Seasonal and Spatial Variations in Bacterial Communities From Tetrodotoxin-Bearing and Non-tetrodotoxin-Bearing Clams

Sudden peak in tetrodotoxin in French oysters during the summer of 2021: Source investigation using microscopy, metabarcoding and droplet digital PCR

Tetrodotoxin (TTX) is a potent neurotoxin causing human intoxications from contaminated seafood worldwide and is of emerging concern in Europe. Shellfish have been shown to contain varying TTX concentrations globally, with concentrations typically higher in Pacific oysters Crassostrea gigas in Europe. Despite many decades of research, the source of TTX remains unknown, with bacterial or algal origins having been suggested. The aim of this study was to identify potential source organisms causing TTX contamination in Pacific oysters in French coastal waters, using three different techniques. Oysters were deployed in cages from April to September 2021 in an estuary where TTX was previously detected. Microscopic analyses of water samples were used to investigate potential microalgal blooms present prior or during the peak in TTX. Differences in the bacterial communities from oyster digestive glands (DG) and remaining flesh were explored using metabarcoding, and lastly, droplet digital PCR assays were developed to investigate the presence of Cephalothrix sp., one European TTX-bearing species in the DG of toxic C. gigas. Oysters analysed by liquid chromatography-tandem mass spectrometry contained quantifiable levels of TTX over a three-week period (24 June-15 July 2021), with concentrations decreasing in the DG from 424 μg/kg for the first detection to 101 μg/kg (equivalent to 74 to 17 μg/kg of total flesh), and trace levels being detected until August 13, 2021. These concentrations are the first report of the European TTX guidance levels being exceeded in French shellfish. Microscopy revealed that some microalgae bloomed during the TTX peak, (e.g., Chaetoceros spp., reaching 40,000 cells/L). Prokaryotic metabarcoding showed increases in abundance of Rubritaleaceae (genus Persicirhabdus) and Neolyngbya, before and during the TTX peak. Both phyla have previously been described as possible TTX-producers and should be investigated further. Droplet digital PCR analyses were negative for the targeted TTX-bearing genus Cephalothrix.

A Hotspot of TTX Contamination in the Adriatic Sea: Study on the Origin and Causative Factors

Tetrodotoxins (TTXs), the pufferfish venom traditionally associated with Indo-Pacific area, has been reported during last decades in ever wider range of marine organisms and ever more geographical areas, including shellfish in Europe. Wild mussels (Mytilus galloprovincialis) grown in the Marche Region (N Adriatic Sea, Italy) were shown to be prone to TTX contamination during the warm season, with a suspected role of Vibrio alginolyticus characterized by non-ribosomal peptide synthetase (NRPS) and polyketide synthase (PKS)-encoding genes. This work aimed to deepen the knowledge about the toxin's origin and the way through which it accumulates in mussels. A two-year study (spring-summer 2020-2021) confirmed the recurrent presence of TTX (11-68 µg kg^-1) in the official monitored natural mussel beds of the Conero Riviera. During 2021, a supplementary nonroutine monitoring of a natural mussel bed in the same area was carried out weekly from June until August for TTXs and/or the presence of V. alginolyticus. Biotic (mussels, mesozooplankton, worms and phytoplankton); abiotic (water and sediment) matrices and phytoplankton assemblage characterizations were studied. Mussels showed relevant TTX contamination levels (9-296 µg kg^-1) with extremely rapid TTX accumulation/depletion rates. The toxin presence in phytoplankton and its distribution in the different mussel tissues supports its possible exogenous origin. The V. alginolyticus count trend overlaps that of TTX contamination in mussels, and similar trends were reported also for some phytoplankton species. The role of V. alginolyticus carrying NRPS or PKS genes as a possible TTX source and of phytoplankton as a "potential vector" should therefore be further investigated.

An Updated Review of Tetrodotoxin and Its Peculiarities

Tetrodotoxin (TTX) is a crystalline, weakly basic, colorless organic substance and is one of the most potent marine toxins known. Although TTX was first isolated from pufferfish, it has been found in numerous other marine organisms and a few terrestrial species. Moreover, tetrodotoxication is still an important health problem today, as TTX has no known antidote. TTX poisonings were most commonly reported from Japan, Thailand, and China, but today the risk of TTX poisoning is spreading around the world. Recent studies have shown that TTX-containing fish are being found in other regions of the Pacific and in the Indian Ocean, as well as the Mediterranean Sea. This review aims to summarize pertinent information available to date on the structure, origin, distribution, mechanism of action of TTX and analytical methods used for the detection of TTX, as well as on TTX-containing organisms, symptoms of TTX poisoning, and incidence worldwide.

Tetrodotoxin in live bivalve mollusks from Europe: Is it to be considered an emerging concern for food safety?

Tetrodotoxins (TTXs) are a group of potent neurotoxins named after the Tetraodontidae fish family (pufferfish). TTXs have been reported in several animal taxa, both terrestrial and marine. The ingestion of TTX-contaminated flesh can cause serious neurotoxic symptomatology and can eventually lead to death. Traditionally, TTXs have been associated with Asian countries, in particular with pufferfish consumption. However, they have also been reported in bivalve mollusks farmed in the Pacific area and, recently, in European seas. In Europe, different countries have reported TTXs, especially those bordering the Mediterranean Sea. As a consequence, in 2017 the European Food Safety Authority (EFSA) released an opinion with reference to TTX present in marine gastropods and bivalves, proposing a safety limit of 44 µg/kg TTXs in shellfish meat, below which no adverse effects should be observed in humans. Nevertheless, this limit has been exceeded on many occasions in European shellfish and, while for bivalves there have been no registered human intoxications, that is not the case for marine gastropods. However, TTXs have not yet been included in the list of marine biotoxins officially monitored in live bivalve mollusks within the European Union (EU). Thus, the aims of this manuscript are to discuss the increasing occurrence of TTXs in live bivalve mollusks from European sea waters, to acknowledge the still ongoing knowledge gaps that should be covered and to stimulate constructive debate on the eventuality of adopting a shared regulatory context, at least in the EU, for monitoring and managing this potential threat to food safety.

Tetrodotoxins in French Bivalve Mollusks-Analytical Methodology, Environmental Dynamics and Screening of Bacterial Strain Collections

Tetrodotoxins (TTXs) are potentially lethal paralytic toxins that have been identified in European shellfish over recent years. Risk assessment has suggested comparatively low levels (44 µg TTX-equivalent/kg) but stresses the lack of data on occurrence. Both bacteria and dinoflagellates were suggested as possible biogenic sources, either from an endogenous or exogenous origin. We thus investigated TTXs in (i) 98 shellfish samples and (ii) 122 bacterial strains, isolated from French environments. We optimized a method based on mass spectrometry, using a single extraction step followed by ultrafiltration without Solid Phase Extraction and matrix-matched calibration for both shellfish and bacterial matrix. Limits of detection and quantification were 6.3 and 12.5 µg/kg for shellfish and 5.0 and 10 µg/kg for bacterial matrix, respectively. Even though bacterial matrix resulted in signal enhancement, no TTX analog was detected in any strain. Bivalves (either Crassostrea gigas or Ruditapes philippinarum) were surveyed in six French production areas over 2.5-3 month periods (2018-2019). Concentrations of TTX ranged from 'not detected' to a maximum of 32 µg/kg (Bay of Brest, 17 June 2019), with events lasting 2 weeks at maximum. While these results are in line with previous studies, they provide new data of TTX occurrence and confirm that the link between bacteria, bivalves and TTX is complex.

A Microencapsulation Method for Delivering Tetrodotoxin to Bivalves to Investigate Uptake and Accumulation

Most marine biotoxins are produced by microalgae. The neurotoxin tetrodotoxin (TTX) has been reported in many seafood species worldwide but its source is unknown, making accumulation and depuration studies in shellfish difficult. Tetrodotoxin is a water-soluble toxin and cannot be directly ingested by shellfish. In the present study, a method was developed which involved binding TTX to solid particles of humic acid and encapsulating them in agar-gelatin capsules. A controlled quantity of TTX-containing microcapsules (size range 20–280 μm) was fed to Paphies australis, a bivalve known to accumulate TTX in the wild. The TTX-containing microcapsules were fed to P. australis every second day for 13 days. Ten P. australis (including five controls fed non-toxic microalgae) were harvested after 7 days and ten after 13 days. Paphies australis accumulated TTX, reaching concentrations of up to 103 µg kg⁻¹ by day 13, exceeding the European Food Safety Authority recommended concentration of 44 μg kg⁻¹ in shellfish. This novel method will allow future studies to explore the effects, accumulation and depuration rates of TTX in different animals and document how it is transferred through food webs.