaccination with commonly administered vaccines against furunculosis. To this end, we examined the immunological response of S. alpinus during infection with Asal, with or without administration of vaccines (Forte Micro®, Forte Micro® + Renogen®, Elanco Animal Health). Artic charr (vaccinated or unvaccinated) were i.p.-injected with a virulent strain of Asal (10⁶ CFUs/mL) and tissues were collected pre-infection/post-vaccination, 8, and 29 days post-infection. Unvaccinated Arctic charr were susceptible to Asal with 72% mortalities observed after 31 days. However, there was 72–82% protection in fish vaccinated with either the single or dual-vaccine, respectively. Protection in vaccinated fish was concordant with significantly higher serum IgM concentrations, and following RNA sequencing and transcriptome assembly, differential expression analysis revealed several patterns and pathways associated with the improved survival of vaccinated fish. Most striking was the dramatically higher basal expression of complement/coagulation factors, acute phase-proteins, and iron hemostasis proteins in pre-challenged, vaccinated fish. Remarkably, following Asal infection, this response was abrogated and instead the transcriptome was characterized by a lack of immune-stimulation compared to that of unvaccinated fish. Furthermore, where pathways of actin assembly and FcγR-mediated phagocytosis were significantly differentially regulated in unvaccinated fish, vaccinated fish showed either the opposite regulation (ForteMicro®), or no impact at all (ForteMicro®Renogen®). The present data indicates that vaccine-induced protection against Asal relies on the pre-activation and immediate control of humoral immune parameters that is coincident with reduced activation of apoptotic (e.g., NF-κB) and actin-associated pathways.

Silver nanoparticles: Their role as antibacterial agent against Aeromonas salmonicida subsp. salmonicida in rainbow trout (Oncorhynchus mykiss)

The rise of bacterial resistance to antibiotics is one of the great challenges of our age. One of the strategies to limit the development of antibiotics resistance is the investigation of alternative antimicrobials. As silver nanoparticles demonstrated a potent bactericidal activity in vitro, the aim of this study was to evaluate the in vivo antibacterial activity of silver nanoparticles against Aeromonas salmonicida subsp. salmonicida. Rainbow trout (n = 120) were divided into four groups of 30 fish each. First group was challenged with A. salmonicida (Positive control), the second group was challenged with A. salmonicida and exposed to silver nanoparticles by immersion for three hours (100 μg/L), the third group was challenged with A. salmonicida and intraperitoneally injected with silver nanoparticles (17 μg/mL) and the fourth group was sham-treated and served as a negative control group. At the 7th day post challenge, histopathology of the positive control group revealed the presence of bacterial aggregates in tissues with degenerative and necrotic changes, while at the 35th day post challenge, only liver necrosis persisted. Silver nanoparticles-treated and negative control groups did not show any clinical signs, mortalities or histopathological alterations and they were tested negative for A. salmonicida. The immersion in silver nanoparticles did not result in detectable residues of silver in the muscles 35 days after treatment. These findings demonstrate the antibacterial properties of silver nanoparticles against A. salmonicida infection. Therefore, they could be used for development of antibacterial agents in aquaculture.