Vulnerability and Resilience of Estuaries to Contamination by Antibiotics and Antibiotic-Resistant Bacteria: a Challenge for the Next Decade - Vulnerability of Coastal Ecosystems and Adaptation - page 68

discharged into the environment via urine and feces as a consequence
of the selective pressure exerted on the intestinal microbiota of
humans and animals treated with antibiotics. Consequently, in parallel
with contamination by antibiotics, fecal antibiotic-resistant bacteria,
notably Escherichia coli and Enterococcus spp , are released into water
[GON 00, KOC 11, LAR 09, LEC 13, OBE 12, WAT 07].
Contamination of surface water is essentially due to the release of
treated effluent from wastewater treatment plants (WWTP), and
surface run-off or soil leaching, to which can be added uncontrolled
waste from factories for pharmaceutical production and from fishing
practices [JIM 12, WAT 09].

Chemical and microbiological contamination of coastal areas and
estuaries reflects the anthropogenic pressure exerted on their
catchments. In this aquatic environment, the fate of antibiotic-resistant
bacteria will depend on their ability to overcome environmental stress
(salinity, temperature and oligotrophy) or grazing by protozoa. When
they are associated with organo-mineral particles, bacteria of fecal
origin and the more stable molecules of antibiotics, follow particle
dynamics and are deposited on mudflats, deposition zones for fine
particles [COR 07, GAO 05, TAM 11, XU 09]. These mudflats, the
habitat of specific microbial communities, are also zones for
allochthonous bacteria [BER 08]. The antibiotic resistance genes
detected in bacterial DNA extracted from highly anthropized river
sediments, shows that these genes can persist in these environments
[KRI 11, PEI 06]. Thus, genes prevalent in clinical strains may be
disseminated to autochtonous bacteria, including phylogenetically
distant bacterial species [GIR 10, GIR 11, WAL 11]. The frequency of
such horizontal gene transfer is difficult to evaluate in environmental
conditions. Depending on the conditions of the experiments, it ranges
from 10 -1 to 10 -3 in optimal conditions, and from 10 -4 to 10 -10 in
oligotrophic conditions, close to those found in the environment [BAL
88, CHA 98, MAR 06, PAU 91]. In the laboratory, a plasmid
conferring co-resistance on seven antibiotics was transferred with a
frequency of 2.8

10 -3 between a strain of E. coli and a strain of
Aeromonas , both isolated in the Seine Estuary [LAR 10]. These

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