Bacteria make shellfish safe to eat!?

Paralytic shellfish toxins (PSTs) are produced by marine dinoflagellates, and pose a serious threat to the human food supply due to their high toxicity and lack of available medical treatment. PSTs concentrate largely in the digestive tract of bivalve molluscs, which filter feed on marine algae. These then pass into the human food chain through ingestion of the molluscs, which are not affected by commercial sterilisation or cooking. There are currently no practical methods for the detoxification of living shellfish.

The presences of toxin-transforming enzymes and/or microorganisms in bivalve molluscs have been suggested, due to several reports noting that their digestive glands have a high capacity for PST transformation. The discoveries of bacterial degradation of toxins suggest that bacteria may play an important role in the elimination of toxins from toxic bivalve molluscs.

It has recently been reported that bacteria found within blue mussels are capable of breaking down marine toxins more rapidly than those of other bivalve species. This report assesses the phenotypic and taxonomic characterisation of these bacteria.

Toxic blue mussels were collected from around Atlantic Canada, and microflora from their digestive glands was cultivated on agar. 69 bacterial isolates were identified and grouped according to the colony appearance. All isolates were then tested for the ability to break down PSTs, and cultures were incubated at 25^oC for 5 days in a shaking incubator. Samples were taken on days 0 and 5, and were analysed for PSTs. It was found that only 7 isolates completely eliminated one type of PST, and reduced the overall toxin by no less than 90% in less than 3 days. These findings were confirmed by further study, which implies true biodegradation. Analysis of the 16S rRNA genes indicated that all 7 isolates were composed of bacteria belonging to a single clade within the genus Pseudalteromonas. These are often found in association with toxic dinoflagellates, and have been reported to produce a number of biologically active metabolites, including antibiotics and antimycotics. Some species within this genus have also shown algalcidal activities against dinoflagellates producing PSTs. Transmission electron microscopy revealed that all 7 isolates were rod-shaped and occurred as single cells and short chains. The isolates also possessed pili and flagella, and outer membrane vesicles.

In conclusion, it is possible that bacteria from within the genus Pseudalteromonas can assist bivalves in breaking down PSTs accumulated in their digestive tracts. Work is currently underway to support these findings, and to develop a biological process that will enable the elimination of these toxins from bivalves in vivo.

This report seems to be a preliminary study that has been used to support further investigation by the Canadian food inspection agency. Its findings in this early stage suggest the possibility of promising results in future studies such as this, and could ultimately lead to a higher level of control over the levels of PSTs present in marine Bivalves fit for human consumption.

A review of Donovan C.J., Garduno R.A., Kalmokoff M., Ku J.C., Quilliam M.A. and Gill T.A. (2009) Pseudoalteromonas Bacteria Are Capable of Degrading Paralytic Shellfish Toxins, Applied and Environmental Microbiology, 75, 6919-6923