measured. Application of antibiotic-polluted manure in
agricultural fields would then lead to the introduction of
various antibiotics in soils. It has been documented for
sulfamethazine, tetracycline family, as well as tylosin
(Schwaiger et al. 2009). Maximum concentrations of
various antibiotics in soils fertilized with animal manure
were 0.3 mg.kg
À1 , 0.015 mg.kg
À1 , and 0.37 mg.kg
À1
for tetracyclines, sulfonamides, and fluoroquinolones,
respectively. Despite antibiotic detection in the amendments, their occurrences and concentrations in soil might
strongly differ due to differences in stability. Such
differences can exist between compounds with structural
similarities. For instance, whereas oxytetracycline was
frequently detected at high levels in soil samples, chlortetracycline has been rarely detected (Karci and
Balciog ˘lu 2009).
Whether environmental concentrations of antibiotics are
sufficient to exert a selective pressure in the environment
and present a risk for human health in terms of resistance,
development is under investigation. Tello et al. (2012)
evaluated that for clinically relevant bacteria, measured
environmental concentrations in some environments,
i.e., river sediments, swine feces lagoons, liquid manure,
and farmed soil, are enough to inhibit wild-type
populations in up to 60 %, 92 %, 100 %, and 30 % of
bacterial genera, respectively. The authors concluded that
concentrations are high enough to exert a selective pressure
on clinically relevant bacteria that may lead to an increase
in the prevalence of resistance. Similarly antibioticresistant bacteria (ARB) present in these amendments can
be added to soils and favor antibiotic resistance gene
(ARG) spread. The frequency of bacteria carrying antimicrobial resistance genes seems to be especially high for
pigs as compared to cattle or sheep which correlates with
the amounts of antibiotics used in the husbandry of these
animal species (Schwaiger et al. 2009). Evidence of
increase ARG genes in soils over time has been
demonstrated by Knapp et al. (2010) in the Netherlands
during their survey of five long-term soil series that spanned
1940–2008. Data showed that ARG from all classes of
antibiotics tested (tetracyclines, vancomycin, erythromycin,
ampicillin, methicillin, and beta-lactams) has significantly
increased since 1940, but especially within the tetracyclines.
A study from Heuer and Smalla (2007) showed that the
manure added to sulfadiazine increased levels of antibiotic
resistance genes and mobile genetic elements carrying these
genes in manured soil over at least 2 months. A review from
Heuer et al. (2011) on ARG spread due to manure application
on agricultural fields concluded on the very likely significant
effect of the agricultural antibiotic use on the spread of
resistance among human beings because of the high exposure
rate to environmental contamination with transferable
antibiotic resistance genes.
Impact of Nonantibiotic Chemicals in the Selection
of Antibiotic Resistance
An increasing number of studies has evidenced the role
of additional chemicals acting as selective pressure for
antibiotic-resistant bacteria in the environment. Some
biocides, detergents, and organic solvents can select for
mutants exhibiting increase expression of multidrugresistant determinants. The most documented ecological
studies are those focusing on the role of metal as a selective
agent in the proliferation of antibiotic resistance. Early
studies on Gram-negative fecal bacteria of primates exposed
to mercury (Hg) consecutively to the use of dental amalgam
demonstrated that mercury-resistant strains are more likely
to be multiresistant to antibiotics than mercury-sensitive
strains are (Ready et al. 2007). Similarly positive correlation
between the presence of metals in ecosystems and the resistance toward both metal and antibiotics in water
(Stepanauskas et al. 2006), sediments, or soils has been
reported. Huysman et al. (1994) observed higher frequencies
of co-resistance to antibiotics and metal among copperresistant bacteria when compared to copper-sensitive
isolates from fields amended with copper-contaminated pig
manure. Similarly Berg et al. (2005b) compared bacterial
communities from copper polluted and nonpolluted soils and
showed an enrichment in copper-resistant populations, an
enrichment in antibiotic-resistant populations and a higher
prevalence of multiresistance (resistance to more than three
antibiotics) in the polluted soil. These authors (Berg et al.
2010) also evidenced that soil Cu exposure provides a
sufficient selection pressure to co-select for resistance to
vancomycin.
Several mechanisms underlie the co-selection process:
co-resistance (different genetic determinants are present on
the same genetic element), cross-resistance (the same determinant responsible for resistance to antibiotic and metal), or
co-regulation (metal or antibiotic act as inducer of a common regulator system responsible of various metal and
antibiotic-resistant determinant expression). These three
mechanisms were shown to be responsible for antibiotic
resistance within the opportunistic bacterial pathogen,
Pseudomonas aeruginosa:
1. Plasmids and/or transposons frequently harbor genes
encoding metal and metalloid resistance as well as
genes encoding antibiotic resistance.
2. Presence of an efflux pump enabling resistance to vanadium and increase resistance to ticarcillin, netilmicin,
and clavulanic acid (Aendekerk et al. 2002).
3. Exposure to zinc select for strains resistant to metal
(zinc, cadmium, and cobalt) and imipenem (Perron et al.
2004). The latter resistance mechanism involves a twocomponent system which activates expression of a metalcontrolling efflux pump and repressed protein membrane
expression.
15 Environmental and Human Pathogenic Microorganisms
637
agricultural fields would then lead to the introduction of
various antibiotics in soils. It has been documented for
sulfamethazine, tetracycline family, as well as tylosin
(Schwaiger et al. 2009). Maximum concentrations of
various antibiotics in soils fertilized with animal manure
were 0.3 mg.kg
À1 , 0.015 mg.kg
À1 , and 0.37 mg.kg
À1
for tetracyclines, sulfonamides, and fluoroquinolones,
respectively. Despite antibiotic detection in the amendments, their occurrences and concentrations in soil might
strongly differ due to differences in stability. Such
differences can exist between compounds with structural
similarities. For instance, whereas oxytetracycline was
frequently detected at high levels in soil samples, chlortetracycline has been rarely detected (Karci and
Balciog ˘lu 2009).
Whether environmental concentrations of antibiotics are
sufficient to exert a selective pressure in the environment
and present a risk for human health in terms of resistance,
development is under investigation. Tello et al. (2012)
evaluated that for clinically relevant bacteria, measured
environmental concentrations in some environments,
i.e., river sediments, swine feces lagoons, liquid manure,
and farmed soil, are enough to inhibit wild-type
populations in up to 60 %, 92 %, 100 %, and 30 % of
bacterial genera, respectively. The authors concluded that
concentrations are high enough to exert a selective pressure
on clinically relevant bacteria that may lead to an increase
in the prevalence of resistance. Similarly antibioticresistant bacteria (ARB) present in these amendments can
be added to soils and favor antibiotic resistance gene
(ARG) spread. The frequency of bacteria carrying antimicrobial resistance genes seems to be especially high for
pigs as compared to cattle or sheep which correlates with
the amounts of antibiotics used in the husbandry of these
animal species (Schwaiger et al. 2009). Evidence of
increase ARG genes in soils over time has been
demonstrated by Knapp et al. (2010) in the Netherlands
during their survey of five long-term soil series that spanned
1940–2008. Data showed that ARG from all classes of
antibiotics tested (tetracyclines, vancomycin, erythromycin,
ampicillin, methicillin, and beta-lactams) has significantly
increased since 1940, but especially within the tetracyclines.
A study from Heuer and Smalla (2007) showed that the
manure added to sulfadiazine increased levels of antibiotic
resistance genes and mobile genetic elements carrying these
genes in manured soil over at least 2 months. A review from
Heuer et al. (2011) on ARG spread due to manure application
on agricultural fields concluded on the very likely significant
effect of the agricultural antibiotic use on the spread of
resistance among human beings because of the high exposure
rate to environmental contamination with transferable
antibiotic resistance genes.
Impact of Nonantibiotic Chemicals in the Selection
of Antibiotic Resistance
An increasing number of studies has evidenced the role
of additional chemicals acting as selective pressure for
antibiotic-resistant bacteria in the environment. Some
biocides, detergents, and organic solvents can select for
mutants exhibiting increase expression of multidrugresistant determinants. The most documented ecological
studies are those focusing on the role of metal as a selective
agent in the proliferation of antibiotic resistance. Early
studies on Gram-negative fecal bacteria of primates exposed
to mercury (Hg) consecutively to the use of dental amalgam
demonstrated that mercury-resistant strains are more likely
to be multiresistant to antibiotics than mercury-sensitive
strains are (Ready et al. 2007). Similarly positive correlation
between the presence of metals in ecosystems and the resistance toward both metal and antibiotics in water
(Stepanauskas et al. 2006), sediments, or soils has been
reported. Huysman et al. (1994) observed higher frequencies
of co-resistance to antibiotics and metal among copperresistant bacteria when compared to copper-sensitive
isolates from fields amended with copper-contaminated pig
manure. Similarly Berg et al. (2005b) compared bacterial
communities from copper polluted and nonpolluted soils and
showed an enrichment in copper-resistant populations, an
enrichment in antibiotic-resistant populations and a higher
prevalence of multiresistance (resistance to more than three
antibiotics) in the polluted soil. These authors (Berg et al.
2010) also evidenced that soil Cu exposure provides a
sufficient selection pressure to co-select for resistance to
vancomycin.
Several mechanisms underlie the co-selection process:
co-resistance (different genetic determinants are present on
the same genetic element), cross-resistance (the same determinant responsible for resistance to antibiotic and metal), or
co-regulation (metal or antibiotic act as inducer of a common regulator system responsible of various metal and
antibiotic-resistant determinant expression). These three
mechanisms were shown to be responsible for antibiotic
resistance within the opportunistic bacterial pathogen,
Pseudomonas aeruginosa:
1. Plasmids and/or transposons frequently harbor genes
encoding metal and metalloid resistance as well as
genes encoding antibiotic resistance.
2. Presence of an efflux pump enabling resistance to vanadium and increase resistance to ticarcillin, netilmicin,
and clavulanic acid (Aendekerk et al. 2002).
3. Exposure to zinc select for strains resistant to metal
(zinc, cadmium, and cobalt) and imipenem (Perron et al.
2004). The latter resistance mechanism involves a twocomponent system which activates expression of a metalcontrolling efflux pump and repressed protein membrane
expression.
15 Environmental and Human Pathogenic Microorganisms
637