Characterization of role of the toxR gene in the physiology and pathogenicity of Vibrio alginolyticus

Characterization of Bacillus thuringiensis isolated from soils in the Jazan region of Saudi Arabia, and their efficacy against Spodoptera littoralis and Aedes aegypti larvae

Pest control in Saudi Arabia depends on applying chemical insecticides, which have many undesirable considerations and impacts on the environment. Therefore, the aim of this study was to isolate Bacillus thuringiensis from different rhizosphere soil samples in the Jazan region for the biological control of Spodoptera littoralis and Aedes aegypti larvae. The samples were collected from the rhizosphere of different plants located in eight agricultural areas in Jazan, Saudi Arabia. Out of 100 bacterial isolates, four bacterial isolates belonging to Bacillus species were selected namely JZ1, JZ2, JZ3, and JZ4, and identified using classical bacteriological and molecular identification using 16S rRNA. JZ1 and JZ2 isolates were identified as Bacillus thuringiensis. SDS-PAGE analysis and the detection of the Cry1 gene were used to describe the two isolates JZ1 and JZ2 in comparison to Bacillus thuringiensis reference strain Kurstaki HD1 (BTSK) were revealed that slightly different from each other due to the place of their isolation and namely Khlab JZ1 and Ayash JZ2. The EC₅₀ of JZ1 and JZ2 isolates, BTSK, and the commercial biopesticide DiPEL 6.4 DF against the second-instar larvae of Aedes aegypti were 207, 932, 400, and 500 ppm respectively, while EC₅₀ against first-instar larvae of Spodoptera littoralis were 193.93, 589.7, 265.108, and 342.9, ppm respectively. Isolate JZ1 recorded the highest mortality while JZ2 isolate gave the lowest mortality. It can be concluded that the local isolate of JZ1 and JZ2 can be developed for bio formulations to be used in Spodoptera littoralis and Aedes aegypti biological control programs.

Effects of intestinal microbiota on physiological metabolism and pathogenicity of Vibrio

Vibrio species are disseminated broadly in the marine environment. Some of them can cause severe gastroenteritis by contaminating seafood and drinking water, such as Vibrio parahaemolyticus, Vibrio cholerae, and Vibrio vulnificus. However, their pathogenic mechanism still needs to be revealed to prevent and reduce morbidity. This review comprehensively introduces and discusses the common pathogenic process of Vibrio including adhesion, cell colonization and proliferation, and resistance to host immunity. Vibrio usually produces pathogenic factors including hemolysin, type-III secretion system, and adhesion proteins. Quorum sensing, a cell molecular communication system between the bacterial cells, plays an important role in Vibrio intestinal invasion and colonization. The human immune system can limit the virulence of Vibrio or even kill the bacteria through different responses. The intestinal microbiota is a key component of the immune system, but information on its effects on physiological metabolism and pathogenicity of Vibrio is seldom available. In this review, the effects of intestinal microorganisms and their metabolites on the invasion and colonization of common pathogenic Vibrio and VBNC status cells are discussed, which is conducive to finding the next-generation prebiotics. The strategy of dietary intervention is discussed for food safety control. Finally, future perspectives are proposed to prevent Vibrio infection in aquaculture.

Transcriptome profiles of genes related to growth and virulence potential in Vibrio alginolyticus treated with modified clay

Vibrio alginolyticus is a globally distributed opportunistic pathogen that causes different degrees of disease in various marine organisms, such as fish, shrimp and shellfish. At present, vibriosis caused by V. alginolyticus has a wide epidemic range and causes frequent outbreaks, resulting in substantial losses in aquaculture. According to previous studies, modified clay (MC) could effectively flocculate and reduce the density of Vibrio in water, but it is still unknown whether MC inhibits growth and how it affects virulence in bottom flocs. Here, we studied the response mechanism of V. alginolyticus in flocs treated with MC at the transcriptome level and verified the transcriptomic data combined with relevant physiological experiments and reverse transcription quantitative real-time PCR (RT-qPCR) for the first time. It was found that the morphology of Vibrio in the MC flocs changed, the membrane function was damaged, the antioxidant system was activated, and the material and energy metabolism also changed. In addition, MC could inhibit the expression of virulence factors of V. alginolyticus; for example, flagella, pilus, siderophores, quorum sensing, and other related genes were significantly downregulated. In general, MC effectively inhibited the growth of Vibrio and reduced its virulence potential in flocs, which could provide theoretical support for a new model of healthy aquaculture.

ToxR modulates biofilm formation in fish pathogen Vibrio harveyi

Vibrio harveyi is a common aquaculture pathogen causing diseases in a variety of aquatic animals. toxR, a conserved virulence-associated gene in vibrios, is identified in V. harveyi 345, a pathogenic strain isolated from diseased fish. In this study, to gain insight into function of ToxR in V. harveyi, an in-frame deletion of the toxR gene was constructed to reveal the role of ToxR in the physiology and virulence of V. harveyi. The statistical analysis showed no significant differences in the growth ability, motility, extracellular protease secretion, antibiotic susceptibility, virulence by intraperitoneal injection and the ability of V. harveyi to colonize the spleen and liver tissues of the pearl gentian grouper between the wild-type (WT) and the toxR mutant. However, the deletion of toxR increased the biofilm formation. The structure of the V. harveyi biofilm was further analysed by using scanning electron microscopy (SEM) and confocal laser scanning microscopy, and the results showed that deletion of toxR increased the number and density of V. harveyi biofilm. Since biofilm production is flagella, exopolysaccharide (EPS) and lipopolysaccharide dependent, 16 of V. harveyi biofilm-related genes were selected for further analysis. Based on quantitative real-time reverse transcription-PCR, the expression levels of these genes, including genes flrB, motY and mshA, flaE, flrA and gmhD, were significantly up-regulated in the ΔtoxR⁺ strain as compared with the WT⁺ and C-ΔtoxR strains during the early and mid-exponential, while epsG, flaA, flaE, flgD, flgE, flrB, flrC, lpxB, motY, mshA and scrG genes were inhibited because of deletion of the toxR gene in the stationary growth phase. Our results indicate that ToxR plays an important role in controlling the biofilm in V. harveyi.

Fast, simple and highly specific molecular detection of Vibrio alginolyticus pathogenic strains using a visualized isothermal amplification method

Background

Vibrio alginolyticus is an important pathogen that has to be closely monitored and controlled in the mariculture industry because of its strong pathogenicity, quick onset after infection and high mortality rate in aquatic animals. Fast, simple and specific methods are needed for on-site detection to effectively control outbreaks and prevent economic losses. The detection specificity towards the pathogenic strains has to be emphasized to facilitate pointed treatment and prevention. Polymerase chain reaction (PCR)-based molecular approaches have been developed, but their application is limited due to the requirement of complicated thermal cycling machines and trained personnel.

Results

A fast, simple and highly specific detection method for V. alginolyticus pathogenic strains was established based on isothermal recombinase polymerase amplification (RPA) and lateral flow dipsticks (LFD). The method targeted the virulence gene toxR, which is reported to have good coverage for V. alginolyticus pathogenic strains. To ensure the specificity of the method, the primer-probe set of the RPA system was carefully designed to recognize regions in the toxR gene that diverge in different Vibrio species but are conserved in V. alginolyticus pathogenic strains. The primer-probe set was determined after a systematic screening of amplification performance, primer-dimer formation and false positive signals. The RPA-LFD method was confirmed to have high specificity for V. alginolyticus pathogenic strains without any cross reaction with other Vibrio species or other pathogenic bacteria and was able to detect as little as 1 colony forming unit (CFU) per reaction without DNA purification, or 170 fg of genomic DNA, or 6.25 × 10³ CFU/25 g in spiked shrimp without any enrichment. The method finishes detection within 30 min at temperatures between 35 °C and 45 °C, and the visual signal on the dipstick can be directly read by the naked eye. In an application simulation, randomly spiked shrimp homogenate samples were 100% accurately detected.

Conclusions

The RPA-LFD method developed in this study is fast, simple, highly specific and does not require complicated equipment. This method is applicable for on-site detection of V. alginolyticus pathogenic strains for the mariculture industry.

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.

The pathogenicity characterization of non-O1 Vibrio cholerae and its activation on immune system in freshwater shrimp Macrobrachium nipponense

Outbreaks of mass mortalities among cultured Macrobrachium nipponense occurred in a commercial hatchery during the autumn of 2017 in Jiangsu province, P. R. China, and non-O1 Vibrio cholerae was isolated and identified as causal agents of M. nipponense, with a LD₅₀ value 4.09 × 10⁴ CFU/mL. Detection of virulence-associated genes by PCR indicated that XL1 was positive for Mp, HlyA, RtxA, OmpU, Ace, Zot and T6SS. Furthermore, the results of extracellular enzyme analysis revealed that the strain can produce lecithinase, amylase, gelatinase and hemolysin. Histopathological analysis revealed that the hepatic tubule lumen and the gap between the hepatic tubules became larger, and the brush border disappeared in the hepatopancreas. Quantitive real-time PCR (qRT-PCR) was undertaken to measure mRNA expression levels for thirteen immune related genes in M. nipponense after non-O1 V. cholerae infection. The transcriptional analysis of these immune related genes demonstrated that the expression levels of dorsal, relish, p38, crustin1, crustin2, crustin3, hemocyanin, i-lysozyme, anti-lipopolysaccharide factors 1, anti-lipopolysaccharide factors 2, prophenoloxidase were significantly up-regulated in hemolymph of M. nipponense post-infection. These results revealed varying expression profiles and clear transcriptional activation of these immune related genes in hemolymph, which will contribute to better understand the pathogenesis and host defensive system in non-O1 V. cholerae invasion.

Quality assessment of Zeus faber (Peter's fish) ovaries regularly commercialized for human consumption

In the last few years, the consumption of fish eggs has increased rapidly, finding widespread use also in mass catering. This increase has involved also those of the Peter's fish (Zeus faber). Females of this species, by their reproductive characteristics, have highly developed gonads in different periods of the year, making the raw material easy to find. The aim of the present study was to perform a quality assessment of Zeus faber ovaries regularly commercialized for human consumption. A total number of 34 samples, divided in fresh (11) and frozen (23), were processed for microbiological characterization, parasitological and histological evaluations. Fresh and frozen samples have significant (P<0.01) differences in total bacterial charge, with values of 4.75±0.5 Log CFU/g and 3.65±0.7 Log CFU/g respectively. The mean value of Enterobacteriaceae was 2.58±0.7 Log CFU/g in fresh products, while 52.17% (12) of frozen samples reported loads of <1 Log CFU/g. No Salmonella spp. and Listeria monocytogenes were found. Aeromonas spp. was detected in two frozen sample (with loads of 2.2 and <1 Log CFU/g) and in 5 fresh ovaries with value ranged from 1.70 to 3.48 Log CFU/g. Vibrio spp. was found in 4 (36.36%) and 3 (13.04%) of fresh and frozen samples respectively, with loads always <1 Log CFU/g. All 31 Vibrio strains isolated, were identified as Vibrio alginolyticus, and 61.29% (19) of them was positive for the ToxRS factor and 6.45% (2) for ToxR. The 47.06% (16) of total samples showed infestations by larvae of Anisakis Type 1 in the serous and inside the ovary. In this last case, histologically it was found to be free larvae. This study attested satisfactory hygiene conditions for Zeus faber ovaries currently marked for human consumption. The presence of potentially pathogenic strains of V. alginolyticus and Aeromonas spp., but above all the frequent infestation by Anisakis larvae, represent a potentially hazard for the consumer.

AcfA is an essential regulator for pathogenesis of fish pathogen Vibrio alginolyticus

V. alginolyticus is an important opportunistic pathogen which causes vibriosis in aquatic animals. AcfA, as an accessory colonization factor, is hypothesized to be involved in the pathogenesis of infection. In this study, a mutant strain with an in-frame deletion removed nucleotides 86 to 561 of the acfA gene was constructed to reveal the role of AcfA in the physiology and virulence from V. alginolyticus. An acfA mutant showed a similar growth level, an obvious decrease in swarming motility and the activity of ECPase, a higher LD₅₀ value by intraperitoneal injection of grouper fish compared to that of the wild-type. Furthermore, the deletion of acfA could enhance the level of biofilm formation and suppress the polar flagellum forming. The comparative proteomic analysis demonstrated the deletion mutation of acfA could up-regulate the expression of 4 proteins of p4alcd, deoD, phb and DctP, and down-regulate the expression of 8 proteins of Clp, hpV36980, ABCtp, pepD, arA, aggp, fla and ompA compared to that of the wild-type. The analysis of RT-qPCR showed the mRNA levels of DctP and deoD were significantly induced, and the mRNA levels of pepD, arA, fla and ompA were significantly reduced in acfA mutant compared with the wild-type. The results suggest that acfA may contribute to the overall success in the pathogenesis of V. alginolyticus by regulating the expression of some relevant genes.

Role of the toxR Gene from Fish Pathogen Vibiro alginolyticus in the Physiology and Virulence

A mutant strain of Vibiro alginolyticus with an in-frame deletion of the toxR gene was constructed to reveal the role of ToxR in the physiology and virulence of V. alginolyticus. The statistical analysis showed no significant difference in the growth ability, swarming motility, activity of extracellular protease and the virulence by injection (the value of LD₅₀) between the wild-type and the toxR mutant. However, the deletion of toxR could decrease the level of biofilm formation. The comparative proteomic analysis demonstrated the deletion mutation of toxR could up-regulate the expression of glutamine synthetase and levansucrase, and down-regulate the expression of 10 proteins such as OmpU, DnaK, etc. These results suggest that ToxR may be involved in the early stages of infection by influencing colonization of the bacteria on the surface of the intestine through enhancing the biofilm information of V. alginolyticus via modulating the expression of glutamine synthetize, levansucrase and OmpU.

The RNA Chaperone Hfq Is Involved in Colony Morphology, Nutrient Utilization and Oxidative and Envelope Stress Response in Vibrio alginolyticus

Hfq is a global regulator that is involved in environmental adaptation of bacteria and in pathogenicity. To gain insight into the role of Hfq in Vibrio alginolyticus, an hfq deletion mutant was constructed in V. alginolyticus ZJ-T strain and phenotypically characterized. Deletion of hfq led to an alteration of colony morphology and reduced extracellular polysaccharide production, a general impairment of growth in both rich medium and minimal media with different carbon sources or amino acids, enhanced sensitivity to oxidative stress and to several antibiotics. Furthermore, a differential transcriptomic analysis showed significant changes of transcript abundance for 306 protein coding genes, with 179 genes being up regulated and 127 down-regulated. Several of these changes could be related to the observed phenotypes of the mutant. Transcriptomic data also provided evidence for the induction of the extracytoplasmic stress response in absence of Hfq. Altogether, these findings point to broad regulatory functions for Hfq in V. alginolyticus cells, likely to underlie an important role in pathogenicity.

An Innovative Method for Rapid Identification and Detection of Vibrio alginolyticus in Different Infection Models

Vibrio alginolyticus is one of the most common pathogenic marine Vibrio species, and has been found to cause serious seafood-poisoning or fatal extra-intestinal infections in humans, such as necrotizing soft-tissue infections, bacteremia, septic shock, and multiple organ failures. Delayed accurate diagnosis and treatment of most Vibrio infections usually result to high mortality rates. The objective of this study was to establish a rapid diagnostic method to detect and identify the presence of V. alginolyticus in different samples, so as to facilitate timely treatment. The widely employed conventional methods for detection of V. alginolyticus include biochemical identification and a variety of PCR methods. The former is of low specificity and time-consuming (2–3 days), while the latter has improved accuracy and processing time. Despite such advancements, these methods are still complicated, time-consuming, expensive, require expertise and advanced laboratory systems, and are not optimal for field use. With the goal of providing a simple and efficient way to detect V. alginolyticus, we established a rapid diagnostic method based on loop-mediated Isothermal amplification (LAMP) technology that is feasible to use in both experimental and field environments. Three primer pairs targeting the toxR gene of V. alginolyticus were designed, and amplification was carried out in an ESE tube scanner and Real-Time PCR device. We successfully identified 93 V. alginolyticus strains from a total of 105 different bacterial isolates and confirmed their identity by 16s rDNA sequencing. We also applied this method on infected mouse blood and contaminated scallop samples, and accurate results were both easily and rapidly (20–60 min) obtained. Therefore, the RT-LAMP assay we developed can be conveniently used to detect the presence of V. alginolyticus in different samples. Furthermore, this method will also fulfill the gap for real-time screening of V. alginolyticus infections especially while on field.

Biofilm Formation and Detachment in Gram-Negative Pathogens Is Modulated by Select Bile Acids

Biofilms are a ubiquitous feature of microbial community structure in both natural and host environments; they enhance transmission and infectivity of pathogens and provide protection from human defense mechanisms and antibiotics. However, few natural products are known that impact biofilm formation or persistence for either environmental or pathogenic bacteria. Using the combination of a novel natural products library from the fish microbiome and an image-based screen for biofilm inhibition, we describe the identification of taurine-conjugated bile acids as inhibitors of biofilm formation against both Vibrio cholerae and Pseudomonas aeruginosa. Taurocholic acid (1) was isolated from the fermentation broth of the fish microbiome-derived strain of Rhodococcus erythropolis and identified using standard NMR and MS methods. Screening of the twelve predominant human steroidal bile acid components revealed that a subset of these compounds can inhibit biofilm formation, induce detachment of preformed biofilms under static conditions, and that these compounds display distinct structure-activity relationships against V. cholerae and P. aeruginosa. Our findings highlight the significance of distinct bile acid components in the regulation of biofilm formation and dispersion in two different clinically relevant bacterial pathogens, and suggest that the bile acids, which are endogenous mammalian metabolites used to solubilize dietary fats, may also play a role in maintaining host health against bacterial infection.

Characterization of blooming algae and bloom-associated changes in the water quality parameters of traditional pokkali cum prawn fields along the South West coast of India

In tropical shrimp farms, especially in traditional pokkali shrimp ponds, poor water quality management can lead to serious threats like heavy algal blooms and frequent disease outbreaks. An investigation regarding the frequent disease outbreaks in selected pokkali shrimp pond adjoining the Cochin backwaters has been conducted. Water quality parameters were analyzed during the study period. Results indicate a considerable fluctuation in the concentration of dissolved oxygen (DO) in the study in the shrimp farm where the algal bloom was observed. Poor water exchange along with nutrient loading from adjacent housing areas resulted in heavy algal bloom in the pond which led to hypoxic conditions in early morning and supersaturation of DO in the afternoon. It also led to considerably high alkaline pH. High levels of total ammonia nitrogen (TAN) were recorded in the sampling sites. Heavy algal bloom was observed throughout the study period in the selected pond especially in the summer. Anabaenopsis elenkinii and Merismopedia elagans which were the dominant species from the culture pond caused the bloom. Characterization and percentage distribution of bacteria present in the water and those associated with the algal blooms were carried out. The algae were found to support greater diversity of bacteria when compared to water. Pathogenic species like Aeromonas hydrophila and Vibrio parahaemolyticus were encountered in the study. Experimental challenge studies using Artemia as a model showed that the V. parahaemolyticus isolates were highly pathogenic. Hence, this study reveals how algal growth supports opportunistic pathogens in great diverse in a shrimp pond and causes frequent disease outbreaks under favourable conditions.

Fitness factors in vibrios: a mini-review

Vibrios are Gram-negative curved bacilli that occur naturally in marine, estuarine, and freshwater systems. Some species include human and animal pathogens, and some vibrios are necessary for natural systems, including the carbon cycle and osmoregulation. Countless in vivo and in vitro studies have examined the interactions between vibrios and their environment, including molecules, cells, whole animals, and abiotic substrates. Many studies have characterized virulence factors, attachment factors, regulatory factors, and antimicrobial resistance factors, and most of these factors impact the organism's fitness regardless of its external environment. This review aims to identify common attributes among factors that increase fitness in various environments, regardless of whether the environment is an oyster, a rabbit, a flask of immortalized mammalian cells, or a planktonic chitin particle. This review aims to summarize findings published thus far to encapsulate some of the basic similarities among the many vibrio fitness factors and how they frame our understanding of vibrio ecology. Factors representing these similarities include hemolysins, capsular polysaccharides, flagella, proteases, attachment factors, type III secretion systems, chitin binding proteins, iron acquisition systems, and colonization factors.