Effectiveness of probiotic Phaeobacter bacteria grown in biofilters against Vibrio anguillarum infections in the rearing of Turbot (Psetta maxima) larvae

Microbial Interactions in Rearing Systems for Marine Fish Larvae

This article reviews the scientific literature discussing the microbial interactions between water microbiota, live food microbiota, fish larvae immune system and gut microbiota, and biofilm microbial communities in rearing systems for marine fish larvae. Fish gut microbiota is the first line of defense against opportunistic pathogens, and marine fish larvae are vulnerable to high mortalities during the first weeks after hatching. The bacterial colonization of fish larvae is a dynamic process influenced by environmental and host-related factors. The bacteria transferred to larvae from the eggs can influence the composition of the gut microbiota in the early stages of fish. Fish larvae ingest free-living microorganisms present in the water, as marine fish larvae drink water for osmoregulation. In marine aquaculture systems, the conventional feeding-rearing protocol consists of zooplankton (rotifers, Artemia, and copepods). These live food organisms are filter-feeders. Once transferred to a new environment, they quickly adopt the microflora of the surrounding water. So, the water microbiota is similar to the microbiota of the live food at the time of ingestion of live food by the larvae. In aquaculture rearing systems, bacterial biofilms may harbor opportunistic pathogenic bacteria and serve as a reservoir for those microbes, which may colonize the water column. The methods applied for the study of fish larvae microbiota were reviewed.

Biosynthesis enhancement of tropodithietic acid (TDA) antibacterial compound through biofilm formation by marine bacteria Phaeobacter inhibens on micro-structured polymer surfaces

Although aquaculture is a major player in current and future food production, the routine use of antibiotics provides ample ground for development of antibiotic resistance. An alternative route to disease control is the use of probiotic bacteria such as the marine bacteria Phaeobacter inhibens which produces tropodithietic acid (TDA) that inhibit pathogens without affecting the fish. Improving conditions for the formation of biofilm and TDA-synthesis is a promising avenue for biocontrol in aquaculture. In this study, the biosynthesis of TDA by Phaeobacter inhibens grown on micro-structured polymeric surfaces in micro-fluidic flow-cells is investigated. The formation of biofilms on three surface topographies; hexagonal micro-pit-arrays, hexagonal micro-pillar-arrays, and planar references is investigated. The biomass on these surfaces is measured by a non-invasive confocal microscopy 3D imaging technique, and the corresponding TDA production is monitored by liquid chromatography mass spectrometry (LC-MS) in samples collected from the outlets of the microfluidic channels. Although all surfaces support growth of P. inhibens, biomass appears to be decoupled from total TDA biosynthesis as the micro-pit-arrays generate the largest biomass while the micro-pillar-arrays produce significantly higher amounts of TDA. The findings highlight the potential for optimized micro-structured surfaces to maintain biofilms of probiotic bacteria for sustainable aquacultures.

Co-Culturing Microalgae with Roseobacter Clade Bacteria as a Strategy for Vibrionaceae Control in Microalgae-Enriched Artemia

Bacterial communities associated with fish larvae are highly influenced by the microbiota of live prey used as feed (rotifers or Artemia), generally dominated by bacterial strains with a low degree of specialization and high growth rates, (e.g., Vibrionaceae), which can be detrimental to larvae. Co-cultivation of microalgae used in the enrichment of Artemia (e.g., Phaeodactylum tricornutum, or Chlorella minutissima) with Vibrio-antagonistic probiotics belonging to the Roseobacter clade bacteria (e.g., Phaeobacter spp. or Ruegeria spp.) was studied. The introduction of the probiotics did not affect microalgae growth or significantly modify the composition of bacterial communities associated with both microalgae, as revealed by DGGE analysis. The inoculation of P. tricornutum with Ruegeria ALR6 allowed the maintenance of the probiotic in the scale-up of the microalgae cultures, both in axenic and non-axenic conditions. Using Ruegeria-inoculated P. tricornutum cultures in the enrichment of Artemia reduced the total Vibrionaceae count in Artemia by 2 Log units, therefore preventing the introduction of opportunistic or pathogenic bacteria to fish larvae fed with them.

Current Status of Probiotics in European Sea Bass Aquaculture as One Important Mediterranean and Atlantic Commercial Species: A Review

European sea bass production has increased in recent decades. This increase is associated with an annually rising demand for sea bass, which encourages the aquaculture industries to increase their production to meet that demand. However, this intensification has repercussions on the animals, causing stress that is usually accompanied by dysbiosis, low feed-conversion rates, and immunodepression, among other factors. Therefore, the appearance of pathogenic diseases is common in these industries after immunodepression. Seeking to enhance animal welfare, researchers have focused on alternative approaches such as probiotic application. The use of probiotics in European sea bass production is presented as an ecological, safe, and viable alternative in addition to enhancing different host parameters such as growth performance, feed utilization, immunity, disease resistance, and fish survival against different pathogens through inclusion in fish diets through vectors and/or in water columns. Accordingly, the aim of this review is to present recent research findings on the application of probiotics in European sea bass aquaculture and their effect on growth performance, microbial diversity, enzyme production, immunity, disease resistance, and survival in order to help future research.

Biotechnological Valorization of Food Marine Wastes: Microbial Productions on Peptones Obtained from Aquaculture By-Products

Based on a biotechnological strategy, in the present work several peptones are produced from the Alcalase hydrolysis (0.1-0.2% v/w, 56-64 °C, pH 8.27-8.98, 3 h) and thermal processing (105 °C, 60 min) of wastes generated from the industrial processing of turbot, salmon, trout, seabream and seabass. These peptones were included (in the range of 2.6-11 g/L of soluble protein) as main source of organic nitrogen (protein substrates) in low-cost media for the culture of lactic acid bacteria (LAB), marine probiotic bacteria (MPB) and ubiquitous Gram+ bacteria. In most cases, batch fermentations conducted in aquaculture peptone media led to the best growth, metabolic productions and yields. Nevertheless, no significant differences between aquaculture peptones and commercial media were generally observed. Kinetic parameters from a logistic equation and used for cultures modeling were applied with the purpose of comparing the bioproduction outcomes. In economical terms, the validity of the aquaculture peptones as substitutives of the peptones (meat extract, casitone, etc.) from commercial media was also compared. The decreasing of the costs for LAB bioproductions ranged between 3-4 times and the growth costs of MPB and Gram+ bacteria were improved more than 70 and 15 times, respectively, in relation to those found in control commercial media.

Production of Marine Probiotic Bacteria in a Cost-Effective Marine Media Based on Peptones Obtained from Discarded Fish By-Products

The industrial production of marine bacteria with probiotic properties is limited by the excessive cost of the culture media adequate for their growth. The present work aimed to study the suitability of 30 marine media formulated with nitrogen sources (fish peptones) from different fish discards and seawater, for the growth of two marine probiotic bacteria (MPB), namely Phaeobacter sp. and Pseudomonas fluorescens. These fish peptones were produced from several discarded fish and by-products (heads, skins, and whole individuals of megrim, mackerel, gurnard, hake, etc.). In all cultivations, no significant differences were found on cell viability when the microorganism grew on commercial or alternative media. In relation to the biomass production, the growth of Phaeobacter sp. on waste media was commonly similar or a 20% lower than observed in the control cultures. For P. fluorescens, various peptones (skin peptones of pouting and blue whiting) showed even higher productive ability than commercial peptones. An initial economical evaluation revealed that low-cost media reduced until 120 times the cost of production of MPB.

Oyster hemolymph is a complex and dynamic ecosystem hosting bacteria, protists and viruses

BACKGROUND

The impact of the microbiota on host fitness has so far mainly been demonstrated for the bacterial microbiome. We know much less about host-associated protist and viral communities, largely due to technical issues. However, all microorganisms within a microbiome potentially interact with each other as well as with the host and the environment, therefore likely affecting the host health.

RESULTS

We set out to explore how environmental and host factors shape the composition and diversity of bacterial, protist and viral microbial communities in the Pacific oyster hemolymph, both in health and disease. To do so, five oyster families differing in susceptibility to the Pacific oyster mortality syndrome were reared in hatchery and transplanted into a natural environment either before or during a disease outbreak. Using metabarcoding and shotgun metagenomics, we demonstrate that hemolymph can be considered as an ecological niche hosting bacterial, protist and viral communities, each of them shaped by different factors and distinct from the corresponding communities in the surrounding seawater. Overall, we found that hemolymph microbiota is more strongly shaped by the environment than by host genetic background. Co-occurrence network analyses suggest a disruption of the microbial network after transplantation into natural environment during both non-infectious and infectious periods. Whereas we could not identify a common microbial community signature for healthy animals, OsHV-1 μVar virus dominated the hemolymph virome during the disease outbreak, without significant modifications of other microbiota components.

CONCLUSION

Our study shows that oyster hemolymph is a complex ecosystem containing diverse bacteria, protists and viruses, whose composition and dynamics are primarily determined by the environment. However, all of these are also shaped by oyster genetic backgrounds, indicating they indeed interact with the oyster host and are therefore not only of transient character. Although it seems that the three microbiome components respond independently to environmental conditions, better characterization of hemolymph-associated viruses could change this picture.

Homologous Recombination in Core Genomes Facilitates Marine Bacterial Adaptation

Acquisition of ecologically relevant genes is common among ocean bacteria, but whether it has a major impact on genome evolution in marine environments remains unknown. Here, we analyzed the core genomes of 16 phylogenetically diverse and ecologically relevant bacterioplankton lineages, each consisting of up to five genomes varying at the strain level. Statistical approaches identified from each lineage up to ∼50 loci showing anomalously high divergence at synonymous sites, which is best explained by recombination with distantly related organisms. The enriched gene categories in these outlier loci match well with the characteristics previously identified as the key phenotypes of these lineages. Examples are antibiotic synthesis and detoxification in Phaeobacter inhibens, exopolysaccharide production in Alteromonas macleodii, hydrocarbon degradation in Marinobacter hydrocarbonoclasticus, and cold adaptation in Pseudoalteromonas haloplanktis Intriguingly, the outlier loci feature polysaccharide catabolism in Cellulophaga baltica but not in Cellulophaga lytica, consistent with their primary habitat preferences in macroalgae and beach sands, respectively. Likewise, analysis of Prochlorococcus showed that photosynthesis-related genes listed in the outlier loci are found only in the high-light-adapted ecotype and not in the low-light adapted ecotype. These observations strongly suggest that recombination with distant relatives is a key mechanism driving the ecological diversification among marine bacterial lineages.IMPORTANCE Acquisition of new metabolic genes has been known as an important mechanism driving bacterial evolution and adaptation in the ocean, but acquisition of novel alleles of existing genes and its potential ecological role have not been examined. Guided by population genetic theories, our genomic analysis showed that divergent allele acquisition is prevalent in phylogenetically diverse marine bacterial lineages and that the affected loci often encode metabolic functions that underlie the known ecological roles of the lineages under study.

Biofilm formation is not a prerequisite for production of the antibacterial compound tropodithietic acid in Phaeobacter inhibens DSM17395

AIMS

The goal of this study was to investigate if biofilm formation on population level is a physiological requirement for antagonism in Phaeobacter inhibens DSM17395, since the antibiotic compound tropodithietic acid (TDA) is produced by several Roseobacter clade species during growth as multicellular aggregates or biofilms at the air-liquid interface and is induced on single cell level upon attachment.

METHODS AND RESULTS

A mutant library was created by Tn5 transposon insertion and 22 TDA-positive (brown) mutants with decreased biofilm formation or adhesion, and eight TDA-negative (white) mutants with increased biofilm formation or adhesion were selected. None of the selected biofilm-overproducing white mutants showed any antibiotic activity, while all brown mutants with reduced or disabled biofilm formation produced the antibacterial compound. Sequencing analysis indicated that genes that are likely involved in EPS/LPS production, motility and chemotaxis, and redox regulation play a role in biofilm formation and/or adhesion in P. inhibens DSM17395.

CONCLUSIONS

Cell aggregation and biofilm formation are not physiological prerequisites for TDA production.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study contributes to the understanding of TDA production in P. inhibens, which has great potential as a probiotic in marine larviculture.

Toxicity of bioactive and probiotic marine bacteria and their secondary metabolites in Artemia sp. and Caenorhabditis elegans as eukaryotic model organisms

We have previously reported that some strains belonging to the marine Actinobacteria class, the Pseudoalteromonas genus, the Roseobacter clade, and the Photobacteriaceae and Vibrionaceae families produce both antibacterial and antivirulence compounds, and these organisms are interesting from an applied point of view as fish probiotics or as a source of pharmaceutical compounds. The application of either organisms or compounds requires that they do not cause any side effects, such as toxicity in eukaryotic organisms. The purpose of this study was to determine whether these bacteria or their compounds have any toxic side effects in the eukaryotic organisms Artemia sp. and Caenorhabditis elegans. Arthrobacter davidanieli WX-11, Pseudoalteromonas luteoviolacea S4060, P. piscicida S2049, P. rubra S2471, Photobacterium halotolerans S2753, and Vibrio coralliilyticus S2052 were lethal to either or both model eukaryotes. The toxicity of P. luteoviolacea S4060 could be related to the production of the antibacterial compound pentabromopseudilin, while the adverse effect observed in the presence of P. halotolerans S2753 and V. coralliilyticus S2052 could not be explained by the production of holomycin nor andrimid, the respective antibiotic compounds in these organisms. In contrast, the tropodithietic acid (TDA)-producing bacteria Phaeobacter inhibens DSM17395 and Ruegeria mobilis F1926 and TDA itself had no adverse effect on the target organisms. These results reaffirm TDA-producing Roseobacter bacteria as a promising group to be used as probiotics in aquaculture, whereas Actinobacteria, Pseudoalteromonas, Photobacteriaceae, and Vibrionaceae should be used with caution.