Uptake and fate of pathogenic microbes in the blue mussel, Mytilus edulis

• Bivalves, like the blue mussel Mytilus edulis, are filter-feeders and can therefore accumulate high numbers of microbes from the surrounding water. These shellfish are considered the main risk for bringing pathogens from domestic outlets in the sea, back to the community. Although Sweden has strict regulations for discharging waste-water, enteric human viruses were frequently found in mussels from the west coast. The viruses occurred even when the present indicator organism for faecal contamination, Escherichia coli, was found in low numbers. The European Commission has suggested bacteriophages (somatic coliphages, F-specific RNA phages, phages infecting Bacteroides fragilis) as more reliable indicators. However, environmental factors, such as water temperature and land run-off, influenced the indicators and human viruses differently and therefore they did not coexist. Furthermore, a series of laboratory experiments were performed in order to investigate if the content of viable bacteria in mussels can be affected by the cell surface properties of the bacteria and by the surrounding temperature. (i) The ingestion of Salmonella typhimurium was significantly increased when the negative cell surface charge of the bacteria was reduced through chemical modification. This treatment also altered the handling of the bacteria in the mussel stomach. Thus, the bacteria were recovered in different amounts and in different organs of the mussels. (ii) Survival of mutants of S. typhimurium when exposed to mussel haemocytes (in vitro) or inoculated into mussel haemolymph, was enhanced for mutants expressing more complete lipopolysaccharide (LPS). These mutants also managed to multiply in vivo and kill mussels when the temperature favoured bacterial growth. (iii) LPS extracted from S. typhimurium showed to be a trigger for antibacterial peptide activity when inoculated in the mussel haemolymph. The activity responded in a dose dependent manner and was enhanced by temperature.It can be concluded that the enrichment in the mussel can differ between microbes due to their cell surface properties. The content of viable microbes in the mussel tissue seemed to be more pronounced for the human bacterial pathogens. Such bacteria had a survival advantage particularly at higher temperatures. Therefore it does not seem adequate to use non-pathogenic E. coli as an indicator for human intestinal pathogens. These results as well as the fact that pathogens circulating in the community can be brought to the mussels via domestic drainage, should be taken into consideration when modelling risk assessment. Indicators should be evaluated with respect to the most risky pathogens present, and how the bivalves can handle these in relation to the particular environmental conditions in the area. Thus, we may have to accept that the choice of reliable indicators will differ between geographical areas and probably also on a seasonal basis.

Ämnesord

NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

Nyckelord

Mytilus edulis

bivalve

immune response

lipopolysaccharide

LPS

antimicrobial peptide

gamma camera

radiolabelling

Salmonella typhimurium

sanitary safety

public health

Publikations- och innehållstyp

vet (ämneskategori)

dok (ämneskategori)