90
3
Conclusion
In the next decades, the estuaries will be exposed
to a great chemical and microbial contamination,
linked to the increase of the demography and
human activity on their watershed. The study carried out in the Seine estuary and at small scale of
a hospital–WWTP–river continuum shows not
only 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 occurrence 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 the degradation or elimination by WWTP
treatments and the subsequent dilution in the
receiving environment. Whatever the antibiotic,
the concentrations observed are lower than the
minimal inhibitory concentrations (MIC ranging
from 0.032 to 256 µg mL
−1
) responsible for the
selection of antibiotic-resistant bacteria demonstrating the resilience of this water environment
to the contamination by antibiotics (Oberlé et al.
2012 ; Leclercq et al. 2013 ). However, possible
effects on bacterial physiology have been
described for subinhibitory or sublethal concentrations (0.9x CMI to 0.25x CMI) (Davies et al.
2006 ; Kohanski et al. 2010 ). Indeed, the most
stable molecules, such as the quinolones, the
macrolides, and the sulfonamides, which persist
longer in water could be accumulated in the environment such as the biofi lms of periphytons or
estuary mudfl ats.
The occurrence of antibiotic-resistant E. coli
and Enterococcus strains released in waters
results from the selective pressure exerted on the
intestinal microbiota of human under antibiotic
treatment. In water environment, no simple relationship exists between the antibiotic detected,
the antibiotic use, and the antibiotic-resistant
phenotypes of fecal bacteria. Indeed, only the
most stable molecules are detectable in water,
and the bacteria can harbor integrons that confer
resistance to several antibiotics (Laroche et al. 2009 ;
Oberlé et al. 2012 ). However, the abundance of
antibiotic-resistant fecal bacteria decreases during their transfer from the source to the estuary,
mainly due to a more important decay of hospital
strains, that harbor gene implicated in the spread
of antibiotic resistance (integrons, erm gene), in
favor of strains less resistant to antibiotic and
probably better adapted to the environment.
These results also underline the resilience capacity of this aquatic environment (Berthe et al.
2013 ; Leclercq et al. 2013 ).
However, the estuarine sediments chronically exposed to multiple chemical contaminants, including antibiotics, to which are added
supplies of antibiotic-resistant bacteria, are
vulnerable environments. Indeed, this area
could be a hot spot zone favorable to the transfer
of antibiotic-resistance genes within the microbial communities.
Acknowledgments These studies were supported by the
project FLASH (GIP Seine-Aval/EC2CO CNRS), by the
SFR SCALE, and by research grants from the HauteNormandie Regional Council (France) for the PhD of
Kenny Oberlé, Mehdy Ratajczak, and Emilie Laroche. We
thank Michel Simon, Caroline Bance, and Michel Auzou
for the excellent technical assistance. We thank also
Michel Leroux, Aurélie Lamy, Yvon Goarvot, Sophie
Coté, and downtown pharmacists for antibiotic consumption data and the people that let us access to the medical
center and to the WWTP.
References
Aarestrup FM (2005) Veterinary drug usage and antimicrobial resistance in bacteria of animal origin. Basic
Clin Pharmacol Toxicol 96:271–81
Berthe T, Ratajczak M, Clermont O, Denamur E, Petit F
(2013) Evidence for co-existence of distinct
Escherichia coli population regarding their survival
in aquatic environment. Appl Environ Microbiol
79:4684–4693
Davies J, Spiegelman GB, Yim G (2006) The world of
subinhibitory antibiotic concentrations. Curr Opin
Microbiol 9(5):445–453
Garcia-Armisen T, Touron A, Petit F, Servais P (2005)
Sources of faecal contamination in the Seine estuary
(France). Estuar Coasts 28(4):627–633
Goni-Urriza M, Capdepuy M, Arpin C, Raymond N,
Caumette P, Quentin C (2000) Impact of an urban
effl uent on antibiotic resistance of riverine
Enterobacteriaceae and Aeromonas spp. Appl Environ
Microbiol 66:125–32
F. Petit et al.
3
Conclusion
In the next decades, the estuaries will be exposed
to a great chemical and microbial contamination,
linked to the increase of the demography and
human activity on their watershed. The study carried out in the Seine estuary and at small scale of
a hospital–WWTP–river continuum shows not
only 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 occurrence 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 the degradation or elimination by WWTP
treatments and the subsequent dilution in the
receiving environment. Whatever the antibiotic,
the concentrations observed are lower than the
minimal inhibitory concentrations (MIC ranging
from 0.032 to 256 µg mL
−1
) responsible for the
selection of antibiotic-resistant bacteria demonstrating the resilience of this water environment
to the contamination by antibiotics (Oberlé et al.
2012 ; Leclercq et al. 2013 ). However, possible
effects on bacterial physiology have been
described for subinhibitory or sublethal concentrations (0.9x CMI to 0.25x CMI) (Davies et al.
2006 ; Kohanski et al. 2010 ). Indeed, the most
stable molecules, such as the quinolones, the
macrolides, and the sulfonamides, which persist
longer in water could be accumulated in the environment such as the biofi lms of periphytons or
estuary mudfl ats.
The occurrence of antibiotic-resistant E. coli
and Enterococcus strains released in waters
results from the selective pressure exerted on the
intestinal microbiota of human under antibiotic
treatment. In water environment, no simple relationship exists between the antibiotic detected,
the antibiotic use, and the antibiotic-resistant
phenotypes of fecal bacteria. Indeed, only the
most stable molecules are detectable in water,
and the bacteria can harbor integrons that confer
resistance to several antibiotics (Laroche et al. 2009 ;
Oberlé et al. 2012 ). However, the abundance of
antibiotic-resistant fecal bacteria decreases during their transfer from the source to the estuary,
mainly due to a more important decay of hospital
strains, that harbor gene implicated in the spread
of antibiotic resistance (integrons, erm gene), in
favor of strains less resistant to antibiotic and
probably better adapted to the environment.
These results also underline the resilience capacity of this aquatic environment (Berthe et al.
2013 ; Leclercq et al. 2013 ).
However, the estuarine sediments chronically exposed to multiple chemical contaminants, including antibiotics, to which are added
supplies of antibiotic-resistant bacteria, are
vulnerable environments. Indeed, this area
could be a hot spot zone favorable to the transfer
of antibiotic-resistance genes within the microbial communities.
Acknowledgments These studies were supported by the
project FLASH (GIP Seine-Aval/EC2CO CNRS), by the
SFR SCALE, and by research grants from the HauteNormandie Regional Council (France) for the PhD of
Kenny Oberlé, Mehdy Ratajczak, and Emilie Laroche. We
thank Michel Simon, Caroline Bance, and Michel Auzou
for the excellent technical assistance. We thank also
Michel Leroux, Aurélie Lamy, Yvon Goarvot, Sophie
Coté, and downtown pharmacists for antibiotic consumption data and the people that let us access to the medical
center and to the WWTP.
References
Aarestrup FM (2005) Veterinary drug usage and antimicrobial resistance in bacteria of animal origin. Basic
Clin Pharmacol Toxicol 96:271–81
Berthe T, Ratajczak M, Clermont O, Denamur E, Petit F
(2013) Evidence for co-existence of distinct
Escherichia coli population regarding their survival
in aquatic environment. Appl Environ Microbiol
79:4684–4693
Davies J, Spiegelman GB, Yim G (2006) The world of
subinhibitory antibiotic concentrations. Curr Opin
Microbiol 9(5):445–453
Garcia-Armisen T, Touron A, Petit F, Servais P (2005)
Sources of faecal contamination in the Seine estuary
(France). Estuar Coasts 28(4):627–633
Goni-Urriza M, Capdepuy M, Arpin C, Raymond N,
Caumette P, Quentin C (2000) Impact of an urban
effl uent on antibiotic resistance of riverine
Enterobacteriaceae and Aeromonas spp. Appl Environ
Microbiol 66:125–32
F. Petit et al.