Isolation of putative probionts from cod rearing environment

Role is in the eye of the beholder-the multiple functions of the antibacterial compound tropodithietic acid produced by marine Rhodobacteraceae

Many microbial secondary metabolites have been studied for decades primarily because of their antimicrobial properties. However, several of these metabolites also possess nonantimicrobial functions, both influencing the physiology of the producer and their ecological neighbors. An example of a versatile bacterial secondary metabolite with multiple functions is the tropone derivative tropodithietic acid (TDA). TDA is a broad-spectrum antimicrobial compound produced by several members of the Rhodobacteraceae family, a major marine bacterial lineage, within the genera Phaeobacter, Tritonibacter, and Pseudovibrio. The production of TDA is governed by the mode of growth and influenced by the availability of nutrient sources. The antibacterial effect of TDA is caused by disruption of the proton motive force of target microorganisms and, potentially, by its iron-chelating properties. TDA also acts as a signaling molecule, affecting gene expression in other bacteria, and altering phenotypic traits such as motility, biofilm formation, and antibiotic production in the producer. In microbial communities, TDA-producing bacteria cause a reduction of the relative abundance of closely related species and some fast-growing heterotrophic bacteria. Here, we summarize the current understanding of the chemical ecology of TDA, including the environmental niches of TDA-producing bacteria, and the molecular mechanisms governing the function and regulation of TDA.

Quorum sensing in Aliivibrio wodanis 06/09/139 and its role in controlling various phenotypic traits

Background

Quorum Sensing (QS) is a cell-to-cell communication system that bacteria utilize to adapt to the external environment by synthesizing and responding to signalling molecules called autoinducers. The psychrotrophic bacterium Aliivibrio wodanis 06/09/139, originally isolated from a winter ulcer of a reared Atlantic salmon, produces the autoinducer N-3-hydroxy-decanoyl-homoserine-lactone (3OHC10-HSL) and encodes the QS systems AinS/R and LuxS/PQ, and the master regulator LitR. However, the role of QS in this bacterium has not been investigated yet.

Results

In the present work we show that 3OHC10-HSL production is cell density and temperature-dependent in A. wodanis 06/09/139 with the highest production occurring at a low temperature (6 °C). Gene inactivation demonstrates that AinS is responsible for 3OHC10-HSL production and positively regulated by LitR. Inactivation of ainS and litR further show that QS is involved in the regulation of growth, motility, hemolysis, protease activity and siderophore production. Of these QS regulated activities, only the protease activity was found to be independent of LitR. Lastly, supernatants harvested from the wild type and the ΔainS and ΔlitR mutants at high cell densities show that inactivation of QS leads to a decreased cytopathogenic effect (CPE) in a cell culture assay, and strongest attenuation of the CPE was observed with supernatants harvested from the ΔlitR mutant.

Conclusion

A. wodanis 06/09/139 use QS to regulate a number of activities that may prove important for host colonization or interactions. The temperature of 6 °C that is in the temperature range at which winter ulcer occurs, plays a role in AHL production and development of CPE on a Chinook Salmon Embryo (CHSE) cell line.

Bioencapsulation and Colonization Characteristics of Lactococcus lactis subsp. lactis CF4MRS in Artemia franciscana: a Biological Approach for the Control of Edwardsiellosis in Larviculture

Predominance of beneficial bacteria helps to establish a healthy microbiota in fish gastrointestinal system and thus to reduce emerging pathogen. In this study, the colonization efficacy of Lactococcus lactis subsp. lactis CF4MRS in Artemia franciscana and its potential as a probiotic in suppressing Edwardsiella sp. infection were investigated in vivo. The colonization extent of the bioencapsulated L. lactis was established through visualization of gfp gene-transformed L. lactis in A. franciscana. Here, we demonstrate that when A. franciscana is administrated with L. lactis at 10⁸ CFU mL^-1 for 8 h, the highest relative percentage of survival (RPS = 50.0) is observed after inoculation with Edwardsiella sp. The total counts of L. lactis entrapped in Artemia were the highest (ranged from 3.2 to 5.1 × 10⁸ CFU mL^-1), when 10⁸-10⁹ CFU mL^-1 of L. lactis was used as starting inoculum, with the bioencapsulation performed within 8-24 h. Fluorescent microscopy showed gfp-transformed L. lactis colonized the external trunk surfaces, mid-gut and locomotion antennules of the A. franciscana nauplii. These illustrations elucidate the efficiency of colonization of L. lactis in the gastrointestinal tract and on the body surfaces of Artemia. In conclusion, L. lactis subsp. lactis CF4MRS shows a good efficacy of colonization in Artemia and has the potential for biocontrol/probiotic activity against Edwardsiella sp. infection.

Isolation of TDA-producing Phaeobacter strains from sea bass larval rearing units and their probiotic effect against pathogenic Vibrio spp. in Artemia cultures

Fish-pathogenic Vibrio can cause large-scale crashes in marine larval rearing units and, since the use of antibiotics can result in bacterial antibiotic resistance, new strategies for disease prevention are needed. Roseobacter-clade bacteria from turbot larval rearing facilities can antagonize Vibrio anguillarum and reduce mortality in V. anguillarum-infected cod and turbot larvae. In this study, it was demonstrated that antagonistic Roseobacter-clade bacteria could be isolated from sea bass larval rearing units. In addition, it was shown that they not only antagonized V. anguillarum but also V. harveyi, which is the major bacterial pathogen in crustaceans and Mediterranean sea bass larvae cultures. Concomitantly, they significantly improved survival of V. harveyi-infected brine shrimp. 16S rRNA gene sequence homology identified the antagonists as Phaeobacter sp., and in silico DNA-DNA hybridization indicated that they could belong to a new species. The genomes contained genes involved in synthesis of the antibacterial compound tropodithietic acid (TDA), and its production was confirmed by UHPLC-TOFMS. The new Phaeobacter colonized live feed (Artemia) cultures and reduced Vibrio counts significantly, since they reached only 10(4)CFUmL(-1), as opposed to 10(8)CFUmL(-1) in non-Phaeobacter treated controls. Survival of V. anguillarum-challenged Artemia nauplii was enhanced by the presence of wild type Phaeobacter compared to challenged control cultures (89±1.0% vs 8±3.2%). In conclusion, TDA-producing Phaeobacter isolated from Mediterranean marine larviculture are promising probiotic bacteria against pathogenic Vibrio in crustacean live-feed cultures for marine fish larvae.

Vibrio lentus protects gnotobiotic sea bass (Dicentrarchus labrax L.) larvae against challenge with Vibrio harveyi

Due to the mounting awareness of the risks associated with the use of antibiotics in aquaculture, treatment with probiotics has recently emerged as the preferred environmental-friendly prophylactic approach in marine larviculture. However, the presence of unknown and variable microbiota in fish larvae makes it impossible to disentangle the efficacy of treatment with probiotics. In this respect, the recent development of a germ-free culture model for European sea bass (Dicentrarchus labrax L.) larvae opened the door for more controlled studies on the use of probiotics. In the present study, 206 bacterial isolates, retrieved from sea bass larvae and adults, were screened in vitro for haemolytic activity, bile tolerance and antagonistic activity against six sea bass pathogens. Subsequently, the harmlessness and the protective effect of the putative probiotic candidates against the sea bass pathogen Vibrio harveyi were evaluated in vivo adopting the previously developed germ-free sea bass larval model. An equivalence trial clearly showed that no harmful effect on larval survival was elicited by all three selected probiotic candidates: Bacillus sp. LT3, Vibrio lentus and Vibrio proteolyticus. Survival of Vibrio harveyi challenged larvae treated with V. lentus was superior in comparison with the untreated challenged group, whereas this was not the case for the larvae supplemented with Bacillus sp. LT3 and V. proteolyticus. In this respect, our results unmistakably revealed the protective effect of V. lentus against vibriosis caused by V. harveyi in gnotobiotic sea bass larvae, rendering this study the first in its kind.

Probiotics, immunostimulants, plant products and oral vaccines, and their role as feed supplements in the control of bacterial fish diseases

There is a rapidly increasing literature pointing to the success of probiotics, immunostimulants, plant products and oral vaccines in immunomodulation, namely stimulation of the innate, cellular and/or humoral immune response, and the control of bacterial fish diseases. Probiotics are regarded as live micro-organisms administered orally and leading to health benefits. However, in contrast with the use in terrestrial animals, a diverse range of micro-organisms have been evaluated in aquaculture with the mode of action often reflecting immunomodulation. Moreover, the need for living cells has been questioned. Also, key subcellular components, including lipopolysaccharides, have been attributed to the beneficial effect in fish. Here, there is a link with immunostimulants, which may also be administered orally. Furthermore, numerous plant products have been reported to have health benefits, namely protection against disease for which stimulation of some immune parameters has been reported. Oral vaccines confer protection against some diseases, although the mode of action is usually linked to humoral rather than the innate and cellular immune responses. This review explores the relationship between probiotics, immunostimulants, plant products and oral vaccines.

Yersiniosis in Atlantic cod, Gadus morhua (L.), characterization of the infective strain and host reactions

A disease outbreak in farmed Atlantic cod caused by Yersinia ruckeri is reported. Mortality started following vaccination of cod reared in two tanks (A and B). The accumulated mortality reached 1.9% in A and 4.8% in B in the following 30 days when treatment with oxytetracycline was applied. Biochemical and molecular analysis of Y. ruckeri isolates from the cod and other fish species from fresh and marine waters in Iceland revealed a high salinity-tolerant subgroup of Y. ruckeri serotype O1. Infected fish showed clinical signs comparable with those of Y. ruckeri -infected salmonids, with the exception of granuloma formations in infected cod tissues, which is a known response of cod to bacterial infections. Immunohistological examination showed Y. ruckeri antigens in the core of granulomas and the involvement of immune parameters that indicates a strong association between complement and lysozyme killing of bacteria. Experimental infection of cod with a cod isolate induced disease, and the calculated LD50 was 1.7 × 10(4) CFU per fish. The results suggest that yersiniosis can be spread between populations of freshwater and marine fish. Treatment of infected cod with antibiotic did not eliminate the infection, which can be explained by the immune response of cod producing prolonged granulomatous infection.

Probiotics-pathogen interactions elicit differential regulation of cutaneous immune responses in epidermal cells of Atlantic cod Gadus morhua

Little is known on the cutaneous immune responses during probiotics-pathogen interactions in fish. Thus, this study employed Atlantic cod primary epidermal (EP) cell cultures as a model to understand this interaction. The probiotics-pathogen interactions in the EP cell cultures were elucidated using Vibrio anguillarum 2133 (VA) as the pathogen and two host-derived bacteria (GP21 and GP12) as the probiotics. There was a regional size difference on the EP cells; i.e., EP cells from the dorsal region were significantly larger than the EP cells at the ventral side. VA significantly decreased viability of EP cells. In the presence of probiotics, this inhibition was mitigated. The probiotics reduced VA-induced cellular apoptosis and the probiotics-pathogen interactions influenced cellular myeloperoxidase activity during the latter stage of co-incubation. The probiotics-pathogen interactions triggered differential regulation of immune-related genes and the effects of the interaction were dependent on the region where the cells were isolated and the length of the co-incubation period. In most cases, the presence of probiotics alone showed no significant change on the mRNA level of immune genes in the EP cells but triggered immunostimulatory activity when incubated with VA. This study showed that the virulence of VA in EP cells could be modulated by host-derived probiotics and the immunomodulatory characteristics of the two candidate probionts advanced their immune-related probiotic potential.

Effects of dietary supplementation of citrus by-products fermented with a probiotic microbe on growth performance, innate immunity and disease resistance against Edwardsiella tarda in juvenile olive flounder, Paralichthys olivaceus (Temminck & Schlegel)

Two consecutive studies were conducted to evaluate the dietary supplementation of citrus by-products (CB) fermented with probiotic bacteria on growth performance, feed utilization, innate immune responses and disease resistance of juvenile olive flounder. In Experiment I, five diets were formulated to contain 0% (control) or 3% four different CB fermented with Bacillus subtilis (BS), Enterococcus faecium (EF), Lactobacillus rhamnosus (LR) and L. plantarum (LP) (designated as CON, CBF-BS, CBF-EF, CBF-LR and CBF-LP, respectively). During 10 weeks of a feeding trial, growth performance and feed efficiency were not significantly different among all the fish groups. However, fish fed CBF containing diets had significantly higher survivals than the CON group. Disease resistance of fish against Edwardsiella tarda was increased by the fermentation of CB. In Experiment II, we chose the BS as a promising probiotic and formulated five diets to contain 0%, 2%, 4%, 6% and 8% CBF-BS. Growth performance was not significantly affected by the CBF-BS supplementation during 6 weeks of a feeding trial. Innate immunity of fish was significantly enhanced by CBF-BS supplementation. Myeloperoxidase and lysozyme activities were increased in a dose-dependent manner by dietary CBF-BS inclusions. In a consecutive challenge test against E. tarda, an increased disease resistance was found by CBF-BS supplementation. These studies indicate that the fermentation process of CB with probiotic has beneficial effects on innate immunity and thereby increases disease resistance of olive flounder against E. tarda. Bacillus subtilis can be used as a promising probiotic microbe for by-product fermentation in fish feeds.

Virulence properties of Moritella viscosa extracellular products

Moritella viscosa is the causative agent of winter ulcer disease of marine fish. Knowledge of its pathogenicity is limited and there are no reports comparing the virulence properties of a collection of bacterial isolates. The in vivo and in vitro virulence of the extracellular products (ECP) of 22 M. viscosa isolates was screened. Two non-virulent Canadian isolates and a Norwegian isolate with reduced virulence produced non-lethal ECP. Correlation was obtained between cytotoxin and haemolysin production of M. viscosa. Isolates from salmon produced ECP with lower cytotoxic and haemolytic activities than ECP of isolates originating from other hosts. Correlation was not found between lethality of ECPs in salmon and cytotoxic or haemolytic activities. All isolates secreted esterases and a metallopeptidase (MvP1), degraded starch and produced siderophores. Variable levels of ECP protein concentration, different enzymatic activities and siderophore production could not explain differences in virulence. The results show that virulent M. viscosa isolates secrete a lethal toxic factor of unknown nature and that cytotoxin production may reflect host adaptation. Cell-culture models may not be optimal for determining the virulence of M. viscosa, as no association between cytotoxicity and bacterial virulence was obtained. Non-virulent strains may be useful in future research on M. viscosa virulence, as construction of mutants has not been successful.

Immunological control of fish diseases

All metazoans possess innate immune defence system whereas parameters of the adaptive immune system make their first appearance in the gnathostomata, the jawed vertebrates. Fish are therefore the first animal phyla to possess both an innate and adaptive immune system making them very interesting as regards developmental studies of the immune system. The massive increase in aquaculture in recent decades has also put greater emphasis on studies of the fish immune system and defence against diseases commonly associated with intensive fish rearing. Some of the main components of the innate and adaptive immune system of fish are described. The innate parameters are at the forefront of immune defence in fish and are a crucial factor in disease resistance. The adaptive response of fish is commonly delayed but is essential for lasting immunity and a key factor in successful vaccination. Some of the inherent and external factors that can manipulate the immune system of fish are discussed, the main fish diseases are listed and the pathogenicity and host defence discussed. The main prophylactic measures are covered, including vaccination, probiotics and immunostimulation. A key element in the immunological control of fish diseases is the great variation in disease susceptibility and immune defence of different fish species, a reflection of the extended time the present day teleosts have been separated in evolution. Future research will probably make use of molecular and proteomic tools both to study important elements in immune defence and prophylactic measures and to assist with breeding programmes for disease resistance.

Microbiota of Atlantic cod (Gadus morhua L.) rearing systems at pre- and posthatch stages and the effect of different treatments

AIMS

To study the effect of ova disinfection, antibiotic and microbial treatments on the dominant cultivable cod rearing microbiota at pre- and posthatch stages, determining some virulence-related phenotypic traits among bacterial isolates and their relation to larval survival.

METHODS AND RESULTS

Sampling of rearing systems (rearing water, ova, larvae, feeds and supplement) for analysis of cultivable microbiota took place at early stages in 2004 and 2005. Cultivation, phenotypic and genotypic (16S rRNA gene) analyses were performed. The production of putative virulence factors (PVFs), including haemolysin, siderophores and quorum-sensing signals, by bacterial isolates was investigated and related to larval survival. The study was performed during two spawning seasons, evaluating current hatchery practices (ova disinfection and antibiotic treatment of unhealthy larvae) and specific putative probiotics applied to ova and larvae or rotifers. A diversified microbiota (75 operational taxonomic units, OTUs) was observed in cod rearing systems influenced by the feeds and treatments, with prevailing γ-Proteobacteria prior to hatching towards a multiphyla microbiota posthatch. Phenotypic tests demonstrated the heterogeneity within some OTUs. Multivariate analysis of survival data in larval silos and the corresponding larval microbiota was used to divide the genotypic groups into beneficial/harmless and detrimental/opportunistic clusters. PVFs were common among the proposed detrimental/opportunistic OTUs.

CONCLUSIONS

The results clearly demonstrate the influence of exogeneous feeding and treatments on larval gastrointestinal microbiota and the role of bacteria in larval survival.

SIGNIFICANCE AND IMPACT OF THE STUDY

Increased understanding of the microbiota in rearing systems may contribute to successful implementation of microbial management in cod aquaculture.

Selection of candidate probionts by two different screening strategies from Atlantic cod (Gadus morhua L.) larvae

Two primary selection criteria were used to collect a pool of nearly 500 candidate probiotic bacteria from Atlantic cod (Gadus morhua L.) larvae, i.e. the dominant intestinal bacterial flora and isolates with antagonistic activity against Vibrio anguillarum. Bacteria were isolated from cod larvae from five rearing groups with variable rearing technologies. The bacteria were brought to pure culture and characterized phenotypically. Based on properties such as uniqueness, dominance and fermentative ability, a selection of approximately 10% of the isolates were chosen from the initial pool of bacteria to reduce the number of candidates. These 55 isolates were characterized further in vitro regarding antagonism, adhesion to mucus, growth in mucus, production of extracellular enzymes, fish bile resistance and haemolytic properties. Based on the results of the in vitro tests, the number of probiotic candidates was further reduced to seven isolates. To evaluate the probiotic potential and to assure that the seven isolates were not harmful to the host, yolk sac larvae of cod were exposed to the isolates in a small-scale in vivo experiment. The in vivo experiment excluded two of the candidate bacteria due to increased mortality of cod larvae, whereas three isolates from the dominant (Vibrio and two different strains of Microbacterium) and two from the antagonistic (Ruegeria and Pseudoalteromonas) group improved the survival of larvae compared to the positive control. Thus, a combination of the two screening methods was suited for making multistrain probiotics with complementary modes of action.