Process Biochemistry 38 (2002) 49 /56
www.elsevier.com/locate/procbio
Aerobic degradation of dichlorodiphenyltrichloroethane (DDT) by
Serratia marcescens DT-1P
Rajkumar Bidlan, H.K. Manonmani *
Department of Food Microbiology, Central Food Technological Research Institute, Mysore-570 013, Karnataka, India
Received 17 August 2001; received in revised form 28 August 2001; accepted 19 February 2002
Abstract
Microbial degradation of 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT)-residues is one of the mechanisms for the removal
of this compound from the environment. A DDT-degrading consortium was isolated by long term enrichment of soil samples
collected from DDT-contaminated fields. This consortium was acclimated by repeated passages through a mineral salt medium
containing increasing concentrations of DDT. This acclimated consortium could degrade 25 ppm of DDT in 144 h. The consortium
consisted of four bacteria. Of these, Serratia marcescens DT-1P was used for further studies. Various factors such as inoculum size,
concentration of DDT, pH, temperature, presence of co-substrates, the type of carbon source used influenced the degradation of
DDT in shake flasks. Complete degradation was observed up to 15 ppm DDT, followed by inhibitory effects at higher
concentrations showing a total loss of degradative ability at 50 ppm DDT. Effective degradation of DDT was obtained with the
inoculum pre-exposed to DDT for 72 h. Degradation was inhibited in the presence of auxiliary carbon sources such as citrate, rice
straw hydrolysate. However, the presence of yeast extract, peptone, glycerol and tryptone soya broth (TSB) showed complete
disappearance of DDT. Mesophilic temperatures (26 /30 8C) and near neutral pH (6.0 /8.0) were most favourable for degradation.
This microbial culture holds the potential for use in bioremediation of DDT-contaminated soils, waste deposits and water bodies.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Dichlorodiphenyltrichloroethane; Biodegradation; Serratia marcescens ; Bioremediation; Acclimation
1. Introduction
1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT)
is one of the most extensively used organochlorine
pesticides. Though the use of DDT was banned in
several advanced countries in the 1970s, India and many
other developing countries continue to use this pesticide
for the control of insect parasites causing malaria,
plague, dengue etc. It is only recently that the agricultural use of DDT has been banned in India. It is still
used in public health programmes as no efficient
alternative to DDT is available. Thus our environment
has been heavily polluted due to the extensive use of
DDT since the 1950s. DDT and its degradation
products have been detected in water, soil and air
[1,2]. Since DDT residues are lipophilic, they tend to
* Corresponding author. Tel.: 91-821-517539; fax: 91-821517233.
E-mail address: [email protected] (H.K. Manonmani).
accumulate in the fatty tissues of the ingesting organisms along the food chain. Almost all the foodstuffs
including processed foods have been shown to contain
high levels of DDT residues [3 /5]. High levels of DDT
and its metabolites have been detected in human adipose
tissues, blood plasma, liver, brain, placenta and even in
breast milk [6 /9]. DDT residues in water and soil are of
concern as their uptake can lead to the accumulation of
primary products. Their removal from water and soil is
therefore a priority. DDT residues have been shown to
persist in the environment predominantly in the form of
DDT;
1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene
(DDE);
1,1-dichloro-2,2-bis(4-chlorophenyl)ethane
(DDD);
1,1,1-trichloro-2-o -chlorophenyl-2-p-chlorophenylethane (o,p ?-DDT). DDD and DDE are the
transformation products of DDT [10], formed due to
microbial action [11] or due to chemical or photochemical reactions [12 /17].
Mechanisms of microbial attack have been described.
Most reports indicate the reductive dechlorination of
0032-9592/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 2 - 9 5 9 2 ( 0 2 ) 0 0 0 6 6 - 3
50
R. Bidlan, H.K. Manonmani / Process Biochemistry 38 (2002) 49 /56
DDT to DDD under reducing conditions [18 /21].
Nadeau et al. [22] has reported degradation of DDT
under aerobic conditions. In this paper we report the
degradation of DDT residues in shake flasks and a study
of the factors affecting the degradation of DDT.
2. Materials and methods
DDT, 5 through 25 ppm. The cells harvested from the
previous batch were used as inoculum to start a fresh
batch with or without increasing the substrate concentration. At each step, complete degradation of DDT in
terms of 100% Cl release and absence of residual
substrate, was assured before going to the next higher
concentration. The microbial consortium thus developed was maintained on M4-agar plates/slopes containing 5 ppm DDT.
2.1. Chemicals
DDT, 99% pure, was purchased from Sigma-Aldrich
Chemical, MO, USA. o -Tolidine and nutrient agar were
purchased from Hi Media, Mumbai, India. Wheat bran
and rice straw were obtained from local market. All
other chemicals used in the study were of analytical
grade and were purchased from standard manufacturers.
2.2. Cultures and media
Microbial consortium used in this study was developed in the laboratory as described later in this section.
The basal mineral medium (M4) used for growing the
consortium and the individual isolates consisted of (per l
of distilled water) 0.675 g KH2PO4; 5.455 g Na2HPO4;
0.25 g NH4NO3; 0.2 g MgSO4 ×/ 7H2O; 0.1 g Ca (NO3)2
and 1.0 ml of trace mineral solution. Trace mineral
solution contained (mg/ml)*/FeSO4 ×/ 7H2O, 1.0;
MnSO4 ×/ H2O, 1.0; CuCl2 ×/ 2H2O, 0.25; Na2MoO4 ×/
2H2O, 0.25; H3BO3, 0.1 and conc. H2SO4, 5.0 ml. The
pH of the medium was 7.5 (stocks of MgSO4, Ca(NO3)2
and trace minerals were separately autoclaved and
added after cooling).
Nutrient agar (NA) containing (g/l of distilled water)
peptone 5.0; beef extract 3.0; NaCl 5.0 and agar /agar
20.0 was used for isolating and purifying individual
bacterial members of the consortium.
All the media were sterilized by autoclaving at 121 8C
for 20 min.
2.3. Isolation, acclimation and maintenance of microbial
consortium
The microbial consortium capable of degrading DDT
was developed by shake flask enrichment of DDTcontaminated soil. An aqueous suspension of the soil
sample was inoculated to M4 medium containing 5 ppm
DDT as a sole carbon source. The culture was regularly
transferred, at weekly intervals, to fresh medium. After
10 /15 transfers, a mixed microbial population was
established, as evidenced by an increase in turbidity
and the ability to degrade 5 ppm of DDT. This
consortium was maintained on minimal agar containing
5 ppm DDT and 1/50 nutrient broth. This consortium
was gradually acclimated to increasing concentrations of
2.4. Resolution of the consortium into individual strains
The individual microbial strains of the acclimated
consortium were resolved on NA by plating appropriately diluted samples of 48 h old cultures, grown with 25
ppm DDT. Four morphologically distinct bacterial
strains were isolated and were purified by repeated
plating. The bacteria were identified using Microbact
Gram Negative Identification System from Medvet
Science Pty; Australia.
2.5. Degradation of DDT
All experiments on the degradation of DDT were
carried out in triplicates. The data presented are an
average of these three values.
The required quantity of DDT as an acetone solution
was dispensed into sterile 250 ml capacity Erlenmeyer
flasks and the acetone was evaporated at room temperature by keeping the flasks open in a UV-sterilized
laminar hood. Sterile M4 medium (25 ml) was added to
each flask which was then inoculated with washed cell
suspension of the consortium or individual isolates,
grown previously in 0.5% glucose /0.2% yeast extract
containing 5 ppm DDT for 72 h. The flasks were
incubated on a rotary shaker (150 rpm) at ambient
temperature (26 /30 8C) unless otherwise indicated.
Samples were removed at regular intervals for the
determination of growth, inorganic chloride, residual
DDT and intermediary metabolites, if any. Uninoculated flasks served as controls.
To study the effect of pH on degradation of DDT (10
ppm), the acclimated cells were inoculated to basal
medium of pH values between 4.0 and 8.0. Acetate
buffer was used for pH values up to 6.0 and phosphate
buffer from 7.0 to 8.0. Flasks were incubated at ambient
temperature on a rotary shaker for 48 h.
Effect of incubation temperature (from 4 through
50 8C) on degradation of DDT (10 ppm) was studied by
maintaining the flasks under stationary condition for 48
h.
The necessity of induction for the degradation of
DDT was studied by inducing the cells for 24, 48 and 72
h by either adding DDT (5 ppm) in the beginning of
incubation period or by incremental addition of DDT (5
R. Bidlan, H.K. Manonmani / Process Biochemistry 38 (2002) 49 /56
ppm) at intervals of 24 h. These induced cells were
washed and inoculated to 10 ppm DDT.
The effect of growth of DDT degrading isolate on
different carbon sources and degradation of DDT was
studied by growing cells in different carbon sources at
0.5% level for 24 h in a rotary shaker at ambient
temperature and the washed cells were inoculated to 10
ppm DDT.
Co-metabolic degradation of DDT was studied by
adding different carbon sources at 0.5% level to the M4
medium containing 10 ppm DDT.
Wheat bran hydrolysate (WBH) and rice straw
hydrolysate (RSH) were prepared by cooking wheat
bran or rice straw with 6 N H2SO4 for 15 min with
constant stirring. pH of the hydrolysate was adjusted to
7.5 with NaOH and centrifuged.
2.6. Analytical methods
Inorganic chloride was estimated by a modified
HNO3 /AgNO3 method of Frier [23]. A total of 1 ml
culture broth was centrifuged and the supernatant
placed in a test tube. The cells were washed in 0.1 N
NaOH (50 ml) and 950 ml mineral medium. The supernatants were pooled with the above supernatant. A total
of 1 ml each of 0.15 N HNO3 and 0.1 N AgNO3 were
added to the supernatant with mixing at each step.
Turbidity was measured at 600 nm after standing for 20
min at room temperature. The amount of chloride was
computed from a standard curve prepared for NaCl in a
similar way.
Growth of the consortium/individual isolates was
determined by estimating total protein in the biomass
by a modified method of Lowry et al. [24] as follows:
Cells were harvested from a suitable quantity of culture
broth, washed with M4 medium, suspended in 3.4 ml
distilled water and 0.6 ml of 20% NaOH. This was
mixed and digested in a constant boiling water bath for
10 min. Total protein, in cooled sample of this hydrolysate, was estimated by using Folin /Ciocalteau reagent. A total of 0.5 ml of the hydrolysate was taken in a
clean test tube. To this was added 5.0 ml of Lowry’s C.
After 10 min 0.5 ml of Lowry’s D [Folin /Ciocalteau
reagent (1:2)] was added and mixed well. The colour was
read using a spectrophotometer (Shimadzu UV 160 A,
Japan) at 660 nm after 20.0 min of standing at room
temperature. Total amount of protein was computed
using the standard curve prepared with BSA. Reducing
sugar was estimated by the method described by Miller
[25] .
Residual DDT was estimated by thin layer chromatography (TLC). The acidified culture broth was extracted thrice with equal volumes of ethyl acetate and
the solvent layers were pooled after passing through
anhydrous sodium sulphate. The sample was concentrated by evaporating the solvent layer and re-suspend-
51
ing in a known volume of acetone. A known amount of
this acetone solution was spotted on Silica Gel-G TLC
plates and these plates were developed in cyclohexane.
Residual DDT spots were identified after spraying the
air-dried plates with 2% o -tolidine in acetone. The
residual substrate spots were delineated by marking
with a needle and the area measured. The concentration
was computed from a standard plot of log concentration
versus square root of the area prepared for standard
DDT.
Residual DDT was also determined by gas chromatography (GC). The extract in ethyl acetate was
evaporated to dryness after passage through florisil
column. Moisture was removed by adding solid
Na2SO4 and evaporated to dryness. The residue was
re-dissolved in known volume of acetone. A Known
quantity of this was injected into GC (Fisons 8000) with
Ni63 electron capture detector. The conditions used
were: SS column (200 cm /2 mm) packed with 1.5%
OV-17 plus QF-1 on 80 /100 mesh chromosorb W. The
column and injector were maintained at 230 8C and the
detector was maintained at 320 8C. Flow rate of the
carrier nitrogen gas was 50 ml/min. Under these
conditions, the retention time of DDT was 7.4 min.
To detect whether any intermediary metabolites were
accumulating in the medium, the concentrated solvent
extract of the culture broth, prepared as above, was
subjected to TLC as well as to GC. GC was done as
described above. TLC of the concentrated solvent phase
was done using cyclohexane as mobile phase. Spots were
visualized with o -tolidine spray and exposure to sunlight. TLC of the concentrated culture broth (aqueous
phase) was done by using benzene:ethanol (19:1).
Developed plates were sprayed either with Folin /
Ciocalteau reagent for phenolic compounds or o tolidine for chloroaromatics. Chlorophenols, chlorobenzenes and catechol were used as reference standards.
3. Results
3.1. Isolation and acclimation of DDT-degrading
microbial consortium
After long (6 months) enrichment of the DDTcontaminated soil, a microbial consortium was enriched
in the shake flasks containing DDT as sole source of
carbon. The consortium thus obtained could degrade up
to 25 ppm DDT. A total of 5 ppm DDT, added as
acetone solution, completely disappeared within 48 h
with stoichiometric release of 100% chloride (Fig. 1).
This consortium was acclimated to higher concentrations of DDT as follows.
The cell biomass of consortium grown on 5 ppm DDT
was washed thoroughly with 0.1% (v/ v) tween 80 (for
dislodging the adsorbed chlorine from the cells), fol-
52
R. Bidlan, H.K. Manonmani / Process Biochemistry 38 (2002) 49 /56
No intermediary metabolites were detected either in
the solvent extract or in the aqueous phase of the culture
broths, from the flasks containing up to 25 ppm DDT,
either by TLC or by GC. However, the samples from 30
ppm DDT showed extra peaks in GC and blue spots in
o -tolidine sprayed TLC plates, indicating the accumulation of intermediary metabolites. (Data not shown.)
3.2. Resolution of microbial consortium and degradation
of DDT by individual strains
The acclimated consortium grown on 25 ppm DDT
was appropriately diluted and plated on NA. Four types
of bacteria were isolated and purified. All these bacterial
cultures were identified by Microbact Identification
System (Medvet Science Pty., Australia) as Serratia
marcescens DT-1P; Pseudomonas fluorescens DT-2;
Pseudomonas aeruginosa DT-Ct1; Pseudomonas aeruginosa DT-Ct2. Of these four bacterial isolates, Pseudomonas aeruginosa DT-Ct1 and Serratia marcescens DT1P were found to have efficient degradative ability
compared to other two isolates. Of these two, S.
marcescens was chosen for further studies, as no reports
are available with this organism.
3.3. Degradation of DDT by S. marcescens DT-1P
The bacterial isolate S. marcescens DT-1P, grown on
glucose-yeast extract medium, when inoculated to 5 ppm
DDT, the degradation was partial. Only 50% of the
initially added DDT was eliminated by 48 h. Growth of
the isolate remained constant up to 24 h and fell
drastically by 48 h. Chloride released was stoichiometrically almost equal to the substrate degraded. However, neither Cl nor growth increased thereafter. The
residual substrate concentration also remained constant
with no further signs of degradation (Fig. 2).
Fig. 1. Degradation of different concentrations of p,p ?-DDT by the
bacterial consortium. (a) Protein; (b) released chloride; (c) residual
DDT. -k-, 5 ppm DDT; - -, 10 ppm DDT; -^-, 15 ppm DDT; -I-,
25 ppm DDT.
lowed by a wash with M4 medium. This was used as
inoculum for the next batch either with the same or a
higher concentration of DDT, until the complete
degradation of the added substrate was obtained. This
acclimated consortium could degrade 10 ppm DDT in
72 h (Fig. 1). A total of 15 ppm DDT and 25 ppm DDT
were degraded by 96 and 144 h, respectively (Fig. 1). In
all cases the stoichiometric release of 100% chloride was
observed. When higher concentrations of DDT were
used, degradation was partial even after 240 h of
incubation (data not shown).
Fig. 2. Degradation of p,p ?- DDT by Serratia marcescens DT-1P
without pre-exposure. -^-, residual DDT; -k-, growth (protein); -m-,
released chloride.
R. Bidlan, H.K. Manonmani / Process Biochemistry 38 (2002) 49 /56
53
3.4. Pre-exposure and DDT degradation
Glucose-yeast extract grown cells, were pre-exposed
to 10 ppm DDT for 24, 48 and 72 h. Degradation of
DDT increased with increase in pre-exposure time. A
total of 80% of the substrate disappeared by 72 h of
incubation when the 48 h pre-exposed inoculum was
used. More than 35% of the substrate disappeared
without any lag period in 24 h and 64% of the remaining
substrate disappeared by next 24 h and 50% of then
remaining substrate disappeared by next 24 h. A total of
75% chloride was released by 72 h of incubation (data
not shown). Degradation of DDT improved with 72 h
pre-exposure. Nearly 90% of the substrate disappeared
by 72 h of incubation (Fig. 3).
3.5. Degradation of different concentrations of DDT by
S. marcescens DT-1P
The glucose-yeast extract grown, DDT-induced cells
of DT-1P could degrade low concentrations of DDT
completely. A total of 5, 10 and 15 ppm DDT
degradation was complete by 96, 100 and 120 h,
respectively (Fig. 4). Nearly 40% of the initially added
substrate disappeared by 24 h in all these cases. The
degradation of 20 ppm DDT was partial. Nearly 6% of
the substrate was found to be remaining even at 144 h of
incubation. Decrease in DDT degradation rate was
observed at 25 ppm level, followed by an inhibitory
effect at 50 ppm DDT level. However, accumulation of
intermediary metabolites was not observed at any time
during the degradation of DDT.
3.6. Inoculum size and DDT degradation
The degradation of DDT increased with increase in
inoculum size. Addition of inoculum at 2 mg protein/ml
to 10 ppm DDT resulted in only 45/65% degradation of
DDT by 72 h of incubation. With further increase of
inoculum level to 50, 100 and 200 mg protein/ml
facilitated the removal of 28, 45 and 50% more substrate
Fig. 4. Degradation of different concentrations of p,p ?-DDT by
Serratia marcescens DT-1P.
respectively in the same period. At 500 mg protein/ml
level, all the substrate was found to have disappeared by
72 h (Fig. 5).
3.7. Effect of pH and temperature on the degradation of
DDT by DT-1P
Fairly good growth was observed at a wide pH range.
Growth increased with increase in pH and reached
maximum at pH 7.0 and 7.5 and decreased thereafter.
At low pH values, i.e. at acidic pH, degradation was low
and even at alkaline pH degradation was low. Maximum degradation was at pH 7.5. Growth, residual
substrate and Cl release were found at a wide range of
temperature. Maximum growth occurred at 37 8C while
maximum degradation and chloride release was observed at 30 8C. (Data not shown.)
3.8. Effect of additional carbon sources
In flasks without co-substrate, the degradation of 10
ppm DDT was only 75% by the end of 72 h (Table 1). In
flasks containing acetate, succinate, glucose and sucrose,
45 /65% degradation was observed. Citrate showed least
effect with 98% of the added 10 ppm DDT still
remaining after 72 h of growth. Presence of glycerol,
peptone, yeast extract and tryptone soya broth (TSB)
showed complete disappearance of DDT by 72 h of
incubation period. Degradation of DDT even with
WBH and RSH as co-substrates was partial with 24
and 14% of DDT respectively remaining after 72 h of
incubation.
4. Discussion
Fig. 3. Pre-exposure to DDT and degradation of p,p ?- DDT by
Serratia marcescens DT-1P. Cells pre-exposed for: 1 24 h; 2 48
h; 3 72 h.
Excessive use of DDT in India and other developing
countries, in both agriculture and public health programmes, has led to widespread contamination of soil,
R. Bidlan, H.K. Manonmani / Process Biochemistry 38 (2002) 49 /56
54
Fig. 5. Effect of inoculum size on p,p ?- DDT degradation. -b-, released chloride (%); -j-, residual DDT (%).
Table 1
Degradation of 10 ppm of DDT by Serratia marcescens DT-1P in
presence of various carbon sources after an incubation period of 72 h
Carbon source
Percent degrada- Growth (mg/
tion
ml)
Acetate sodium salt
Succinate sodium salt
Citrate sodium salt
Glucose
Sucrose
Glycerol
Peptone
Yeast extract
Tryptone soya broth
Wheat bran hydrolysate
Rice straw hydrolysate
Control (without any additional
carbon source)
43.33
43.33
6.67
53.33
65.00
100.00
100.00
100.00
100.00
78.33
86.67
80.00
40.00
43.00
50.00
60.00
76.00
87.50
64.00
52.00
74.00
62.00
60.00
41.30
water and air. Residues of this chemical persists in soil
and in turn became a threat to human and animal
health. To eliminate these contaminants from the
environment, attempts have been made in our laboratory by isolating pesticide-degrading native microorganisms and improving their degradative ability. The
present microbial consortium is one such potent degrader of DDT that was isolated from DDT-contaminated
soil, by long-term shake flask-enrichment technique.
This consortium when acclimated with increasing concentrations of DDT, could degrade 5, 10, 15, and 25
ppm DDT by 48, 72, 96 and 144 h, respectively.
It can be assumed that continuous exposure of this
consortium to increasing concentrations of DDT improved its DDT-degrading ability. Similar observations
have been made in studies with HCH-degrading consortium, where acclimation improved the degradative
ability of HCH-degrading consortium [26]. The acclimated consortium showed a higher rate of HCHdegradation, with higher concentrations of HCH. Bhuyan et al. [27] and Wada et al. [28] have also made
similar observations, where g-HCH degradation improved after every successive application of the compound. However, in our studies, acclimation to still
higher concentrations did not show any further improvement in degradation rate. During acclimation, a
succession of microbial members had taken place
resulting ultimately in the survival of four members at
the end of acclimation period. Aislabie et al. [29] has
reported that it is difficult to isolate from the environment, microbes that can attack a compound co-metabolically. A technique known as analogue enrichment
was adapted by Bartha [30] in which a structural
analogue was substituted for the compound of interest.
Focht and Alexander [31], applied diphenyl methane, a
structural analogue of DDT to sewage samples and
isolated a DDT metabolizer, Hydrogenomonas sp. A
similar technique was adapted using 4-chlorobiphenyl,
another structural analogue of DDT to isolate DDT
degrading bacteria [22,32,33]. Among the four members
of the DDT-degrading microbial community, S. marcescens DT-1P was chosen for further studies. Many
Gram-negative and Gram-positive bacteria have been
reported to have metabolic capabilities of attacking
DDT. Alcaligenes eutrophus A5 [22], Hydrogenomonas
sp. [17,34], Pseudomonas putida [35] and fungi such as
Aspergillus niger , Penicillium brefeldianum [36] and
Phanerochaete chrysosporium [37] have been reported
R. Bidlan, H.K. Manonmani / Process Biochemistry 38 (2002) 49 /56
to degrade DDT. The minimum and maximum biodegradable concentration is an important factor. Some
biodegradative strains when inoculated into the environmental samples are unable to metabolize the pollutant. Among the reasons proposed for this observation
is that the presence of very low concentrations of the
substrate limits enzyme induction [29]. For some chemicals there is a threshold concentration below which
the biodegradation rate is negligible. Katayama et al.
[38] isolated two strains of bacteria, Bacillus sp. B75 and
an unidentified Gram-variable rod B116, which degraded DDT at extremely low level of 10 pg/ml. In
our studies S. marcescens DT-1P was able to degrade 5/
15 ppm DDT by 120 h and degradation of higher
concentrations was partial.
Environmental factors such as pH, temperature and
other substrates in the environment may affect the
growth of microorganisms and their degradative abilities. In the present study, 5% of added DDT was
degraded at 4 8C and as the temperature was increased,
the degradation rate increased. Thus the degradation of
DDT was observed at temperatures as low as 4 8C and
as high as 50 8C, maximum degradation of DDT
occurring at 30 8C, under stationary conditions. This
indicates that the isolate can be used at wide ranging
temperatures of different climatic regions. S. marcescens
DT-1P could degrade DDT at a pH range of 4 /8 with
an optimum pH 7.5. The isolate showed more tolerance
and degradation at acidic pH that increased towards
neutral pH. Information on the effect of pH on
degradation of DDT is rather scanty. However, reports
on degradation of other pesticides show the degradation
of HCH isomers in soils of acidic pH by a strain of S.
paucimobilis [39]. However, the degradation was very
poor in the soil with low pH of 2.98.
Carbon sources other than the target chemicals may
influence the degradation rates. The presence of sodium
acetate and sodium succinate inhibited the degradation
of endosulfan [40]. The presence of more favourable
carbon sources has been shown to impede the degradation of xenobiotics [41,42]. This could be due to
catabolic repression [41,43] or decrease in the rates of
transcription either due to supercoiling of promoter
DNA [44] or by decreased binding of transcription
factors [45]. A Gram-positive bacterium could degrade
DDT, DDD and DDE in the presence of biphenyl [40].
In our studies, the presence of glycerol, peptone, yeast
extract and TSB completely eliminated DDT. Other cosubstrates did not show much degradation with citrate
giving the least support.
Studies on the microbial degradation of DDT will be
useful for the development of methods for the remediation of the contaminated sites. In situ biodegradation
may be limited because of a complex set of environmental conditions. To enhance the biodegradation,
these engineered microorganisms may have to be
55
introduced into the contaminated soil. Knowledge of
the various optimized parameters would facilitate an
easy and more effective translation of the laboratory
results to the fields. Although the bioremediation of
DDT-contaminated sites is difficult, all avenues for
research have not been closed.
Acknowledgements
The authors thank Dr V. Prakash, Director, Central
Food Technological Research Institute, India for his
support to this work; Dr M.S. Prasad, Head, Food
Microbiology Dept., CFTRI, for his encouragement;
Mr Prakash, CIFS, CFTRI for his support in analytical
work; CSIR, New Delhi for financial assistance in the
form of Research Fellowship to Mr Rajkumar Bidlan.
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