In this study the blue mussel M. edulis has been used for investigations on faecal pollution.
The aim was to detect the uptake, accumulation and persistence of two human specific Enterococcus species -that are often the most abundant species in wastewater- in the blue mussels.
To this purpose, the authors investigated the presence of E. faecium and E. faecalis in a natural population of mussels collected at 4 sites potentially affected by human faecal pollution, as they were located less then 1km from discharges of local wastewater treatment plants.
To study the uptake and accumulation of bacteria mussels have been exposed to a culture of E. faecalis in a microcosm experiment.
To estimate the persistence of bacteria, contaminated mussels were tranferred to aquaria with flowing seawater without bacteria for depuration and then transplanted to coastal sites to study in situ persistence of human associated Enterococcus.
The presence of the two Enterococcus species was detected targetting two specific molecular markers:
- esp gene which encodes the enterococcal surface protein, a putative virulence factor in human associated E. faecium strains
- M66 gene which encodes a protein in E. faecalis with unknown function but it may serve a functional role during survival in human hosts
In laboratory experiments the authors found that after 24h of exposure to contaminated water, the culturability of the E. faecalis declined exponentially, and culturable cells were not detected after 21 days of incubation. At the same time the concentration of E. faecalis in mussels was a twofold greater than in the surrounding seawater, suggesting that the uptake and bioaccumulation of bacteria was massive.
A substancial decline in the concentrations of E. faecalis in M. edulis occured after the initial 24h of depurationin aquaria.
The in situ experiment on depuration showed that the human associated E. species in mussels were detectable for at least 96h.
These results suggest that M. edulis can accumulate and retain elevated concentration of putative source specific E. species, hence these filtrating bivalve molluscs may be considered as additional targets in microbial source tracking studies.
Additionally, noncontaminated mussels may be transplanted to contaminated sites for time-integrated accumulation of markers relevant for microbial source tracking.
A review of: Roslev, P., Iversen, L., Sønderbo, H., Iversen, N. and Bastholm, S. (2009), Uptake and persistence of human associated Enterococcus in the mussel Mytilus edulis: relevance for faecal pollution source tracking. Journal of Applied Microbiology, 107: 944–953. doi: 10.1111/j.1365-2672.2009.04272.x
1 comment:
I found this paper good overall, but I do notice that methods and analysis were not always clear, especially the section on enumeration of Enterococci.
There were also some inconsistencies regarding the strains used in the experiment: they investigated both E. faecium and E. faecalis in water and mussels collected from the 4 sites, while in the mesocosm experiment they used just E. faecalis. A possible explanation could be that they did not detect the esp marker (E. faecium gene) in the seawater sample, but this is not a valid reason anyway.
Although the results seems encouraging and suitable for water quality assessment, they are referred to two particular species - E. faecalis and M. edulis- so these results could not eventually be valid for other bacteria or mussel species, as the response can vary.
As the authors suggest Mytilids produce extracellular enzymes capable of catalyzing degradation of bacterial cell walls, but previous studies showed that Gram positive bacteria including enterococci may be less susceptible to digestive degradation than Gram negative bacteria such as E. coli. This is supported by observation by Marino et al (2005) who concluded that Enterococcus durans persists longer in marine mussels than E. coli and that uptake and persistence of enterococci are less influenced by temperature making them potentially better indicators in mussels.
Furthermore it would be interesting to detect the presence of E. species in different tissues as the depuration can vary according to a specific organ or tissue of mussels; even if bacteria are not detectable in the surrounding water they can be present in mussels, hence the site is polluted anyway.
Additional references:
Marino, A., L. Lombardo, et al. (2005). "Uptake of Escherichia coli, Vibrio cholerae non-O1 and Enterococcus durans by, and depuration of mussels (Mytilus galloprovincialis)." International Journal of Food Microbiology 99(3): 281-286.
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