good candidates to be used as inoculums for
bioaugmentation. Their inability to carry out the metabolic process completion does not allow them to convert
their metabolic peculiarity in a selective advantage favoring their implanting within indigenous soil microbial
community. However, several reports have shown that
these degrading strains can express their catabolic potential in complex microbial consortia in which each microorganism brings its metabolic competence to reconstruct
a coherent set of steps leading to the mineralization of the
pesticide (Smith et al. 2005; Kolic et al. 2007). For
atrazine biodegradation, the literature gives a number of
examples of such microbial associations where no individual species has a complete set of catabolic gene, but
together they have the full degrading pathway (UdikovicKolic et al. 2012). These associations have various taxonomic compositions (Gram + and Gram – negative bacteria) and have also various atrazine-degrading-genetic
combinations (atzABC, trzNatzBC, trzNatzBCtrzD, etc.)
(Martin-Laurent et al. 2006). To some extent, pesticidedegrading-microbial consortia might be considered more
resilient than pure degrading strain, as they are hosting
various metabolic pathways that may constitute a selective
advantage favoring their survival in oligotrophic
conditions found in the soil environment (Soulas 2003).
However, despite their ecological interest, until now the
recourse to the use of degrading-microbial consortia for the
bioaugmentation is compromised by two yet unresolved
questions. The first concerns a practical issue on how to
produce pesticide-degrading consortia inoculum having
stable taxonomic- and degrading-genetic compositions
for ensuring the maintenance of its degrading ability.
Indeed, the transience of pesticide-degrading ability
observed in pure strains portends major difficulties when
the degrading ability will have to be held in several strains
simultaneously. The second concerns their stability in the
natural environment where the selection pressure that
allowed their isolation and their maintenance under laboratory conditions may decrease and where ambient nutrient
conditions may drastically change with consequences on
the cohesion of the whole. This is an important point
knowing that the loss of a single member of the consortium
may be sufficient to stop all the degrading activity of the
functional unit.
However, it has to be noticed that beside technical
limitations to use living microorganisms for bioaugmentation to bioremediate pesticide-contaminated soils,
there are some limitations due to the regulations in particular in Europe. Indeed, prior to its commercial use, any
microorganism inoculum has to get an official agreement delivered by EU authorities after the constitution
of a homologation dossier made of several elements
including a deep toxicological analysis of the impact
of the inoculum on animal and human health. Several
studies report the use of dead microorganisms or cellfree enzymes to avoid this strict regulation. As an example, field-scale atrazine bioremediation has been achieved
using killed and stabilized transgenic E. coli (Strong et al.
2000). More recently, the use of cell-free TrzN (as a
DNA-free cell lysate) to treat a 1.5-ML dam reservoir
contaminated with atrazine was shown to be successful
(Scott et al. 2010). Interestingly, the encapsulation of
AtzA and of nonviable atrazine degraders was found to
be successful to treat atrazine-contaminated water under
laboratory conditions (Rea ´tegui et al. 2012).
• What solutions for the future?
To improve the efficiency of soil bioremediation and
remove obstacles sometimes found when trying to apply
this technology, solutions have been proposed. The most
immediate is that of improving the microbial-assisted
phytoremediation by associating bioaugmentation to
rhizostimulation. The beneficial effects of this double
treatment on atrazine biodegradation were demonstrated
long ago (Alvey and Crowley 1996). These authors
showed that maize seedlings increased atrazine mineralization catalyzed by a composite microbial inoculated to
the soil. A better survival of the composite inoculum was
reported in the maize rhizospheric soil as compared to the
bare soil (Alvey and Crowley 1996). This first observation was later on confirmed by several other studies
confirming that the activity of degrading-microbial
communities was stimulated in the maize rhizosphere
(Piutti et al. 2002). This process combining bioaugmentation to rhizostimulation where plant rhizosphere is
used both as vector exploration of the soil environment
and as source of carbon and electrons has not yet
been applied for field-scale bioremediation of atrazinepolluted sites. However, its feasibility was shown in a
recent study reporting the stimulation of the degradation of α-, β-, γ-, and δ-HCH in the rhizosphere of
maize inoculated with four Sphingomonas sp. strains
previously isolated from maize rhizospheric soil (Bo ¨ltner
et al. 2008).
Molecular ecology is offering some interesting
perspectives to the issue of the efficiency, the survival,
and/or the maintenance of the pesticide-degrading
populations inoculated to the contaminated site.
On atrazine, the possibility of introducing degrading
genes of different origins in a microbial host was
illustrated by the transfer in Rhodococcus sp. TE1, naturally possessing the atrA gene encoding enzyme
catalyzing N-dealkylations, the trzA gene from
Rhodococcus corallinus encoding enzyme dechlorinating
dealkylated derivatives of atrazine (Shao et al. 1995).
16 Applied Microbial Ecology and Bioremediation
703
bioaugmentation. Their inability to carry out the metabolic process completion does not allow them to convert
their metabolic peculiarity in a selective advantage favoring their implanting within indigenous soil microbial
community. However, several reports have shown that
these degrading strains can express their catabolic potential in complex microbial consortia in which each microorganism brings its metabolic competence to reconstruct
a coherent set of steps leading to the mineralization of the
pesticide (Smith et al. 2005; Kolic et al. 2007). For
atrazine biodegradation, the literature gives a number of
examples of such microbial associations where no individual species has a complete set of catabolic gene, but
together they have the full degrading pathway (UdikovicKolic et al. 2012). These associations have various taxonomic compositions (Gram + and Gram – negative bacteria) and have also various atrazine-degrading-genetic
combinations (atzABC, trzNatzBC, trzNatzBCtrzD, etc.)
(Martin-Laurent et al. 2006). To some extent, pesticidedegrading-microbial consortia might be considered more
resilient than pure degrading strain, as they are hosting
various metabolic pathways that may constitute a selective
advantage favoring their survival in oligotrophic
conditions found in the soil environment (Soulas 2003).
However, despite their ecological interest, until now the
recourse to the use of degrading-microbial consortia for the
bioaugmentation is compromised by two yet unresolved
questions. The first concerns a practical issue on how to
produce pesticide-degrading consortia inoculum having
stable taxonomic- and degrading-genetic compositions
for ensuring the maintenance of its degrading ability.
Indeed, the transience of pesticide-degrading ability
observed in pure strains portends major difficulties when
the degrading ability will have to be held in several strains
simultaneously. The second concerns their stability in the
natural environment where the selection pressure that
allowed their isolation and their maintenance under laboratory conditions may decrease and where ambient nutrient
conditions may drastically change with consequences on
the cohesion of the whole. This is an important point
knowing that the loss of a single member of the consortium
may be sufficient to stop all the degrading activity of the
functional unit.
However, it has to be noticed that beside technical
limitations to use living microorganisms for bioaugmentation to bioremediate pesticide-contaminated soils,
there are some limitations due to the regulations in particular in Europe. Indeed, prior to its commercial use, any
microorganism inoculum has to get an official agreement delivered by EU authorities after the constitution
of a homologation dossier made of several elements
including a deep toxicological analysis of the impact
of the inoculum on animal and human health. Several
studies report the use of dead microorganisms or cellfree enzymes to avoid this strict regulation. As an example, field-scale atrazine bioremediation has been achieved
using killed and stabilized transgenic E. coli (Strong et al.
2000). More recently, the use of cell-free TrzN (as a
DNA-free cell lysate) to treat a 1.5-ML dam reservoir
contaminated with atrazine was shown to be successful
(Scott et al. 2010). Interestingly, the encapsulation of
AtzA and of nonviable atrazine degraders was found to
be successful to treat atrazine-contaminated water under
laboratory conditions (Rea ´tegui et al. 2012).
• What solutions for the future?
To improve the efficiency of soil bioremediation and
remove obstacles sometimes found when trying to apply
this technology, solutions have been proposed. The most
immediate is that of improving the microbial-assisted
phytoremediation by associating bioaugmentation to
rhizostimulation. The beneficial effects of this double
treatment on atrazine biodegradation were demonstrated
long ago (Alvey and Crowley 1996). These authors
showed that maize seedlings increased atrazine mineralization catalyzed by a composite microbial inoculated to
the soil. A better survival of the composite inoculum was
reported in the maize rhizospheric soil as compared to the
bare soil (Alvey and Crowley 1996). This first observation was later on confirmed by several other studies
confirming that the activity of degrading-microbial
communities was stimulated in the maize rhizosphere
(Piutti et al. 2002). This process combining bioaugmentation to rhizostimulation where plant rhizosphere is
used both as vector exploration of the soil environment
and as source of carbon and electrons has not yet
been applied for field-scale bioremediation of atrazinepolluted sites. However, its feasibility was shown in a
recent study reporting the stimulation of the degradation of α-, β-, γ-, and δ-HCH in the rhizosphere of
maize inoculated with four Sphingomonas sp. strains
previously isolated from maize rhizospheric soil (Bo ¨ltner
et al. 2008).
Molecular ecology is offering some interesting
perspectives to the issue of the efficiency, the survival,
and/or the maintenance of the pesticide-degrading
populations inoculated to the contaminated site.
On atrazine, the possibility of introducing degrading
genes of different origins in a microbial host was
illustrated by the transfer in Rhodococcus sp. TE1, naturally possessing the atrA gene encoding enzyme
catalyzing N-dealkylations, the trzA gene from
Rhodococcus corallinus encoding enzyme dechlorinating
dealkylated derivatives of atrazine (Shao et al. 1995).
16 Applied Microbial Ecology and Bioremediation
703