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the anthropized river sediments, the most persistent antibiotics are
detectable (macrolides, quinolones, sulfonamides and tetracyclines) at
concentrations that can reach 100 µg.kg -1 for antibiotics from the
quinolones family [MAS 10]. In the sediments of the Seine, antibiotics
can persist for more than 30 years, at concentrations that can reach 32
µg.kg -1 for quinolones, 15 µg.kg -1 for sulfonamides and 20 µg.kg -1 for
nalidixic acid [TAM 11]. Laboratory experiments have shown that a
continuous exposure of sediments to concentrations of antibiotics of 1
mg.L -1 and 250 mg.L -1 (erythromycin, clarithromycin, amoxicillin and
oxytetracycline) inhibits the activity of nitrifying and denitrifying
microbial communities [LUI 01]. At the lowest concentrations of
vancomycin or flumequine (100 and 1,000 ng.L -1 ), the diversity of
microbial communities is modified, whereas the rate of nitrate
reduction in denitrifying bacterial communities does not vary
significantly [YAN 13]. This phenomenon of resilience, which is
explained by a functional redundancy within microbial community,
has also been observed among sulfate-reducing microbial
communities present in the sediments contaminated by antibiotics in
marine fish farms [HAN 92].
2.5. Vulnerability and resilience in the estuary environment
In the decades to come, estuaries will be exposed to a great
chemical and microbial contamination, in correlation to the increase of
the demography and human activity on their watershed. The study
carried out in the Seine Estuary, and at the small-scale of a hospital-
WWTP-river continuum, shows the vulnerability, but also the
resilience, of this environment, to the contamination by antibiotics and
antibiotic-resistant bacteria.
Thus, both the concentrations of antibiotic and the occurence of
antibiotic-resistant bacteria greatly decrease during their transfer from
the main source of contamination (WWTP, hospital) to the estuary.
The substantial decrease of antibiotic concentration along this
continuum is mainly due to degradation or elimination by WWTP
treatments, and the subsequent dilution in the receiving environment.
Whatever the antibiotic, the concentrations observed are lower than
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