Development of novel aptamer‐based enzyme‐linked apta‐sorbent assay (ELASA) for rapid detection of mariculture pathogen Vibrio alginolyticus

As the major opportunistic pathogen to both marine animals and humans, Vibrio alginolyticus (V. alginolyticus) has caused heavy economic losses to mariculture. ssDNA aptamer VA2 targeting live V. alginolyticus was generated by systematic evolution of ligands by exponential enrichment (SELEX) technology in our previous study. In this study, we first developed aptamer (VA2)‐based enzyme‐linked apta‐sorbent assay (VA2‐ELASA) for rapid detection of mariculture pathogen V. alginolyticus. The VA2‐ELASA could achieve the rapid detection for V. alginolyticus infection with high specificity and sensitivity. The VA2‐ELASA could specifically identify V. alginolyticus, but not other non‐target bacterial strains. VA2‐ELASA could detect V. alginolyticus at the concentration of 5 × 10⁴/ml, the incubation time short to 1 min and the incubation temperature as high as 45°C, which proved sensitivity and stability of the novel VA2‐ELASA in this study. It took less than one hour to accomplish the detection process by VA2‐ELASA. The characteristics of specificity, sensitivity and easy operation make VA2‐ELASA a novel useful technology for the rapid diagnosis of pathogen V. alginolyticus in mariculture.     

Identification of novel sRNAs involved in oxidative stress response in the fish pathogen Vibrio alginolyticus by transcriptome analysis

Vibrio alginolyticus as an important pathogen in aquaculture can encounter the oxidative stress produced by the immune system during infection. Previous studies showed that sRNAs have important functions in response to oxidative stress in bacteria; however, less of sRNAs related to oxidative stress response were identified in V. alginolyticus. In this study, a total of 749 novel sRNAs were identified by RNA sequencing; among them, 128 sRNAs were up‐ or downregulated in response to oxidative stress. In addition, 1,870 genes exhibited variation on mRNA levels in oxidative stress response. By analysing the target genes of the sRNAs, we concluded that these sRNAs could regulate expressions of genes responsible for iron transport, catalase, GSH‐dependent defence system, electron transferred and stress response. Moreover, the functions of the sRNAs are also seemed related to the pathogenicity in V. alginolyticus. Based on the results, we constructed the oxidative stress model in V. alginolyticus. This study provides us the first outlook of sRNAs function in oxidative stress response in V. alginolyticus. Furthermore, this study can help us to prevent and control this important opportunistic pathogen in aquaculture.     

Histopathological changes in giant freshwater prawn Macrobrachium rosenbergii (de Man 1879) fed with probiotic Bacillus licheniformis upon challenge with Vibrio alginolyticus

The effect of dietary supplementation of probiotic bacterium Bacillus licheniformis on the histopathological changes in Macrobrachium rosenbergii juveniles (4.0 ± 0.02 g) challenged with known pathogenic strain of Vibrio alginolyticus are reported. Two isocaloric basal diets supplemented with probiotic bacteria B. licheniformis (1.0 × 10⁹ cfu/g feed) and other without probiotic supplementation were fed to the M. rosenbergii juveniles for 45 days. The histological observations revealed no significant changes in the hepatopancreas and gut tissues of both the experimental and the control groups which indicate that the present bacterium is a safe candidate probiont for the host. Prawns were challenged with V. alginolyticus after 45 days of feeding with probiotic diet. The histopathological studies of the hepatopancreas revealed that M. rosenbergii fed with probiotic‐supplemented diet showed less changes as compared to the prawns fed with control diet on second and fourth day of post‐experimental challenge with V. alginolyticus. Histopathological observations revealed that the gills of the prawns fed with control diet were severely affected in comparison to the prawns fed with probiotic‐supplemented diet after challenging with V. alginolyticus. Results from this study revealed the improved protection by dietary incorporation of B. licheniformis in reducing the histopathological manifestations due to V. alginolyticus infection in freshwater prawn.     

The role of rpoS in the regulation of Vibrio alginolyticus virulence and the response to diverse stresses

Vibrio alginolyticus is a leading aquatic pathogen, causing huge losses to aquaculture. rpoS has been proven to play a variety of important roles in stress response and virulence in several bacteria. In our previous study, upon treatment with Cu²⁺, Pb²⁺, Hg²⁺ and low pH, the expression levels of rpoS were downregulated as assessed by RNA‐seq, while impaired adhesion ability was observed, indicating that rpoS might play roles in the regulation of adhesion. In the present study, the RNAi technology was used to knockdown rpoS in V. alginolyticus. In comparison with wild‐type V. alginolyticus, RNAi‐treated bacteria showed significantly impaired abilities of adhesion, growth, haemolytic, biofilm production, movement and virulence. Meanwhile, alterations of temperature, salinity, pH and starvation starkly affected rpoS expression. The present data suggested that rpoS is a critical regulator of virulence in V. alginolyticus; in addition, rpoS regulates bacterial adhesion in response to temperature, pH and nutrient content changes. These are helpful to explore its pathogenic mechanism and provide reference for disease control.     

Cloning,expression of Vibrio alginolyticus outer membrane protein‐OmpU gene and its potential application as vaccine in crimson snapper,Lutjanus erythropterus Bloch

The outer membrane proteins of the marine aquatic animal pathogen, Vibrio alginolyticus, play an important role in the virulence of the bacterium and are potential candidates for vaccine development. In this study, the gene encoding an outer membrane protein‐OmpU was cloned and expressed in Escherichia coli. Polyclonal antibodies were raised in rabbits against the purified recombinant OmpU, and the reaction of the antibody was confirmed by Western blotting using the isolated OmpU and the recombinant OmpU of V. alginolyticus. To analyze the immunogenicity of the recombinant OmpU, crimson snapper, Lutjanus erythropterus Bloch, were immunized by intraperitoneal injection, and antibody response was assessed by the enzyme‐linked immunosorbent assay (ELISA). The results demonstrated that the recombinant OmpU produced an observable antibody response in all sera of the vaccinated fish. The vaccinated fish were challenged by virulent V. alginolyticus and observed to have high resistance to infection. These results indicate that the recombinant OmpU is an effective vaccine candidate against V. alginolyticus in L. erythropterus.     

Differences in uptake and killing of pathogenic and non‐pathogenic bacteria by haemocyte subpopulations of penaeid shrimp,Litopenaeus vannamei, (Boone)

Phagocytosis is an important function of both invertebrate and vertebrate blood cells. In this study, the phagocytic activity of haemocyte subpopulations of penaeid shrimp, Litopenaeus vannamei, (Boone), against pathogenic and non‐pathogenic particles was investigated in vitro. The haemocytes of penaeid shrimp were firstly separated by centrifugation on a continuous density gradient of iodixanol into four fractions with five subpopulations (sub), of which sub 1 (hyalinocytes) and sub 4 (semi‐granulocytes) have the main function in phagocytosis of both pathogenic and non‐pathogenic bacteria as well as fluorescent polystyrene beads. It was found that these haemocyte subpopulations engulfed virulent Vibrio campbellii and Vibrio harveyi at a higher rate than non‐virulent Escherichia coli and polystyrene beads. When these bacteria were mixed with shrimp haemocyte subpopulations and incubated for 180 min, the percentage of viable intracellular V. campbellii (25.5 ± 6.0%) recovered was significantly higher than the percentage recovered from V. harveyi (13.5 ± 1.1%). No viable intracellular E. coli was observed in this study. In contrast to V. harveyi and E. coli, V. campbellii containing endosomes did not acidify in time. Incubation of haemocyte subpopulations with the most virulent V. campbellii strain resulted in a significant drop in haemocyte viability (41.4 ± 6.3% in sub 1 and 30.2 ± 15.1% in sub 4) after 180 min post‐inoculation in comparison with the less virulent V. harveyi (84.1 ± 5.6% in sub 1 and 83.4 ± 4.1% in sub 4) and non‐virulent E. coli (92.7 ± 2.8% in sub 1 and 92.3 ± 5.6% in sub 4) and polystyrene beads (91.9 ± 1.6% in sub 1 and 84.4 ± 3.4% in sub 4). These findings may be a valuable tool for monitoring shrimp health and immunological studies.     

Generation and characterization of aptamers against grass carp reovirus infection for the development of rapid detection assay

Grass carp reovirus (GCRV) causes devastating viral haemorrhagic disease in farmed grass carp (Ctenopharyngon idellus). As novel molecular probes, aptamers have been widely applied in rapid diagnosis and efficient therapies against virus or diseases. In this study, three single‐stranded DNA (ssDNA) aptamers were selected against GCRV‐infected CIK cells via SELEX (systematic evolution of ligands by exponential enrichment technology). Secondary structures predicted by MFOLD indicated that aptamers formed stem‐loop structures, and GVI‐11 had the lowest ΔG value of ?30.84 KJ/mol. Three aptamers could specifically recognize GCRV‐infected CIK cells, with calculated dissociation constants (Kd) of 220.86, 176.63 and 278.66 nM for aptamers GVI‐1, GVI‐7 and GVI‐11, respectively, which indicated that they could serve as specific delivery system for antiviral therapies. The targets of aptamers GVI‐1, GVI‐7 and GVI‐11 on the surface of GCRV‐infected cells could be membrane proteins, which were trypsin‐sensitive. Furthermore, FAM‐labelled aptamer GVI‐7 could be applied to detect GCRV infection in vivo. It is the first time to generate and characterize aptamers against GCRV‐infected cells. These aptamers have great potentials in development of rapid diagnosis technology and antiviral agents against GCRV infection in aquaculture.     

Outer membrane protein A (OmpA) is a potential virulence factor of Vibrio alginolyticus strains isolated from diseased fish

Vibrio alginolyticus is one of the most serious causative agents of diseases in cultured marine fish and shellfish. However, the characteristics of virulence factors in pathogenic V. alginolyticus are poorly known. To gain insight into fish diseases caused by V. alginolyticus, we carried out two-dimensional gel electrophoresis (2-DE) combined with MALDI-TOF mass spectrometry to identify uniquely expressed proteins in the disease-causing V. alginolyticus. V. alginolyticus strains were isolated from marine environments and diseased fish obtained from southern Thailand. We identified seven unique proteins in the disease-causing V. alginolyticus strain. Among those, the outer membrane protein A (OmpA) had the strongest expression. Therefore, the function of this protein was further analysed. To investigate the role of OmpA protein, an in-frame deletion mutant of ompA was constructed using the homologous recombination method. Although the ompA mutant V. alginolyticus strain (ΔompA) grew normally, the mutant exhibited a significant defect in the swarming ability and the biofilm formation. Furthermore, Galleria mellonella larvae injected with the mutant bacteria had a significantly greater survival percentage than those injected with the wild-type strain, demonstrating that OmpA protein is required for the pathogenicity of V. alginolyticus. Together, this study suggests a potential target for vaccine development against pathogenic V. alginolyticus strain.     

基于核酸适配体的PCR法检测溶藻弧菌及其灭活菌

溶藻弧菌(Vibrio alginolyticus)分布广,数量多,发病率高,是水产养殖中常见的条件致病菌,而对溶藻弧菌进行快速准确的识别鉴定是其病害防治的前提和基础。核酸适配体,因为具有较高的亲和特异性,在微生物的识别鉴定方面展现出了巨大的优势。本文利用核酸适配体和适配体筛选产物,通过结合、洗涤、加热分离、PCR扩增以及电泳检测等步骤,对溶藻弧菌进行了检测鉴定。结果表明,适配体和筛选产物都能对溶藻弧菌及其灭活菌进行较好的识别鉴定,适配体筛选产物对溶藻弧菌的检测下限为10~3cfu/mL,而对其灭活菌的检测下限为10~2cfu/mL,适配体对溶藻弧菌及其灭活菌的检测下限都可达到10 cfu/mL。该方法对溶藻弧菌有较好的亲和特异性,并能较好地区分溶藻弧菌与哈维氏弧菌等水产常见病原菌,在水产病害的检测中显示了较好的应用前景。     

Effects of crude extracellular protein and crude intracellular polysaccharides of Vibrio alginolyticus on the growth,energy metabolism regulation and WSSV resistance of Litopenaeus vannamei

This study aimed to investigate the effects of adding crude extracts of the extracellular protein (Ex‐Pro) and intracellular polysaccharides (In‐Poly) of Vibrio alginolyticus to diets, at a dosage of 10 g/kg, on the growth, energy metabolism and WSSV (White Spot Syndrome Virus) resistance of Litopenaeus vannamei (initial weight, 0.88 ± 0.04 g/shrimp). Growth and survival rate were not significantly affected by the crude extracts (p > 0.05). Shrimp fed Ex‐Pro crude extracts showed higher succinate dehydrogenase (SDH) activity in the hepatopancreas and muscle but lower lactate dehydrogenase (LDH) activity in the muscle (p < 0.05). The activities of hexokinase (HK), pyruvate kinase (PK), LDH and SDH in the hepatopancreas and the activity of SDH in the muscle were significantly increased by feeding In‐Poly crude extracts (p < 0.05). In contrast, the content of fatty acid synthase (FAS) in the muscle was significantly reduced by the crude extracts (p < 0.05). The contents of glucose and triglyceride and the activity of the electron transport system in the hepatopancreas were significantly increased by the crude extracts (p < 0.05), and the WSSV resistance of the shrimp was increased. These results indicated that the Ex‐Pro and In‐Poly crude extracts of V. alginolyticus could affect energy metabolism, and there was a correlation between WSSV resistance and energy metabolism in L. vannamei.     

Identification of novel immunogenic proteins of Vibrio alginolyticus by immunoproteomic methodologies

Vibrio alginolyticus is one of the most serious diseases in cultured marine and freshwater fish and shellfish. The absence of suitable vaccine or virulent marker can be the bottleneck to control V. alginolyticus infection. In this study, immunoproteomic approaches were undertaken to study the immunogenicity of the whole‐cell protein of V. alginolyticus HY9901. The whole‐cell proteins were analysed by two‐dimensional gel electrophoresis and subsequent immunoblotting using the rabbit anti‐V. alginolyticus HY9901 serum. A total of 55 immunogenic proteins were identified by immunoproteomic analysis. Of the 55 proteins, 51 are specific immunoreactive proteins and four are nonspecific immunoreactive proteins. Furthermore, outer membrane protein N (spot 2) was used as immunogens to immunize Epinephelus coioides for investigation of protective abilities and activities. The E. coioides immunized with OmpN has abilities to fight against infections caused by V. alginolyticus. The other novel immunogenic proteins may be developed as alternative antigens for further study of V. alginolyticus vaccine and diagnostics. These data show that immunoproteomics methods can be successfully applied in identifying immunogenic proteins of V. alginolyticus, which helps to search for the protective antigens in future.     

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.     

Enhanced immune responses and effectiveness of refined outer membrane protein vaccines against Vibrio harveyi in orange‐spotted grouper (Epinephelus coioides)

Vibriosis is a severe infection occurring in many commercially important marine fish species. In this study, vaccines containing Vibrio harveyi recombinant outer membrane protein K (rOmpK), outer membrane protein U (rOmpU) and rOmpK‐OmpU fusion protein in addition to the metabolizable Montanide^TM ISA 763 A VG adjuvant were developed and evaluated in the orange‐spotted grouper. The results indicate that recombinant V. harveyi protein‐based vaccines resulted in a remarkably higher expression of IL‐1β and IL‐8 at 24 hr, and greater antibody production, as early as 2 weeks postimmunization. Notably, enhanced immune responses and significant protective efficacy against V. harveyi infections were observed in the fusion protein vaccine‐injected fishes with relative per cent survival value of 81.8%. Additionally, the rOmpK‐OmpU antisera presented a high bactericidal effect on not only V. harveyi, but also Vibrio parahaermolyticus and Vibrio alginolyticus. Our results demonstrated that the fusion protein rOmpK‐OmpU was an effective vaccine candidate that exhibited potentially great versatility for controlling vibrio infections.     

Recent update on the prevalence of Vibrio species among cultured grouper in Peninsular Malaysia

Vibrio infections are common among marine fish and lead to serious problems in the aquaculture sector. This study reports a recent occurrence of Vibrio species (spp.) isolated from cultured groupers in Peninsular Malaysia using the gyrB and pyrH genes. A total of 147 Vibrio strains were successfully isolated from 77 (64%) groupers using culture method and subjected to gyrB and pyrH sequencing for species identification and confirmation. Results showed that 89% of Vibrio strains were identified and clustered to six groups of Vibrio spp., while 11% were not clustered to any Vibrio spp. using the gyrB sequences. Meanwhile, by analysis of the pyrH sequences all the 147 Vibrio strains (100%) were successfully identified and clustered into 11 groups of Vibrio spp., including the gyrB non‐identified strains. The pyrH gene provides a better resolution for identification of Vibrio spp. compared with the gyrB gene. Thus, the pyrH gene was more suitable for a rapid determination of Vibrio spp. distribution in Peninsular Malaysia. Using the pyrH gene, our study found higher prevalence of Vibrio vulnificus (33%), V. alginolyticus (24%) and V. parahaemolyticus (22%), followed by V. rotiferianus (5%), V. harveyi (3%), V. tubiashii (2%), V. campbellii (2%), V. ponticus (1%), V. diabolicus (1%), V. owensii (1%) and others Vibrio sp. (7%). Thus, the results of this study revealed that the occurrence of pathogenic vibrios among grouper fish is still high in Malaysian aquaculture. In addition, the pyrH gene was proved as a suitable marker for rapid identification of Vibrio species compared with the gyrB gene.     

Development of a colloidal gold immunochromatographic assay (GICA) for the rapid detection of Spiroplasma eriocheiris in commercially exploited crustaceans from China

Spiroplasma eriocheiris is an emerging pathogen in freshwater crustaceans. In recent years, Eriocheir sinensis, Procambarus clarkii, Litopenaeus vannamei, Macrobrachium rosenbergii and Macrobrachium nipponense had been infected by this pathogen in China. An immunochromatographic strip test using gold nanoparticles was developed for rapidly detecting this pathogen. The strip test based on the principle of sandwich immunoassay by the specific combination between the pathogen and polyclonal antibody on a nitrocellulose membrane. Positive samples were displayed as red lines at the test and control zones of the nitrocellulose membrane, while negative samples resulted in a red line only at the control zone. The limit of detection was proved to be 10⁶ Color Change Unit/ml. The test strip could be visually detected within 15 min and do not have cross‐reaction with other aquatic bacteria. This test strip allows on‐site rapid detection of S. eriocheiris in crustacean without the requirement of specialized equipment and professional personnel. The one‐step test strips developed in our study had high sensitivity, specificity, reproducibility and stability. In conclusion, this method was proved to be convenient, feasible, rapid and effective for detecting S. eriocheiris.     

Replacing fish meal with vegetable protein sources in feed for juvenile red swamp crayfish,Procambarus clarkii: Effects of amino acids supplementation on growth and feed utilization

An 8‐week growth trial was conducted to investigate the effects of dietary fish meal replacement with a vegetable mixture of soybean meal and rapeseed meal (1:1) on growth of juvenile red swamp crayfish. Nine isonitrogenous diets were designed: V0, V34, V50, V65, V73 and V81 with six levels of vegetable proteins, and VA48, VA63 and VA78 by further adding crystalline lysine and methionine into V50, V65 and V81. Compared with V0, V34 significantly improved the specific growth rate (SGR), while V65, V73, V81 and VA78 depressed the SGR (p < .05). Feeding rate showed a decreasing trend as dietary vegetable protein level increased (p < .05), except that in VA48 group. Significantly lower FCR and higher PER were observed in V34 group, whereas all vegetable protein diets depressed the feed utilization of crayfish (p < .05). Crayfish fed with diets containing vegetable proteins showed significantly lower hepatosomatic indices and higher condition factors than the control (p < .05). Muscle lipid content was significantly (p < .05) lowered in V81 group, but not in VA78 group. The results suggested that 338 g/kg vegetable protein improved growth performance of crayfish. Excessive vegetable protein depressed the growth of crayfish, which could be prevented by lysine and methionine supplementation except for the all vegetable protein diets.     

The inhibitory activities and antiviral mechanism of Viola philippica aqueous extracts against grouper iridovirus infection in vitro and in vivo

Grouper iridovirus (GIV) is one of the most serious pathogens in mariculture and causes high mortality rates in cultured groupers; then, effective medicines for controlling GIV infections are urgently needed. Viola philippica is a well‐known medicinal plant, and the application of V. philippica aqueous extracts against GIV infection was assessed by different methods in this study. The results showed that the working concentration of V. philippica aqueous extracts was 10 mg/ml. V. philippica aqueous extracts below 10 mg/ml have no significant cytotoxic effects on cell viability, while extracts over 15 mg/ml decreased cell viability and showed cytotoxic activity. V. philippica aqueous extracts had excellent inhibitory effects against GIV infection in vitro and in vivo. The possible antiviral mechanism of V. philippica was further analysed, which indicated that V. philippica did no damages to GIV particles, but it could disturb GIV binding, entry and replication in host cells. V. philippica had the best inhibitory effects against GIV during viral infection stage of binding and replication in host cells. Overall, the results suggest that appropriate concentration of V. philippica aqueous extracts has great antiviral effects, making it an interesting candidate for developing effective medicines for preventing and controlling GIV infection in farmed groupers.     

A selective and differential medium for Vibrio alginolyticus

In this paper, we present a selective and differential medium, termed Vibrio alginolyticus (VAL) agar, developed for the isolation and identification of V. alginolyticus. The presence of bile salts, high salinity and high incubation temperature allows the selective growth of moderately halophilic Vibrio species. Differentiation of bacteria is achieved by identifying species capable of sucrose fermentation, made visible by the pH indicator bromocresol purple. In this study, all of the 26 strains of V. alginolyticus and only three of the 99 strains representing 30 species (including 19 Vibrio species) other than V. alginolyticus were able to grow in the VAL medium. The remaining three strains could be further differentiated from V. alginolyticus according to colour or the diameter of colonies produced on VAL agar plates. Colonies isolated from shellfish rearing water and infected shrimp through the use of VAL agar plates were all positively identified as V. alginolyticus by conventional tests and 16S rDNA sequencing. The testing of specificity and differentiation capability of VAL shows the potential of the agar as a medium for the primary isolation of V. alginolyticus from pathological and environmental samples.     

Antimicrobial activity of partially characterized analytes from Bacillus pumilus(B2)

This study was carried out to characterize the antimicrobial substance produced by the strain of Bacillus pumilus (B2) obtained from Novozymes Biologicals Inc. to compare its antimicrobial activity by agar well diffusion assay and bacteriocin activity assay via critical dilution method against seven different strains of Vibrio spp., specifically V. alginolyticus (A01), V. cholerae (C01), V. fluvialis (F01, F02), V herveyii (H), V. mimicus (M01), V. parahaemolyticus (P01) and V. vulnificus (V01, V02). All Vibrio spp. were isolated from the hemolymph and intestine of the white faeces disease‐infected moribund pacific white‐leg shrimp Litopenaeus vannamei (Boone 1931) and one strain (V. harveyi) from its diseased postlarva. The cell‐free neutralized supernatant (CFNS) of B2 showed moderate thermo‐stability being stable up to 70°C for 60 min with, however, reducing activity above 80°C for 20 min. B2 antimicrobials showed a stable activity within the pH ranging from 6 to 10 at room temperature and at 4°C, while residual antimicrobial activity of crude CFNS showed tolerance to salinity up to 7% of sodium chloride below 4°C. No B2 activity was obtained while exposed to proteolytic enzyme, such as proteinase k and pepsin, while its activity kept stable exposed to lipase. Initial B2 characterization for antimicrobial substance in CFNS revealed proteinaceous in nature owing to activity loss against proteolytic enzymes and no lipid moiety activity against lipase, which could be categorized as bacteriocin‐like inhibitory substance having potential application against several strains of Vibrio spp. in aquaculture.     

Biocontrol of saprolegniosis in rainbow trout (Oncorhynchus mykiss Walbaum) using two bacterial isolates (LE89 and LE141) of Pseudomonas fluorescens

The probiotic activity of 15 bacterial isolates that inhibit Saprolegnia parasitica in vitro was tested for the biocontrol of saprolegniosis in rainbow trout (Oncorhynchus mykiss Walbaum), adding the bacteria to tank water for 14 days at a concentration of 10⁶ bacteria ml^?1 water. Pseudomonas fluorescens LE89 and Pseudomonas fluorescens LE141 were effective in controlling experimental infection with S. parasitica since of the fish treated with LE89, 24.5% ± 16.27% (p < 0.05) became infected, as did 42.8% ± 8.41% (p < 0.05) of those treated with LE141. Given their protective effect when administered in water, their effect was also studied when administered in feed before and after experimental infection. Both bacterial isolates survived low pH levels and the action of bile, grew in skin and intestinal mucus, were resistant to several antibiotics and survived in feed; however, neither of the two isolates prevented S. parasitica infection when administered in feed.