Zebra mussels (Dreissena polymorpha) for assessing microbial contamination and antibiotic resistant bacteria in freshwaters

Sammanfattning: The suitability of zebra mussels (Dreissena polymorpha) as biomonitoring organisms for faecal contamination of surface waters was examined in both laboratory and field experiments. In addition, the presence of antibiotic resistant bacteria was investigated. In the first two laboratory experiments, the mussels were first exposed to a known concentration of Escherichia coli and Enterococcus facecalis and then depurated. In the other two experiments, the mussels were exposed to a tenfold dilution of sewage treatment plant effluent and then depurated. Zebra mussels showed a maximum uptake and elimination rate during the first hours after exposure to contaminated, and respectively, clean water. The field experiment was conducted at 6 sites located upstream and downstream from the sewage treatment plant outlet in the Fyris river (Uppsala, Sweden). During the 19 days of exposing caged mussels, the presence of faecal indicators, as well as Salmonella spp. was investigated. All the experiments demonstrated the ability of the zebra mussels to accumulate much higher concentrations of bacteria than in the surrounding water. This is in accordance with the work of previous studies and sustains the usefulness of zebra mussels in detecting bacterial peaks. Salmonella spp. was found on three occasions at the outlet, in both mussels and water, and on two occasions in the water downstream. All the E. coli and 41.3 % of the Enterococcus spp. isolates from both water and mussels were resistant to at least one antibiotic. A large percentage (48.5 %) of those found in mussels was multi drug resistant, thus providing evidence for this emerging problem that needs to be controlled.