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Journal of Clinical Microbiology, January 2001, p. 279-284, Vol. 39, No. 1
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.1.279-284.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Detection and Initial Characterization of Novel Capsular
Polysaccharide among Diverse Campylobacter jejuni
Strains Using Alcian Blue Dye
Andrey V.
Karlyshev and
Brendan W.
Wren*
Department of Infectious and Tropical
Diseases, London School of Hygiene and Tropical Medicine,
University of London, London WC1E 7HT, United Kingdom
Received 5 July 2000/Returned for modification 22 September
2000/Accepted 20 October 2000
 |
ABSTRACT |
We have recently demonstrated that most strains of
Campylobacter jejuni produce capsular polysaccharide (CPS),
which can be detected by immunoblotting with homologous Penner antisera
on polyvinylidene difluoride membranes (A. V. Karlyshev, D. Linton, N. A. Gregson, A. J. Lastovica, and B. W. Wren,
Mol. Microbiol. 35:529-541, 2000). In this report, we describe a
universal and rapid staining procedure using Alcian blue for C. jejuni CPS, which does not rely on the availability of antisera
and identifies CPS in untypeable strains. Furthermore, Alcian blue
staining identified CPS in its lipid-free form directly on Tricine
gels, and we demonstrate that CPS is thermostable and is accumulated in
the culture supernatant in a lipid-free form. The identification of a
newly described CPS and its lipid-free form in C. jejuni
should prove invaluable in studying the pathogenesis and epidemiology
of this important pathogen.
 |
INTRODUCTION |
Campylobacter jejuni is
the principal cause of food poisoning in the United States and other
developed countries (reviewed in reference 23).
C. jejuni is also increasingly recognized for its
association with the serious postinfection neurological complications
of Guillain-Barré syndrome (26). Significantly, prior infections with C. jejuni Penner serotypes O:19
and O:41 are more commonly associated with these devastating
neuropathies (11). The Penner serotyping system has been
widely adopted for the epidemiological analysis of C. jejuni and is based on passive hemagglutination using heat-stable
antigens, which were shown previously to consist of lipopolysaccharide
(LPS) (14). C. jejuni produces
lipooligosaccharide (LOS) and in some cases also high-molecular-weight polysaccharide, which in most cases could not be detected using the
silver staining technique (18). We have recently
demonstrated that, in contrast to previous reports, the
high-molecular-weight polysaccharide is ubiquitous in C. jejuni and is genetically and biochemically similar to capsular
polysaccharides (CPSs) in other gram-negative bacteria
(12). This was a surprising finding, as a capsule had not
previously been identified in C. jejuni using traditional staining techniques such as ruthenium red. C. jejuni CPSs from a range of strains could be detected by blotting
with homologous Penner antiserum. However, some C. jejuni strains, especially fresh clinical isolates, do not
interact with available Penner antisera and are untypeable, thus making
detection of CPS in these strains extremely difficult. Moreover, the
structure of the monomers of CPS can vary between different strains
(4), presumably due to the presence of at least five
hypervariable genes in the polysaccharide biosynthesis locus (12,
16).
Alcian blue has been used previously to stain and identify CPSs from
several bacterial species (1). We report that, in all nine
C. jejuni strains tested, the newly identified CPS was stained with Alcian blue dye. This rapid and reliable staining procedure does not depend on the availability of homologous antisera and, in addition, allows detection in culture supernatant of the lipid-free form of CPS. We demonstrate that, in contrast to the cell-bound CPS, the secreted molecules lack a phospholipid moiety. These data and the finding that deoxycholate (DOC) can inhibit conversion of the CPS into a lipid-free form suggest the presence of a
phospholipase distinct from that of outer membrane-bound detergent-resistant phospholipase PldA. This was further supported by
the analysis of CPS production in defined C. jejuni
pldA mutants.
 |
MATERIALS AND METHODS |
Bacterial strains and growth conditions.
C.
jejuni strain X is a recent isolate from a patient with enteritis;
strain G1 is an isolate from a Guillain-Barré syndrome patient.
Other strains are described in the legend to Fig. 1. The strains were
grown at 37°C on blood agar plates (Columbia agar [Oxoid,
Basingstoke, United Kingdom], supplemented with 7% horse blood) or on
a minimal broth medium (MEM Alpha medium; Gibco BRL, Life Technologies,
Paisley, United Kingdom) with a growth supplement (Oxoid) in a
microaerobic atmosphere for 2 days. Medium for growth of the mutant
strain G1 kpsM::Kanr was supplemented
with kanamycin at a concentration of 50 µg ml
1.
Preparation of CPS samples.
Crude extracts of C. jejuni CPS were prepared from all strains essentially as described
previously (10). Bacteria from 2-day blood agar plates
were directly solubilized in 100 µl of lysis buffer containing 31.25 mM Tris-HCl (pH 6.8), 4% sodium dodecyl sulfate, 0.025% bromophenol
blue, and 20% glycerol. Samples were heated at 100°C for 5 min.
Twenty-microliter aliquots of the supernatant were removed to a fresh
tube, to which 1 µl of a 20-mg/ml solution of proteinase K
(Sigma) was added, and the mixture was incubated at 50°C for 1 h. Cell extracts were prepared in a similar way, except that no boiling
was used and 1 volume of the lysis buffer was added to the samples
prior to proteinase K treatment.
In order to prepare fractions, bacteria were treated subsequently with
saline at room temperature and at 50°C. One loop of bacteria
from 2-day blood agar plates was washed with 0.5 ml of saline;
supernatant was collected (F1); the pellet was resuspended in 0.5 ml of
saline, incubated at 50°C for 1 h, and centrifuged, and
supernatant (F2) and pellet (P) were collected. Supernatant obtained
after growth in minimal medium (10 ml) was filter sterilized using
0.2-µm-pore-size membranes (Pall, Portsmouth, United Kingdom), concentrated using the Centriplus 3 system (Millipore, Watford, United
Kingdom) to 300 µl, diluted to 5 ml, and concentrated again to the
final volume of 250 µl followed by dialysis against distilled water.
Electrophoresis and Western blotting.
The samples were
fractionated on 12.5% Tricine-sodium dodecyl sulfate-polyacrylamide
gels (20) using a Hoefer minigel system (Pharmacia
Biotech, St. Albans, United Kingdom). Prestained protein markers (New
England BioLabs) were used. Following electrophoresis, the gels were
stained with either Alcian blue (0.1% Alcian blue in 40% ethanol-5%
acetic acid) or silver (22). In the combined Alcian
blue-silver staining procedure, the gels were initially stained with
Alcian blue.
CPS and LOS were detected in Western blotting experiments using a
polyvinylidene difluoride (PVDF) membrane (Millipore) and
a semidry
electroblotting apparatus (Hoefer; Pharmacia Biotech).
Blots were
blocked for 1 h at ambient temperature in Tris-buffered
saline
containing 0.01% Tween 20 (TBST) and 3% skimmed milk, followed
by
incubation for 1 h with Penner antiserum at a dilution of 1:100
in
TBST containing 1% bovine serum albumin, washed three times
in TBST,
and incubated for a further hour with peroxidase-labeled
anti-rabbit
immunoglobulin G (Sigma-Genosys, Pampisford, United
Kingdom) diluted
1:1,000 in TBST-bovine serum albumin. Following
washing as described
above, blots were developed using the diaminobenzidine
staining kit
with nickel enhancement according to the manufacturer's
instructions
(Vector Laboratories, Burlingame, Calif.).
Western blotting for the detection of lipid A and
3-deoxy-
D-manno-octulosonic acid (Kdo) moieties was
performed essentially
as described above according to the protocol
described in reference
15. For the detection of
lipid A, the membranes were incubated
in 1% acetic acid for 1 h.
After treatment with the corresponding
monoclonal antibodies (MAbs),
the membranes were incubated with
a 1:1,000 dilution of
peroxidase-labeled anti-mouse immunoglobulin
G (Sigma-Genosys), and the
blots were developed as described
above.
Construction of defined pldA mutants.
PCRs were
performed using 1 U of Taq polymerase (Gibco BRL, Life
Technologies) in 20-µl volumes containing 0.1 µg of primers and DNA
at 94°C for 1 min and 25 cycles of 94°C for 45 s, 50°C for
45 s, and 72°C for 2 min, followed by 7 min of extension at 72°C. The PCR products were purified using S300 microspin
columns (Bio-Rad, Hemel Hempstead, United Kingdom) and/or analyzed on agarose gels. A 0.9-kb fragment of the NCTC 11168 pldA gene
was PCR amplified using primers ak113 (GGC TAG TGA TTT ACA ACA AGC) and
ak114 (CCA GTG GAA AGT CTT TGC AAG TG) and cloned into vector pGEM-T
Easy (Promega, Southampton, United Kingdom). DNA extracted from one of
the recombinant clones was digested with BglII enzyme and
ligated with a BamHI fragment containing a Kanr
cassette (21). After transformation into Escherichia
coli XL2-Blue (Stratagene, Amsterdam, The Netherlands),
Kanr clones were selected and analyzed for the presence of
the inserts using PCR with primers ak113 and a Kanr
cassette-derived primer, DL4 (TGT TGC TGT CTC CCA GGT CG). DNA recovered from clones with a Kanr cassette oriented in the
same direction as the pldA gene was used for electroporation
of strains X and G1 as described previously (24). Allelic
replacement in the transformants was confirmed using PCR with ak113 and
ak114 primers.
Treatment with phospholipases.
Treatment with a mixture of
phospholipase C (type IX from Bacillus cereus
[Sigma-Genosys]), phospholipase A2 (from bee venom [Sigma-Genosys]), and phospholipase D (type I from cabbage
[Sigma-Genosys]) was performed as described previously
(12). One unit of each phospholipase was added to the
samples adjusted to 50 mM Tris-HCl (pH 8.0) and incubated for 1 h
at 30°C followed by incubation at 37°C for 12 to 14 h.
 |
RESULTS |
Detection of C. jejuni CPS using Alcian blue
dye.
Figure 1 shows the use of
Alcian blue to detect CPS in nine C. jejuni strains,
including the untypeable strain X (Fig. 1, lane 9). The size and
intensity of the CPS band vary between the strains. For example, the
intensity of the CPS band of strain NCTC 11168 (Fig. 1, lane 7) is
usually much weaker than that of strain 81116 (lane 6) and G1 (lane 8).
Strain G1, a clinical isolate from a patient with Guillain-Barré
syndrome, was chosen for further analysis.

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FIG. 1.
Detection of CPS using Alcian blue in different
C. jejuni strains. Lanes: 1, NCTC 12501 (HS:2); 2, NCTC
12502 (HS:3); 3, NCTC 12561 (HS:4); 4, NCTC 12509 (HS:10); 5, NCTC
12517 (HS:19); 6, 81116 (NCTC 11828, HS:6, 7); 7, NCTC 11168 (HS:2); 8, G1 (HS:1); 9, × (untypeable); 10, prestained markers.
|
|
CPS is thermostable and can be selectively extracted after
incubation of cells at 50°C.
Production of C. jejuni extracellular proteins was found to be stimulated by the
addition of normal horse serum (19). As at least some
extracellular proteins are essential for bacterial pathogenesis
(13), and due to a possible biological role of CPSs, we
hypothesized that these molecules could be coinduced with other
virulence factors. The effects of growth temperature and the addition
of horse blood, horse serum, and lysed blood to Columbia base agar were
tested. None of the additives had any stimulatory effect on the
synthesis of CPS (Fig. 2A). Similar results were obtained for cultures grown at 42°C (data not shown). In
subsequent experiments, bacteria were routinely grown at 37°C on
Columbia agar supplemented with horse blood.

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FIG. 2.
Analysis of CPS extracted from C. jejuni
strain G1. (A) Blotting with Penner 1 antiserum. Shown are effects of
supplements to the Columbia agar base medium on CPS production at
37°C. Lane 1, no supplements; lane 2, normal horse serum; lane
3, horse blood; lane 4, lysed horse blood. (B to D) Efficiency of CPS
staining using blotting with Penner 1 antiserum (B), Alcian blue (C),
and Alcian blue followed by silver staining according to the method of
Tsai and Frasch (22) (D). Lanes 1, supernatant of cells
washed with saline; lanes 2, 50°C extract; lanes 3, lysate; lane 4, prestained markers.
|
|
CPS was found to be relatively tightly bound to the bacteria and could
not be released by vortexing in saline at room temperature
(Fig.
2B,
lane 1). However, after incubation of bacterial suspension
at 50°C in
saline, the polysaccharide appeared in the supernatant
with very little
contamination from LOS (Fig.
2B, lane 2), the
latter being mainly
associated with the bacteria (Fig.
2B, lane
3). This fraction is
designated F2 (fraction 2). The results demonstrate
that the highest
yield and purity of CPS can be achieved by saline
extraction at 50°C
and are in agreement with previously reported
data (
6).
Filter sterilization of the extracts using 0.2-µm-pore-size
membranes
significantly reduced contamination with LOS without
an effect on CPS
(data not shown). This is probably due to the
tendency of LOS to form
micelles. Although both CPS and LOS can
be stained with Alcian blue,
the CPS/LOS signal ratio is much
higher with the Alcian blue staining
procedure (Fig.
2C) than
with Western blotting (Fig.
2B).
Alcian blue staining followed by prolonged silver staining using the
Tsai-Frasch procedure (
22) can increase the intensity
of
the CPS band with significant increase of the background level
(Fig.
2D). No CPS could be detected with silver staining alone
(data not
shown). No changes in the intensity of the CPS band
and no accumulation
of a possible lipid-free product of hydrolysis
could be detected after
incubation of samples at 100°C (Fig.
3),
confirming that these molecules are
thermostable, as one would
expect given the heat treatment used in the
Penner serotyping
procedure (
17).

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FIG. 3.
Thermal stability of the C. jejuni G1
CPS. Western blotting with Penner 1 antisera of fraction F2 (see text)
after incubation at 100°C for 0 (lane 1), 5 (lane 2), and 10 (lane 3)
min. Lane 4 shows prestained markers.
|
|
Phospholipid moiety, but not polysaccharide chain, is responsible
for membrane binding.
We previously reported that after treatment
with phospholipases CPS could not be detected by Western blotting
(12). Removal of the phospholipid could have prevented the
molecules from either entering the Tricine gel or binding to PVDF
membranes. To determine whether lipid-free CPS (CPS-PL) enters the gel,
we stained membranes with Alcian blue. As shown in Fig.
4, both CPS (lane 1) and CPS-PL (lane 2)
can be visualized, indicating that the CPS-PL does migrate into Tricine
gels but seems to have lost its ability to bind hydrophobic PVDF
membranes, probably due to the absence of a hydrophobic lipid moiety.

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FIG. 4.
Gel detection of C. jejuni G1 CPS (lane
1) and its lipid-free form CPS-PL (lane 2) by a combination of Alcian
blue-silver staining procedures. S. enterica serovar
Typhimurium LPS is present in lane 3 for comparison. Lane 4 shows
prestained markers.
|
|
The staining properties of Alcian blue are considered to be associated
with the interaction of this cationic dye with negatively
charged
macromolecules (
5). The data presented here demonstrate
that the polysaccharide moiety of CPS binds Alcian blue, suggesting
that these molecules are also negatively charged. A polysaccharide
extraction procedure based on reversible binding of negatively
charged
polysaccharides, containing sulfate groups, to positively
charged
membranes has been described previously (
1). To determine
if this method could also be used for purification of
C. jejuni CPS, we investigated binding of these molecules to
positively
charged Hybond N
membranes.
Hybond N can effectively bind CPS (Fig.
5A, lane 1). However, it was not possible
to extract bound polysaccharide even in
the presence of as high a
concentration of sodium chloride as
5 M. Interestingly, no
binding of lipid-free CPS to the membrane
could be detected (Fig.
5A, lane 2). The results suggest that
hydrophobic rather than
electrostatic interaction is a major factor
for binding CPS to the
membrane and that the repeating units do
not contain strongly acidic
groups such as sulfates and phosphates
which are required for
electrostatic interaction with the positively
charged membrane.

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FIG. 5.
Effect of phospholipase treatment on binding of CPS to a
Hybond N membrane. After blotting, the membrane was stained with Alcian
blue as described in the text. (A) Lane 1, CPS; lane 2, prestained
markers. (B) Lane 1, CPS-PL; lane 2, prestained markers.
|
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CPS is accumulated in the culture medium in a lipid-free form.
It was observed that CPS recovered from culture supernatant of strain
G1 grown in the absence of DOC migrates faster (Fig. 6A, lane 1) than cell-associated CPS
(Fig. 6A, lane 7). However, in the presence of DOC in the medium
extracellular and cell-bound CPSs have similar sizes (Fig. 6A, lanes 3 and 7). Brief treatment of the culture with DOC also results in the
accumulation of the full-sized CPS (Fig. 6A, lane 5). Treatment of the
CPS from the culture supernatant after growth in the presence of DOC
(Fig. 6C, lane 2) results in the formation of a form similar to that produced in the absence of DOC (Fig. 6C, lanes 1 and 4). This experiment suggests that a smaller size of extracellular CPS is the
result of removal of a phospholipid moiety by a bacterial phospholipase.

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FIG. 6.
Production of extracellular and cell-bound CPSs by
C. jejuni grown on minimal medium. (A and B) The gel
was stained with either Alcian blue (A) or Alcian blue and silver (B).
Lanes 1, 3, 5, and 7, wild-type strain; lanes 2, 4, 6, and 8, kpsM mutant (control); lanes 1 to 6, culture supernatant;
lanes 7 and 8, total cell lysate; lanes 1 and 2, no DOC present; lanes
3 and 4, DOC present in the growth medium; lanes 4 and 5, DOC added
3 h before the end of incubation. A sharp ~18-kDa band is
present in all lanes in panel B and comigrates with CPS-PL,
corresponding to proteinase K. (C) Effect of phospholipases on the
product (CPS) accumulated in the supernatant in the presence of DOC
(from panel A, lane 3). Lane 1, CPS-PL (control); lane 2, CPS; lanes 3 and 4, limited and complete hydrolysis with PL, respectively; lane M,
prestained markers.
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An outer membrane phospholipase, PldA, is not involved in the
conversion of the CPS to its lipid-free form in liquid culture.
A
phospholipase A (PldA), belonging to a family of bacterial enzymes
found in several bacteria, has been genetically and biochemically characterized for the related species Campylobacter coli
(9). However, no biological function could be assigned to
this enzyme (7). The gene was also found in C. jejuni strain NCTC 11168 (16). To determine if this
enzyme is responsible for processing CPS, we constructed defined
pldA mutants for strains G1 and X. Interestingly, all
extracellular CPSs accumulated in the supernatant of both mutants grown
in liquid medium were present in the lipid-free form, as in the
wild-type strains (data not shown), indicating that PldA is not
involved in the processing of CPS.
 |
DISCUSSION |
We recently demonstrated that the previously described O antigen
in many C. jejuni strains is CPS and is a major
component of the Penner antigen used for serotyping C. jejuni (12). In this study, Alcian blue dye was used
to stain the newly identified CPS in all C. jejuni
strains tested. This rapid and reliable staining procedure, which does
not depend on the availability of homologous antisera, identified CPS
in untypeable strains and allowed detection of the lipid-free form of CPS.
The Penner serotyping procedure involves passive hemagglutination based
on extracts of bacterial suspensions heated at 100°C (14). The phosphodiether linkage in many CPSs was found to
be heat labile (25). Our finding demonstrates that this is
not the case for C. jejuni CPS. Resistance of the CPS
to heat treatment implies that the sugar residue linking polysaccharide
to the phospholipid moiety is different from those present in other
CPSs. The reason for the heat stability could be the lack of Kdo
linkage to the phospholipid moiety, which would make the molecule
unstable. The thermal stability of C. jejuni CPS makes
it an unusual representative of type II or type III CPSs, which are
invariably thermolabile (25).
MAbs against E. coli lipid A (MAb 43) and branched-terminal
Kdo residues (MAb 20) failed to cross-react with C. jejuni CPS (data not shown). By contrast, C. jejuni intermediates of LOS containing a terminal Kdo
cross-reacted with MAb 20, as well as LPS of Salmonella
enterica serovar Typhimurium, generating a typical ladderlike
pattern after interaction with MAb 43. Although these results were
inconclusive regarding the presence or absence of a lipid A in CPS, the
ability to detect a lipid-free product after phospholipase treatment
using Alcian blue dye is a further demonstration that CPS is
substituted with a phospholipid, rather than with lipid A.
The present study suggests that C. jejuni CPS can be
released from the cell surface presumably due to the action of a
phospholipase which is independent of PldA, suggesting the presence of
a further phospholipase in C. jejuni. Production of a
second phospholipase by the related species C. coli has
been postulated by Grant et al. (9). In contrast to
strains G1 and X, no extracellular form of CPS could be detected in
strain NCTC 11168 (data not shown), suggesting the absence of an
additional phospholipase in this strain. This is consistent with the
absence of a second identifiable phospholipase gene in the NCTC 11168 genome sequence (16).
Our data demonstrate that the CPS can be released from the cell surface
in either native or lipid-free form depending on the presence or
absence of DOC. DOC is a component of bile salts found in large
quantities in the intestine of birds and mammals where C. jejuni resides. DOC and bile salts have been reported previously to regulate the production of surface appendages in
Campylobacter species and may have an effect on the
expression and processing of CPS in vivo (8). The role of
the released extracellular polysaccharide is unclear, but it may be
important for the survival and pathogenesis of enteric
Campylobacter in the host environment.
It has been shown previously that the binding of the cationic dye
Alcian blue to polysaccharides is due to the presence of negative
charges on the macromolecules (5). In some cases such as
HS:3 (2) and HS:19 (3), high-molecular-weight
LPSs of C. jejuni were found to contain no negatively
charged sugar residues. However, we were able to demonstrate CPS in
these strains using Alcian blue (Fig. 1, lanes 2 and 5). A lipid-free
form of CPS in strain G1 could not be detected after blotting on a
Hybond N membrane even after gentle staining with Alcian blue at a
higher pH to prevent desorption from the membrane (data not shown). The absence of binding to the membrane after the removal of the lipid moiety may indicate that the repeating unit of the CPS lacks strongly charged acidic groups (e.g., phosphate and sulfate) required for electrostatic interaction with a positively charged membrane. On the
other hand, the presence of weakly charged acidic groups, such as
carboxyl, would ensure interaction with the Alcian blue dye. Sugars
containing carboxyl groups (e.g., uronic acids and their derivatives)
are common components of CPSs from different pathogenic bacteria.
Despite the apparent variability in the intensity of CPS bands among
different C. jejuni strains (probably the result of
variable affinities of the dye for different CPSs due to the
differences in their chemical structures), the present study
demonstrates that the Alcian blue staining procedure is an efficient
way of detecting C. jejuni CPS and will be invaluable
for structural and epidemiological investigations of this newly
identified polysaccharide molecule.
 |
ACKNOWLEDGMENTS |
This work was supported by the BBSRC and the Wellcome Trust,
United Kingdom.
We are grateful to Ben Appelmelk for a gift of monoclonal anti-lipid A
and anti-Kdo antibodies, Normal Gregson for antisera, and Dennis Linton
for useful discussions.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, University of London, Keppel Street, London WC1E 7HT, United
Kingdom. Phone: 44 (0) 207 927 2288. Fax: 44 (0) 207 637 4314. E-mail:
brendan.wren{at}lshtm.ac.uk.
 |
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Journal of Clinical Microbiology, January 2001, p. 279-284, Vol. 39, No. 1
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.1.279-284.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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