Antibiotics and Antibiotic Resistance in Water Environments

Abstract

Antibiotic-resistant organisms enter into water environments from human and animal sources. These bacteria are able to spread their genes into water-indigenous microbes, which also contain resistance genes. On the contrary, many antibiotics from industrial origin circulate in water environments, potentially altering microbial ecosystems. Risk assessment protocols for antibiotics and resistant bacteria in water, based on better systems for antibiotics detection and antibiotic-resistance microbial source tracking, are starting to be discussed. Methods to reduce resistant bacterial load in wastewaters, and the amount of antimicrobial agents, in most cases originated in hospitals and farms, include optimization of disinfection procedures and management of wastewater and manure. A policy for preventing mixing human-originated and animal-originated bacteria with environmental organisms seems advisable.

Full text

A
vailable online at www.sciencedirect.com
Antibiotics and antibiotic resistance in water environments
Fernando Baquero
1,3
, Jose
´-Luis Martı
´nez
2,3
and Rafael Canto
´n
1,3
Antibiotic-resistant organisms enter into water environments
from human and animal sources. These bacteria are able to
spread their genes into water-indigenous microbes, which also
contain resistance genes. On the contrary, many antibiotics
from industrial origin circulate in water environments,
potentially altering microbial ecosystems. Risk assessment
protocols for antibiotics and resistant bacteria in water, based
on better systems for antibiotics detection and antibiotic-
resistance microbial source tracking, are starting to be
discussed. Methods to reduce resistant bacterial load in
wastewaters, and the amount of antimicrobial agents, in most
cases originated in hospitals and farms, include optimization of
disinfection procedures and management of wastewater and
manure. A policy for preventing mixing human-originated and
animal-originated bacteria with environmental organisms
seems advisable.
Addresses
1
Department of Microbiology, Ramo
´n y Cajal University Hospital,
CIBER-ESP, Spain
2
National Center for Biotechnology, CSIC, Spain
3
Joint Unit for Antimicrobial Resistance and Virulence, 28034 Madrid,
Spain
Corresponding author: Baquero, Fernando (baquero@bitmailer.net)
Current Opinion in Biotechnology 2008, 19:260–265
This review comes from a themed issue on
Environmental Biotechnology
Edited by Carla Pruzzo and Pietro Canepari
Available online 4th June 2008
0958-1669/$ – see front matter
#2008 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.copbio.2008.05.006
Introduction
Human and animal pathogenic and potentially patho-
genic bacteria are constantly released with wastewater
into the water environment. Many of these organisms
harbor antibiotic-resistance genes, eventually inserted
into genetic mobile platforms (plasmids, transposons,
integrons) able to spread among water and soil bacterial
communities [1]. Water constitutes not only a way of
dissemination of antibiotic-resistant organisms among
human and animal populations, as drinking water is
produced from surface water, but also the route by which
resistance genes are introduced in natural bacterial eco-
systems. In such systems, nonpathogenic bacteria could
serve as a reservoir of resistance genes and platforms.
Moreover, the introduction (and progressive accumu-
lation) in the environment of antimicrobial agents, deter-
gents, disinfectants, and residues from industrial
pollution, as heavy metals, contributes to the evolution
and spread of such resistant organisms in the water
environment. The heavy use of prophylactic antibiotics
in aquaculture [2] can be particularly relevant. On the
contrary, environmental bacteria act as an unlimited
source of genes that might act as resistance genes when
entering in pathogenic organisms. Note that many of
these genes are not primarily resistance genes, but belong
to the hidden ‘resistome’ [3], the set of genes able to be
converted in antibiotic-resistance genes. Human health
risk assessment protocols for antibiotic and resistant bac-
teria in water are starting to be discussed [4]. Certainly it
is difficult to believe why public health officers and eco-
toxicologists have failed for more than a century to
seriously propose the absolute need of preventing the
mix between microorganisms from human–animal and
environmental compartments.
The four genetic reactors in antibiotic
resistance
Antibiotic resistance evolves in bacteria because of the
effect of industrially produced antimicrobial agents on
bacterial populations and communities. Genetic reactors
are places in which the occasion occurs for genetic evol-
ution, particularly because of high biological connectivity,
generation of variation, and presence of specific selection.
Beyond mutational events, significant genetic variation
occurs as a consequence of recombinatorial events, fre-
quently resulting from genetic exchanges among organ-
isms inside populations and communities. There are four
main genetic reactors in which antibiotic resistance
evolves (Figure 1). The primary reactor is constituted
by the human and animal microbiota, with more than 500
bacterial species involved, in which therapeutic or pre-
ventive antibiotics exert their actions. The secondary
reactor involves the hospitals, long-term care facilities,
farms, or any other place in which susceptible individuals
are crowded and exposed to bacterial exchange. The
tertiary reactor corresponds to the wastewater and any
type of biological residues originated in the secondary
reactor, including for instance lagoons, sewage treatment
plants, or compost toilets, in which bacterial organisms
from many different individuals have the opportunity to
mix and genetically react. The fourth reactor is the soil
and the surface or ground water environments, where the
bacterial organisms originated in the previous reactors
mix and counteract with environmental organisms. Water
is involved as a crucial agent in all four genetic reactors,
but particularly in the last ones. The possibility of redu-
cing the evolvability of antibiotic resistance depends on
Current Opinion in Biotechnology 2008, 19:260–265 www.sciencedirect.com

the ability of humans to control the flow of active anti-
microbial agents, bacterial clones, and genetically based
biological information along these genetic reactors.
Industrial antibiotics in soil–water
environments
Water dissolves industrial antibiotics that are bound to
environmental matrices. Binding to soil particles (and
sediments) delays its biodegradation and explains long-
term permanence of the drugs in the environment. Of
course, soil particles also remove antibiotics from water,
so that a kind of water–soil pharmacokinetics might be
considered. Antimicrobial agents are retained in soil by its
association with soil chemicals. For instance, Elliot soil
humic acids produce complexation of antibiotics [5].
Interestingly, heavy metals (as methyl-mercury) also
associate with humic acids, so that in the water film
associated with soil organic particles several antimicrobial
effects might be simultaneously present. Indeed it
appears that in the presence of humic substances, in both
dissolved and mineral-bound forms, environmental mobi-
lity of antibiotics [5] might increase. Aluminum and iron
oxides might alter these interactions by changing surface
charge. For instance, sorption to such oxides results in
different types of ciprofloxacin-surface complexes [5]
probably changing the reactivity of fluoroquinolones in
the soil–water interphase. It is to be noticed that general
alterations in water or in soil (as pH changes, or ionic
strength) might alter these antibiotic–soil–water inter-
actions, producing different levels of antibiotic release
(dissolution) from soil particles. In a study, half-lives in
soil have been estimated in 20–30 days for erythromycin
or oleandomycin.
Industrial antibiotics in water–sludge
environments
Antimicrobial agents as sulfonamides, macrolides, tri-
methoprim, cephalosporins, or fluoroquinolones can be
found at potentially active concentrations in activated
sludge treatment, and the antibiotic load along the year
correlates with the variation in annual consumption data,
being higher in the winter [6]. The wastewater concen-
tration of antimicrobials depends on the sludge–waste-
water partition coefficient, but with fluoroquinolones field
experiments of sludge application to agricultural land
confirmed long persistence of these compounds, but with
limited mobility into the subsoil [7]. Very high concen-
trations of sulfonamides (20 10
3
ng/ml) have been
found in pig farm wastewater, and detection of sulfa-
methazine has been suggested to serve as a marker for
livestock-source contamination in Vietnam [8]. In Japa-
nese urban rivers a high number of antibiotic agents can
be detected, including sulfonamides, trimethoprim, and
macrolides. In Hong Kong and Shenzhen sewage
samples, penicillin levels (as penicillin V) were undetect-
able, but that was not the case for cephalosporins, as
cefalexin or cefotaxime reached concentrations exceed-
ing 1 mg/ml [9], probably sufficient to select organisms
producing extended-spectrum beta-lactamases, as CTX-
M enzymes. If selection of ESBL organism will produce a
reduction in the antibiotic concentration is controversial.
In compost toilets, amoxicillin decay is negligible, even in
the presence of beta-lactamase producing bacteria.
Hydrophobic antibiotics, as tetracycline or ciprofloxacin
were detected in all sludge samples from two Oslo city
hospitals, but not in the collected influent samples,
suggesting binding to effluent particles [10]. Similarly,
fluoroquinolones were consistently found in hospital
effluents [11

]. The extensive use of antibiotics in human
medicine, animal farming, and agriculture leads to anti-
biotic contamination of manure, which can be used as
fertilizer. Leaching tests indicate that in general less than
1% of fluoroquinolones in the sludge reached the aqueous
phase, which might indicate a relatively reduced mobility
when sludge is used to fertilize soil [7]. Nevertheless, that
does not exclude localized biological effects on particu-
lated material. Indeed high concentrations of fluoroqui-
nolones were found in secondary sludge (sorption).
Macrolides were frequently resistant to the processes
carried out in sewage treatment plants in South China,
and even higher concentrations were found in the final
effluents than in the raw sewages [12].
Antibiotics and antibiotic resistance in water environments Baquero, Martı
´nez and Canto
´n 261
Figure 1
The four genetic reactors in antibiotic resistance, where genetic
exchange and recombination shapes the future evolution of resistance
determinants. Particularly in the lowest reactors, bacteria from human-
associated or animal-associated microbiota (in black) mix with
environmental bacteria (in white), increasing the power of genetic
variation and possible emergence of novel mechanisms of resistance
that are re-introduced in human or animal environments (back arrows).
www.sciencedirect.com Current Opinion in Biotechnology 2008, 19:260–265

Water disinfection and wastewater treatment
influence antibiotic concentrations
Water disinfection by ClO(2) might contribute to the
removal to beta-lactam agents. The water-degradation
of beta-lactams (penicillin G) has been recently explored,
being penicilloic acid the main degradation product [13].
Aqueous chlorination of drinking water and wastewater
removes trimethoprim activity [14]. Wastewater treat-
ment might eliminate nearly 80% of fluoroquinolones
or tetracyclines before they enter rivers, and are suscept-
ible to photodegradation [7,9]. Antibiotic removal effi-
ciencies by wastewater treatment are less effective for
macrolides that are relatively persistent in the environ-
ment [9]. The application of techniques for antibiotic
removal by coagulation and granular activated carbon
filtration, ionic treatment or micelle–clay systems are
promising for the removal of tetracycline and sulfona-
mides [15].
Measuring water-antibiotic concentrations
The progress in instrumental analytical chemistry, using
electrophoretic and chromatographic techniques, as
liquid chromatography–tandem mass spectrometry
enables to detect many different types of antibiotics at
concentrations of nanograms/liter, after solid-phase
extraction [16,17]. Alternatively, immunochemical
approaches are also useful for inexpensive quick screen-
ing [17]. Voltammetry and amperometric detection of
tetracyclines using multiwall carbon nanotube modified
electrodes have been recently proposed for monitoring
water samples [18]. Even commercially available test kits
have been successfully used for rapid screening of anti-
biotic activity in effluents and surface water samples [19].
It is to be noticed that natural organic matter might
significantly impact the results of the analysis of some
antibiotics.
Reducing antibiotic-resistant bacteria in
wastewater
Antibiotic-resistant organisms from humans and animals
are released into the sewage by contaminated sites (in-
cluding urine), feces, eventually corpses and manure. In
particular, wastewater from hospitals and intensive farm-
ing facilities (under concentrated animal feeding oper-
ations) is probably a major source of pathogenic and
antibiotic-resistant organisms and antibiotic-resistance
genes that are released into the environment. It is essen-
tial to increase our knowledge on effective barrier
measures preventing the incorporation of resistant and
pathogenic bacteria into the environment (see Introduc-
tion). Wastewater could be disinfected in many ways,
including chlorine (2–3 logs bacterial reduction with
chlorine dose of 30 mg/L), ozone (3–4 logs reduction at
100 mg/L), or ultraviolet light (effective but expensive)
[20]. These treatments might differ in different circum-
stances, as for instance ammonia present in wastewaters
might compete for free chlorine to form monochloramine.
In these cases effective chlorine doses might require
concentrations of 100 mg/L. Filtering technologies in-
cluding surface-modified ones activated carbon filter
media are promising (more than 6 logs reduction). The
results of some studies [20] indicate the possibility that
chlorination might result in the alteration of wastewater
populations, with the selection of chlorine-resistant bac-
teria (related to Bacillus subtilis and Bacillus licheniformis),
which might contribute to the selection of particular
resistance genes and genetic platforms. A number of
studies have addressed the influence of different types
of manure management into the environmental fate of
resistance genes. High-intensity manure management
(with amending, watering, and turning) was more effec-
tive in reducing permanence of resistance genes than low-
intensity management. Different genes had diverse
kinetics of maintenance [21,22,23

], probably in relation
to the organisms harboring them.
Influence of water-antibiotics on antibiotic
resistance
Land application of manure can result in the dispersion of
resistant bacteria to water sources. The effect of a con-
centrated swine feeding operation on surface water and
groundwater situated up and down gradient of the swine
facility was studied [24]. The proportion of macrolide and
tetracycline-resistant enterococci was significantly
increased in down-gradient surface waters. Similarly,
samples of surface water sites near wastewater treatment
plants in Australia had a significant increase of antibiotic-
resistant Escherichia coli [25]. Nevertheless, the quanti-
tation of the effects of antibiotics on water bacteria
remains a difficult question, which has been addressed
by using model systems as mesocosms, soil-filled lysi-
meters representing the leachfield of a septic system.
Under these conditions, the local release of tetracycline
(5 mg/ml) had a very small effect in this model on the
development of antibiotic resistance [26

]. In another
model, using river water in continuous flow chemostat
system, similar results were found, but with high tetra-
cycline concentrations a greater diversity of tetracycline-
resistance genes was detected [27]. In a third model, using
a soil microcosm supplemented with pig manure slurry
and an Enterococcus faecalis strain harboring a tet(M) resist-
ance gene, stable tetracycline concentrations were unable
to influence the local prevalence of antibiotic-resistant
bacteria, but tetracycline-resistance genes persisted for a
long time, probably because of horizontal gene transfer to
other organisms. Obviously these models do not take into
account the possible concentration of drugs and bacteria
in particulated surfaces or sediments. A final concern
regards the utilization of prophylactic antibiotics in aqua-
culture. The heavy use of these compounds, several of
which are nonbiodegradable increases antibiotic selective
pressure in water, facilitates the transfer of antibiotic-
resistance determinants between aquatic bacteria,
including fish and human pathogens, and allows the
262 Environmental Biotechnology
Current Opinion in Biotechnology 2008, 19:260–265 www.sciencedirect.com

presence of residual antibiotics in commercialized fish
and shellfish products [2].
Antibiotic-resistant bacteria in water
Water bacteria might be indigenous to aquatic environ-
ments, or exogenous, transiently and occasionally present
in the water as a result of shedding from animal, vegetal,
or soil surfaces. More than 90% of bacterial strains origi-
nated in seawater are resistant to more than one antibiotic,
and 20% are resistant at least to five [28]. The study of
antibiotic resistance in indigenous water organisms is
important, as it might indicate the extent of alteration
of water ecosystems by human action. Aeromonas strains
from Portuguese estuarine water carry less frequently
beta-lactamase genes than Enterobacteriaceae (10% ver-
sus 78%) [29]. In water reservoirs a-half of Aeromonas
strains might present multiple antibiotic resistance [30].
Resistance profiles of aquatic pseudomonads depend on
the species composition, but also from the site in which
they were isolated, being more antibiotic-resistant along
shorelines and in sheltered bays than in the open water,
indicating the influence of nonaquatic organisms or pol-
lutants. Nevertheless, such influences can be found in the
more remote water environments; psychrotrophic bac-
teria from Antarctic show various degrees of resistance
to industrial antibiotics and metals [31]. The association
of antibiotic-resistance and resistance to heavy metals is
very frequent in the same organism (also in the same
plasmid, transposon, or integron) so that industrial pollu-
tion probably selects for antibiotic-resistance and vice
versa [32

]. Indeed metal contamination represents a
long-standing, widespread, and recalcitrant selection
pressure for multiresistant organisms. For the nonaquatic
organisms, obviously the density of antibiotic-resistance
organisms and antibiotic-resistance genes in fresh water
varies with the proximity to areas with increased anti-
biotic consumption, metal pollution, and between sea-
sons, being more frequently found in rainy seasons [23

].
Very little work has been done to elucidate the role of
bacterial biofilms in water environments and its role in
antibiotic resistance. Phenotypic antibiotic resistance in
bacterial biofilms might indeed protect the water environ-
ment from selective events caused by the antibiotic
release, which probably are acting more effectively on
planktonic bacteria.
Tracking the sources for antibiotic-resistant
organisms in water
The accessibility of modern molecular techniques for
subspecific characterization of bacterial organisms (clonal
detection) should readily increase our possibilities for
antibiotic-resistance microbial source tracking. Such
approach will provide only useful results after a much
more comprehensive knowledge of population biology of
bacterial organisms, as the genetic diversity of potential
organisms entering in water is very high [33]. The same is
true for tracking the plasmids and other genetic mobile
platforms involved in antibiotic resistance, but it is clear
that genetic techniques are providing a much more accu-
rate image of the real diversity and complexity of anti-
biotic resistance in water-borne bacteria, if compared with
cultivation-depending approaches [29]. Nevertheless, for
local purposes, phenotypic techniques, as those based on
carbon-utilization and antibiotic-resistance patterns
might still be useful for bacterial source tracking [34].
Horizontal gene transfer of antibiotic
resistance in water environments
Estuarine water-borne Aeromonas strains carry almost as
frequently as Enterobacteriaceae class 1 integron plat-
forms carrying antibiotic-resistance genes [29]. Exclu-
sively environmentally based organisms, as Delftia, also
harbor class 3 integrons [35

]. The persistence of such
genetic structures cannot probably be explained solely by
antibiotic selection, suggesting that activities resulting in
antibiotic resistance might have other physiological roles,
or that they are placed in multifunctional plasmids. The
most frequent gene cassette found involves aminoglyco-
side-resistance genes, rarely under positive selection in
our days, and there is a suspicion that some other resist-
ance genes, as integron sul genes, might provide benefits
for the bacteria, unrelated with resistance. However,
some of these mobile gene cassettes in Aeromonas might
involve important mechanisms of resistance, as Qnr,
involved in fluoroquinolone resistance, which might be
horizontally propagated by IncU-type plasmids [36

].
Certainly the dense bacterial populations in sewage treat-
ment plants favor genetic exchange among bacterial
populations and communities, integrons predating trans-
posons and plasmid dissemination. Multiresistance plas-
mids of broad host-range are consistently recovered in
sewage [37]. Interestingly, antibiotic-resistance genes
from manure influence the lagoons and groundwater gene
pool, but this pool also contains antibiotic-resistance
genes from indigenous bacteria [38]. Aeromonas from
aquaculture water systems (fish, eel farming) are particu-
larly resistant to antibiotics [39], and frequently contain
plasmids and integrons with multiple genes for antibiotic
resistance [40], and the association with heavy-metal
resistance is not uncommon [32

]. Water originated in
transgenic plant fields may constitute a matter of concern,
but no significant differences have been found in bacterial
antibiotic-resistance levels between transgenic and non-
transgenic corn fields [41].
Environmental damage mediated by
antibiotics in water environments
Pharmaceuticals are introduced in the environment from
human and veterinary applications at volumes comparable
with total pesticide loadings [42

]. Antibiotic resistance is
not the only possible adverse effect of antibiotic release in
water environments, and ecotoxicity tests are starting to be
introduced to document these effects [43]. Antibiotics
might act, at very low concentrations, as signaling agents
Antibiotics and antibiotic resistance in water environments Baquero, Martı
´nez and Canto
´n 263
www.sciencedirect.com Current Opinion in Biotechnology 2008, 19:260–265

(a kind of hormones) in microbial environments [44–46].
Common receptors have been identified in plants for a
number of antibiotics and disinfectants affecting chloro-
plast replication (fluoroquinolones), transcription–trans-
lation (tetracyclines, macrolides, lincosamides,
aminoglycosides, pleuromutilins), folate biosynthesis (sul-
fonamides, and probably trimetoprim), fatty acid synthesis
(triclosan), and sterol biosynthesis (azoles, statins) [42

].
During the past years the environmental consequences of
the release of triclosan in freshwater environment has been
considered [47]. Ciprofloxacin affects stream microbial
communities, including those colonizing senesced leaf
materials [48]. A matter of major future concern is the
effect of antibiotics and disinfectants released into the
environment on Cyanobacteria, largely susceptible to anti-
microbial agents, as such type of organisms accounts for
more than 70% of the total phytoplankton mass, and are
responsible for more than a third of the total free O
2
production, or CO
2
fixation. Amazonia is green, visible,
and attractive, but there are much bigger microscopic
Amazonias! What seems certain is that such alterations
in microbial ecosystems, either produced by antimicrobial
release or by the unexpected effective dispersal in water
environments of resistant pathogenic organisms [49] might
be relevant for public health. Future prediction and pre-
vention of antibiotic resistance [50] depends on the
research investments in the ecology, including water
ecology, of antibiotic-resistant microorganisms.
Conclusions
An important part of the dispersal and evolution of anti-
biotic-resistant bacterial organisms depends on water
environments. In water, bacteria from different origins
(human, animal, environmental) are able to mix, and
resistance evolves as a consequence of promiscuous
exchange and shuffling of genes, genetic platforms, and
genetic vectors. At the same time, antibiotics, disinfec-
tants, and heavy metals are released in water, and might
exert selective activities, as well as ecological damage in
water communities, resulting in antibiotic resistance.
Methods should be developed for cheap and reliable: first,
bacterial clones and resistance genes source tracking; sec-
ond, detection of antibiotics in water environments; third,
disinfection of water from antibiotic-resistant populations
and the resistance gene pool, and removal of antibiotics
from wastewater; and fourth,prevention policies for mixing
human–animal-originated and soil–water bacteria.
References and recommended reading
Papers of particular interest, published within the period of review,
have been highlighted as:
of special interest
1. Alonso A, Sanchez P, Martinez JL: Environmental selection of
antibiotic resistance genes.Environ Microbiol 2001, 3:1-9.
2. Cabello FC: Heavy use of prophylactic antibiotics in
aquaculture: a growing problem for human and animal health
and for the environment.Environ Microbiol 2006, 8:1137-1144.
3. D’Acosta VM, McGrann KM, Hughes DW, Wright GD: Sampling
the antibiotic resistome.Science 2006, 311:374-377.
4. Kim S, Aga DS: Potential ecological and human health impacts
of antibiotics and antibiotic-resistant bacteria from
wastewater treatment plants.J Toxicol Environ Health B: Crit
Rev 2007, 10:559-573.
5. Gu C, Karthikeyan KG: Sorption of the antibiotic tetracycline
to humic–mineral complexes.J Environ Qual 2008,
37:704-711.
6. Go
¨bel A, Thomsen A, McArdell CS, Joss A, Giger W: Occurrence
and sorption behavior of sulfonamides, macrolides, and
trimethoprim in activated sludge treatment.Environ Sci
Technol 2005, 39:3981-3989.
7. Sukul P, Spiteller M: Fluoroquinolone antibiotics in the
environment.Rev Environ Contam Toxicol 2007,
191:131-162.
8. Managaki S, Murata A, Takada H, Tuyen BC, Chiem NH:
Distribution of macrolides, sulfonamides and trimethoprim in
tropical waters: ubiquitous occurrence of veterinary
antibiotics in the Mekong delta.Environ Sci Technol 2007,
41:8004-8010.
9. Gulkowska A, Leung HW, So MK, Taniyasu S, Yamashita N,
Yeung LW, Richardson BJ, Lei AP, Giesy JP, Lam PK: Removal of
antibiotics from wastewater by sewage treatment facilities in
Hong Kong and Shenzhen, China.Water Res 2008, 42:395-403.
10. Thomas KV, Dye C, Schlabach M, Langford KH: Source to sink
tracking of selected human pharmaceuticals from two Oslo
city hospitals and a wastewater treatment works.J Environ
Monit 2007, 12:1410-1418.
11.

Lindberg RH, Bjo
¨rklund K, Rendahl P, Johansson MI, Tysklind M,
Andersson BA: Environmental risk assessment with emphasis
on sewage treatment plants.Water Res 2007, 41:613-619.
A synthetic view of the environmental effects of antibiotics and its
potential risks.
12. Xu W, Zhang G, Li X, Zou S, Li P, Hu Z, Li J: Occurrence and
elimination of antibiotics at four sewage treatment plants in
the Pearl River Delta, South China.Water Res 2007, 19:4526-
4534.
13. Li D, Yang M, Hu J, Zhang Y, Chang H, Jin F: Determination of
penicillin G and its degradation products in a penicillin
production wastewater treatment plant and the receiving
river.Water Res 2008, 42:307-317.
14. Dodd MC, Huang CH: Aqueous chlorination of the antibiotic
agent trimethoprim: reaction kinetics and pathways.Water
Res 2007, 41:647-655.
15. Choi KJ, Kim SG, Kim SH: Removal of antibiotics by coagulation
and granular activated carbon filtration.J Hazard Mater 2008,
151:38-43.
16. Dı
´az-Cruz MS, Barcelo
´D: Determination of antimicrobial
residues and metabolites in the aquatic environment by liquid
chromatography ta
´ndem mass spectrometry.Anal Bioanal
Chem 2006, 386:973-985.
17. Buchberger WW: Novel analytical procedures for screening of
drug residues in water, wastewater, sediment, and sludge.
Anal Chim Acta 2007, 19:129-139.
18. Vega D, Agu
¨i L, Gonzalez-Corte
´sA,Ya
´n
˜ez-Seden
˜oP,
Pingarron JM: Voltammetry and amperometric detection of
tetracyclines at multi-wall carbon nanotube modified
electrodes.Anal Bioanal Chem 2007, 389:951-958.
19. Smith AJ, Balaam JL, Ward A: The development of a rapid
screening technique to measure antibiotic activity in
effluents and surface water samples.Mar Pollut Bull 2007,
54:1940-1946.
20. Macauley JJ, Quiang Z, Adams CD, Surampalli R, Mormile MR:
Disinfection of swine wastewater using chlorine, ultraviolet
light and ozone.Water Res 2006, 10:2017-2026.
21. Pei R, Cha J, Carlson KH, Pruden A: Response of antibiotic
resistance genes to biological treatment in dairy lagoon water.
Environ Sci Technol 2007, 41:5108-5113.
264 Environmental Biotechnology
Current Opinion in Biotechnology 2008, 19:260–265 www.sciencedirect.com

22. Auerbach EA, Seyfried EE, McMahon KD: Tetracycline
resistance genes in activated sludge wastewater treatment
plants.Water Res 2007, 41:1143-1151.
23.

Peak N, Knapp CW, Yang RK, Hanfelt MM, Smith MS, Aga DS,
Graham DW: Abundance of six tetracycline resistance
genes in wastewater lagoons at cattle feedlots with
different antibiotic use strategies.Environ Microbiol 2007,
9:143-151.
The approach of this paper is to explore gene diversity instead of
organism diversity in contaminated water, and how this ‘gene population
biology’ is altered by the use of antibiotics.
24. Sapkota AR, Curriero FC, Gibson KE, Schwab KJ: Antibiotic-
resistant enterococci and fecal indicators in surface water and
groundwater impacted by a concentrated Swine feeding
operation.Environ Health Perspect 2007, 115:1040-1045.
25. Watkinson AJ, Micalizzi GR, Bates JR, Costanzo SD: Novel
method for rapid assessment of antibiotic resistance in
Escherichia coli isolates from environmental waters by use of
a modified chromogenic agar.Appl Environ Microbiol 2007,
73:2224-2229.
26.

Atoyan JA, Patenaude EL, Potts DA, Amador JA: Effects of
tetracycline on antibiotic resistance and removal of fecal
indicator bacteria in aerated and unaerated leachfield
mesocosms.J Environ Sci Health A: Tox Hazard Subst Environ
Eng 2007, 42:1571-1578.
This paper proposes the use of model systems to predict the environ-
mental effect of antibiotic release in wastewater treatment facilities.
27. Mun
˜oz-Aguayo J, Lang KS, LaPara TM, Gonzalez G, Singer RS:
Evaluating the effects of chlortetracycline on the proliferation
of antibiotic-resistant bacteria in a simulated river water
ecosystem.Appl Environ Microbiol 2007, 17:5421-5425.
28. Martinez JL: Recent advances on antibiotic resistance genes.
In Recent Advances in Marine Biotechnology. Molecular Genetics
of Marine Organisms, vol 10. Edited by Fingerman,
Nagabhushanam. 2003:13-32.
29. Henriques IS, Fonseca F, Alves A, Saavedra MJ, Correia A:
Occurrence and diversity of integrons and beta-lactamase
genes among ampicillin-resistant isolates from estuarine
waters.Res Microbiol 2006, 157:938-947.
30. Blasco MD, Esteve C, Alcaide E: Multiresistant waterborne
pathogens isolated from water reservoirs and cooling
systems.J Appl Microbiol 2008.
31. De Souza MJ, Nair S, Loka Bharati PA, Chandramohan D: Metal
and antibiotic resistance in psychotrophic bacteria from
Antarctic marine waters.Ecotoxicology 2006, 15:379-384.
32.

Baker-Austin C, Wright MS, Stepanauskas R, McArthur JV: Co-
selection of antibiotic and metal resistance.Trends Microbiol
2006, 14:176-182.
This is an excellent review of the role of metal pollution in selecting
antibiotic resistance and vice versa.
33. Olivas Y, Faulkner BR: Fecal source tracking by antibiotic
resistance analysis on a watershed exhibiting low resistance.
Environ Monit Assess 2008, 139:15-25.
34. Moussa SH, Massengale RD: Identification of the sources of
Escherichia coli in a watershed using carbon-utilization
patterns and composite data sets.J Water Health 2008,
6:197-207.
35.

Xu H, Davies J, Miao V: Molecular characterization of class 3
integrons from Delftia spp.J Bacteriol 2007, 189:6276-6283.
The investigation of the integron dispersal in environmental bacterial
communities is crucial to understand the antibiotic-resistance gene
flow among free-leaving and human-associated or animal-associated
bacteria.
36.

Cattoir V, Poirel L, Aubert C, Soussy CJ, Nordmann P:
Unexpected occurrence of plasmid-mediated quinolone
resistance determinants in environmental Aeromonas spp.
Emerg Infect Dis 2008, 14:231-237.
This group of authors explore in this and other papers the origin of
clinically relevant antibiotic resistances in environmental bacteria.
37. Schlu
¨ter A, Szczepanowski R, Kurz N, Schneiker S, Krahn I,
Pu
¨hler A: Erythromycin resistance-conferring plasmid
pRSB105, isolated from a sewage treatment plant, harbors a
new macrolide resistance determinant, an integron-
containing Tn402-like element, and a large region of unknown
function.Appl Environ Microbiol 2007, 73:1952-1960.
38. Koike S, Krapac IG, Oliver HD, Yannarell AC, Chee-Sanford JC,
Aminov RI, Mackie RI: Monitoring and source tracking of
tetracycline resistance genes in lagoons and groundwater
adjacent to swine production facilities over a 3-year period.
Appl Environ Microbiol 2007, 73:4813-4823.
39. Penders J, Stobbering EE: Antibiotic resistance of motile
aeromonads in indoor catfish and eel farms in the southern
part of The Netherlands.Int J Antimicrob Agents 2007,
31:261-265.
40. Jacobs L, Chenia HY: Characterization of integrons and
tetracycline resistance determinants in Aeromonas spp.
isolated from South African aquaculture systems.Int J Food
Microbiol 2007, 114:295-306.
41. Demane
´che S, Sanguin H, Pote
´J, Navarro E, Bernillon D,
Mavingui P, Wildi W, Vogel TM, Simonet P: Antibiotic-resistant
soil bacteria in transgenic plant fields.Proc Natl Acad Sci U S A
2008, 105:3957-3962.
42.

Brain RA, Hanson ML, Solomon KR, Brooks BW: Aquatic plants
exposed to pharmaceuticals: effects and risks.Rev Environ
Contam Toxicol 2008, 192:67-115.
This paper deals with larger ecological and environmental effects of
pharmaceuticals, and particularly antibiotics.
43. Yamashita N, Yasojima M, Nakada N, Miyajima K, Komori K,
Suzuki Y, Tanaka H: Effects of antibacterial agents,
levofloxacin and clarithromycin, on aquatic organisms.Water
Sci Technol 2006, 53:65-72.
44. Fajardo A, Martı
´nez JL: Antibiotics as signals that trigger
specific bacterial responses.Curr Opin Microbiol 2008,
11:161-167.
45. Linares JF, Gustafsson I, Baquero F, Martinez JL: Antibiotics as
intermicrobial signaling agents instead of weapons.Proc Natl
Acad Sci U S A 2006, 103:19484-19489.
46. Fajardo A, Martı
´nez-Martı
´n N, Mercadillo M, Gala
´n JC, Ghysels B,
Matthijs S, Cornelis P, Wiehlmann L, Tu
¨mmler, Baquero F,
Martı
´nez JL: The neglected intrinsic resistome of bacterial
pathogens.PLoS ONE 2008, 3:e1619.
This paper reveals the wealth of genes potentially involved not only in
antibiotic resistance in bacterial pathogens but also in environmental
organisms, genes able to contribute to the evolution of antibiotic
resistance.
47. Capdevielle M, van Egmond R, Whelan M, Versteeg D, Hofmann-
Kamensky M, Inauen J, Cunningham V, Woltering D:
Consideration of exposure and species sensitivity of triclosan
in the freshwater environment.Integr Environ Assess 2008,
4:15-23.
48. Maul JD, Schuler LJ, Beiden JB, Whiles MR, Lydy MJ: Effects of
the antibiotic ciprofloxacin on stream microbial communities
and detritivorous macroinvertebrates.Environ Toxicol Chem
2006, 25:1598-1606.
49. Quinteira S, Peixe L: Multiniche screening reveals the clinically
relevant metallo-beta-lactamase VIM-2 in Pseudomonas
aeruginosa far from the hospital setting: an ongoing
dispersion process? Appl Environ Microbiol 2006, 72:3743-3745.
50. Martı
´nez JL, Baquero F, Andersson DI: Predicting antibiotic
resistance.Nat Rev Microbiol 2007, 5:958-965.
Antibiotics and antibiotic resistance in water environments Baquero, Martı
´nez and Canto
´n 265
www.sciencedirect.com Current Opinion in Biotechnology 2008, 19:260–265