Characterization of Chinese Haemophilus parasuis Isolates by Traditional Serotyping and Molecular Serotyping Methods
Characterization of Chinese Haemophilus parasuis Isolates by Traditional Serotyping and Molecular Serotyping Methods
Editor: Ray Borrow
Public Health England
Lina Ma 0
Liyan Wang 0
Yuefeng Chu 0
Xuerui Li 0
Shengli Chen 0
Jianhua Zhou 0
Zhongxin Lu 0
Jixing Liu 0
Yongsheng Liu 0
0 State Key Laboratory of Veterinary Etiological Biology, Lanzhou Veterinary Research Institute, Chinese Academy of Agricultural Sciences , Lanzhou, Gansu , China , 2 State Key Laboratory of Pathogen and Biosecurity, Beijing Institute of Microbiology and Epidemiology , Beijing , China , 3 Institute of Animal Health, Guangdong Academy of Agricultural Sciences , Guangzhou, Guangdong , China
Haemophilus parasuis is classified mainly through serotyping, but traditional serotyping always yields non-typable (NT) strains and unreliable results via cross-reactions. Here, we surveyed the serotype prevalence of Chinese H. parasuis isolates using traditional serotyping (gel immuno-diffusion test, GID) and molecular serotyping (multiplex PCR, mPCR). We also investigated why discrepant results between these methods were obtained, and investigated mPCR failure through whole-genome sequencing. Of the 100 isolate tested, 73 (73%) and 93 (93%) were serotyped by the GID test and mPCR, respectively, with a concordance rate of 66% (66/100). Additionally, mPCR reduced the number of NT isolates from 27 (27%) for the GID testing, to seven (7%). Eleven isolates were sequenced, including nine serotype-discrepant isolates from mPCR and GID typing (excluding strains that were NT by GID only) and two NT isolates from both methods, and their in silico serotypes were obtained from genome sequencing based on their capsule loci. The mPCR results were supported by the in silico serotyping of the seven serotype-discrepant isolates. The discrepant results and NT isolates determined by mPCR were attributed to deletions and unknown sequences in the serotype-specific region of each capsule locus. Compared with previous investigations, this study found a similar predominant serotype profile, but a different prevalence frequency for H. parasuis, and the five most prevalent serotypes or strain groups were serotypes 5, 4, NT, 7 and 13 for mPCR, and serotypes 5, NT, 4, 7 and 13/10/14 for GID. Additionally, serotype 7 was recognized as a principal serotype in this work.
Haemophilus parasuis, the causative agent of GlaÈsser's disease in pigs, has multiple clinical
manifestations, including pneumonia, meningitis, arthritis, polyserositis, and septicemia [1±
4]. GlaÈsser's disease outbreaks are seriously damaging to pigs and cause devastating economic
losses to the swine industry worldwide, either on their own, or when co-infections with other
Funding: This work was supported by
National Nature Science Foundation of China
(No.31172335) for YL, the Special Fund for
Agro-scientific Research in the Public Interest
(Grant No. 201303034-2) for Zl and State Key
Laboratory of Veterinary Etiological Biology,
Lanzhou Veterinary Research Institute, CAAS
(No. SKLVEB2012KFKT010) for YC. The funders
had no role in study design, data collection and
analysis, decision to publish, or preparation of the
swine pathogens occur [4±8]. Accurate serotype identification is critical for epidemiological
investigations or vaccine selection studies in H. parasuis infections.
Generally, H. parasuis is classified by serotyping, and fifteen serotypes are recognized
according to the current serotyping scheme [
]. Global serological surveys of H. parasuis have
been carried out using traditional serotyping methods, and the prevalent serotypes are as
follows: types 5, 4, 2, and 13 in Spain [
], 5, 4, and 13 in Denmark [
], 4, 5, 13, and 7 in North
], 1, 2, 4, 5, and 13 in the Netherlands [
], and 4, 5, 14, 13, and 2 in Brazil [
Epidemiological studies in China indicated that the prevalent serotypes were 4, 5, 13, 14 and
12 in 2005 [
] and 4, 5, 13, 15 and 2 in 2011 [
]. Regardless of whether H. parasuis is typed
by the gel-immuno-diffusion (GID) test or the indirect haemagglutination assay (IHA), 10±
40% of the non-typable (NT) strains [9±12, 15, 17±19] and frequent cross-reactions [
12, 15, 16,
] were found in previous studies. Additionally, lack of the 15 serotype reference strains,
difficulties in serovar-specific antigen and antiserum preparation [
9, 17, 19
between antiserum batches [
], and the variable sensitivities of the detection methods [
] greatly decrease the capabilities of the traditional serotyping assays for typing H. parasuis.
Among the serotyping protocols available for H. parasuis, the capsular polysaccharide is
assumed to be the dominant component of the serotyping antigen [9, 23±25]. The capsule loci
for the 15 H. parasuis serotype reference strains have been annotated, and a strong correlation
between the capsule locus type/in silico serotype and serotyping result was observed [
Surprisingly, the capsule locus was also found in NT strains . Therefore, the capsule locus
offers a potential target for molecular serotyping of H. parasuis. Based on the concept of the
capsule locus being responsible for the phenotype of the capsule, a multiplex PCR (mPCR) was
developed by Howell et al. [
] for rapid molecular serotyping of H. parasuis [
]. All the
isolates tested were typed by this method in that research study and a high concordance was
gained between the mPCR and IHA results [
The aim of this study was to investigate H. parasuis serotype prevalence in Chinese pig
herds. For this purpose, both mPCR and GID tests were performed, and whole-genome
sequencing was used to validate the discrepant results between the mPCR and GID tests and
to survey the cause of the mPCR serotyping failure. We found that the mPCR serotyping
detection rate was superior to that of GID typing, and where discrepancies existed in the mPCR
serotyping they were attributable to deletions and unknown sequences in the serotype-specific
capsule locus region.
Materials and Methods
This study was approved by the Animal Ethics Committee of Lanzhou Veterinary Research
Institute, Chinese Academy of Agricultural Sciences (Permit No. LVRIAEC2007-003). All the
experimental protocols in this study were conducted in strict accordance with the
requirements of the Animal Ethics Procedures and Guidelines of the People's Republic of China. All
animals were humanely sacrificed under sodium pentobarbital anesthesia, and all efforts were
made to minimize any suffering.
A panel of 100 H. parasuis field isolates (S1 Table) was included in this study. The H. parasuis
reference strains were kindly provided by Dr Patrick Blackall (Animal Research Institute,
Queensland, Australia) and Dr Albert Rovira (Veterinary Diagnostic Laboratory, University of
Minnesota, Minnesota, USA). All the isolates were collected from diseased pigs from
Guangdong, Jiangsu, Shanghai, Qinghai, Gansu, Heilongjiang, and Jiangxi provinces of China
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between February 2007 and September 2014. The isolates were all characterized as H. parasuis
in accordance with their colony characteristics [
], their Gram staining properties,
nicotinamide adenine dinucleotide (NAD)-dependent tests [
], and 16S RNA sequence
identification . The majority of the isolates originated from organs or tissues, including lung
(n = 43), brain (n = 4), joint fluid (n = 2), cardiac blood (n = 10), lymph node (n = 3),
pericardial effusion (n = 2), and abdominal effusion (n = 1), but 35 of the isolates lacked information
about the isolation site.
The serotypes of all the field isolates were identified using the GID test, as originally described
by Morozumi and Nicolet [
]. Reference strain antisera were prepared as described
] using cells grown overnight on tryptic soy agar (TSA, Becton, Dickinson and
Company, Sparks, USA) supplemented with 5% horse serum and 10 μg/ml NAD. The serotyping
antigen/heat-stable antigens from the field isolates were prepared by autoclaving at 121ÊC for
2 h as described by Morozumi and Nicolet [
]. The serotyping procedure was performed as
described previously [
]. The test was repeated once more if no definitive serotype was
obtained for an isolate. NT strains were defined as isolates whose antigens did not react with
antiserum against the 15 serotype reference strains.
Multiplex PCR assay
The test procedure for the one-step mPCR was performed as previously described [
some modifications. Briefly, a loopful of bacteria from a pure culture plate was suspended in
30μl of UltraPure H2O. The mixture was boiled for 30 min, and the supernatant collected for
each isolate after centrifugation at 4,000 × g for 1 min. A 1μl aliquot of genomic DNA for each
sample was added to an mPCR mixture, and the 25μl total volume consisted of 12.5 μl premix
Taq (Ex Taq version 2.0 plus dye), 0.5μl of primer mix (50 μM), 0.25μl DMSO (added at 1% of
the total reaction volume), and 10.75μl of UltraPure H2O. All the samples were examined
according to the serotype order, and the RNAse-free ddH2O and genomic DNA of the
corresponding serotype reference strain were used as negative and positive controls, respectively.
The dominant serotype 4 reference strain was used as the positive control for the NT strains.
The mPCR was heated at 94ÊC for 5 min, followed by 30 cycles of 94ÊC for 30 s, 58ÊC for 30 s,
68ÊC for 60 s, and a final extension at 68ÊC for 5 min. The amplified products were
electrophoresed in 2.0% agarose gels run in 1 x Tris-borate buffer with a Quick-Load1 100 bp DNA
Ladder (New England BioLabs) as the molecular size standard. The procedure was repeated twice
for each isolate.
DNA extraction and genome sequencing
Eleven isolates were sequenced, including the 9 isolates with serotype discrepancies by mPCR
and GID (excluding strains that were NT by GID only) and the 2 isolates that were NT by both
methods. Genomic DNA was extracted from overnight cultures grown in supplemented TSB
using a DNeasy Blood & Tissue Kit (Qiagen, Valencia, CA) according to the manufacturer's
instructions. The concentrations of the extracted genomic DNAs were measured using the
Nanodrop 2000/2000C system (Thermo Scientific Company, Waltham, UK). All the isolates
were sequenced on an Illumina HiSeq 2000 platform with a paired-end (PE) strategy. The
SOAPdenovo assembly was performed using PE reads with quality filtering (Q20), first 5
nucleotides of the 5'-end, and adapter trimming. The average effective sequencing depth for all
the isolates was 120-fold.
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Capsule locus identification and in silico serotype analysis
The in silico serotypes of the serotype-discrepant isolates and common NT isolates were
determined by comparing their capsule contents and compositions with those of the reference
strains. The capsule locus was identified for the 9 isolates with serotype discrepancies by
mPCR and GID (excluding strains that were NT by GID only) and the 2 isolates that were NT
by both methods according to a previous description [
] with some modifications. Briefly,
the locus sequences were acquired by using the first gene (funA) and last gene (iscR) of the H.
parasuis SH0165 capsule locus (GenBank accession No. CP001321.1) [
] as the query
sequences. The gene name was determined for each coding sequence within the capsule locus
by a nucleotide Basic Local Alignment Search Tool (BLASTn) interrogation of the NCBI
database (https://www.ncbi.nlm.nih.gov/). The predicted gene names were recorded according to
the highest matched nucleotide identity score. When more than one significant BLAST match
sequence was found for a single isolate or various isolates, their identities were aligned further
by BLASTn to determine whether the same sequence was obtained. Identical sequences were
defined as described previously [
], using a threshold of >80% nucleotide identity over
>80% of coverage length. For simplicity, the capsule locus genes from each isolate were
ordered from funA to iscR.
Comparing the detection performances of mPCR and GID
The mPCR and GID performances were evaluated using the results from both of these
analyses, and the in silico serotyping results. Because the in silico serotyping was 100% concordant
with the mPCR results, it was considered to be a potential new gold standard for replacing
traditional serotyping methods [
], and was used to validate the results of the above mentioned
nine serotype-discrepant isolates. For the serotype-discrepant isolates from the GID and
mPCR tests, the serotype category that agreed with the in silico serotyping was considered to
be correct. The detection rate for the typable strains was calculated for the GID test and the
mPCR assay, and the concordance between both methods was analyzed using the above
Nucleotide sequence accession numbers
The genome sequences from this study were deposited in GenBank under the accession
numbers MNAP00000000, MNAQ00000000, MNAR00000000, MNAS00000000, MNAT00000000,
MNAU00000000, MNAV00000000, MNAW00000000, MNAX00000000, MNAY00000000,
Comparison of the prevalence and serotype profiles between mPCR and GID
A total of 100 Chinese H. parasuis clinical strains were tested using mPCR and GID
methodologies. Of the 100 field isolates, 93 were typable, but the remaining seven were confirmed as NT
by mPCR (Table 1). Regarding serotypes 5 and 12 as being the same serotype [
] (in this
study, if not specified, serotype 5 refers to the serotype 5 and 12 combination), the most
prevalent serotype identified by mPCR was serotype 5 (40% of isolates), followed by serotype 4
(33%), NT (7%), serotype 7 (6%), 13 (4%), 11 (3%), 1 (2%), 2 (2%), 10 (2%), and 14 (1%) (Fig
1). In contrast, only 73 field isolates were typable and 27 isolates were confirmed as NT strains
by GID. The dominant serotype was serotype 5 (38%), followed by NT serotypes (27%),
serotype 4 (15%), 7 (7%), 10 (3%), 13 (3%), 14 (3%), 1 (2%), 2 (1%), and 15 (1%) (Fig 1).
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Of the 73 serotyped isolates from the GID test, nine had results discrepant with mPCR.
These serotype-discrepant isolates comprised a serotype 1 identified as a serotype 11 by
mPCR, three serotype 7s identified as NT by mPCR, two serotype 10s identified as NT by
mPCR, two serotype 14s identified as serotype 4 by mPCR, and a serotype 15 identified as
serotype 4 by mPCR (Table 1). For the 73 typable isolates identified by GID, more than 87%
(64/73) concordance was acquired between the mPCR and GID serotyping.
Fig 1. Serotype distribution of 100 Chinese isolates as determined by GID (blue) and mPCR (red).
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Of the 27 NT isolates identified by GID, 25 were identified as typable strains by mPCR and
these were classified as the following 8 serotypes: serotype 1 (n = 1), 2 (n = 1), 4 (n = 15), 5
(n = 2), 7 (n = 2), 10 (n = 1), 11 (n = 2), and 13 (n = 1) (Table 1). H47 and K3, the remaining
two NT isolates in the GID test, were also confirmed as NT by the mPCR method.
Although the distribution frequency of each serotype varied extremely between mPCR and
GID, almost identical serotype profiles were identified by both methods. Additionally, mPCR
and the GID showed nearly identical prevalences of the dominant serotypes or isolate group,
and the first five most predominant groups, covering 90% of the total number of isolates, were
completely identical for the two methods.
In silico serotype analysis based on the capsule locus
Because nine isolates displayed different serotype results for GID and mPCR and two isolates
were NT by both methods, the molecular basis of the inconsistent results and mPCR failures
were investigated by whole-genome sequencing and analysis of the in silico serotypes obtained.
Genome sequencing and assembly were finalized (Table 2), and the capsule loci were identified
for the above mentioned serotype-discrepant isolates and NT isolates (Table 3) from the genome.
In silico serotypes were obtained from the content and composition of each capsule locus.
Compared with the capsule loci of the reference strains, these isolates displayed obvious deletions and/
or unknown sequences (no significant similarity sequence in BLASTn search, NSSS) in their
capsule loci. Deletions and NSSSs both occurred in the serotype-specific region of each capsule locus
of these isolates. Furthermore, these deletions or NSSSs covered not only the serotype-specific
gene position of the mPCR scheme, but also the signal regions of the in silico serotype analysis.
H12, H35, and H36 share some common capsule locus features with the serotype 4
reference strain, SW124, and their capsule loci obviously differ from those of the serotype 14
reference strain 22113 and the serotype 15 reference strain 15995 (Table 3). Compared with
SW124, the three isolates only lacked the lstB gene in their capsule loci, so they were defined as
belonging to capsule locus type 4 or in silico serotype 4.
H38, H39, and K3 share similar capsule loci with the C5 serotype 8 reference strain but
these loci differed markedly from the H555 serotype 10 reference strain locus. Compared with
C5, these three isolates lacked the scdA gene, but H38 and H39 share NSSS10 in this position
(Table 3). Based on the capsule composition analysis, H38, H39 and K3 can be defined as in
silico serotype 8.
Compared with the serotype 1 reference strain No.4, the capsule locus of HPS6 is more
closely related to that of the serotype 11 reference strain H465 (Table 3). HPS6 shares gltO,
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bstA and amtA with H465; therefore, HPS6 is related to in silico serotype 11 based on its
HPS4, 16, and YT were identified separately as serotype 7 and NT strains by GID and
mPCR; they share four NSSSs (NSSS6-NSSS9, with 100% nucleotide identity) and a sequence
encoding a hypothetical protein named fun, as described previously [
] (Table 3). Moreover,
HPS4 and 16 share NSSS5 with 100% nucleotide identity. Another gene, amtA, which is
common in the capsule locus of the serotype 11 reference strain H465 [
], also appears in the
capsule loci of YT and 16. In these two isolates the funP-funQ-gltJ-cap5E-ndeA-naeA-gltA gene
cluster, which is part of the serotype-specific region for serotype 7 [
], is replaced by NSSSs;
among these missing genes, funQ is the serotype-specific target gene for serotype 7 in the
mPCR scheme [
]. Based on the capsule composition, HPS4, 16 and YT cannot be identified
as a definite serotype in the in silico serotype analysis, so we defined them here as NT strains.
Compared with six other NT strains (YT, 16, HPS4, H38, H39 and K3) from the mPCR,
H47 differs markedly in its capsule locus, which contains four continuous and totally different
NSSSs (NSSS1-NSSS4) (Table 3). These NSSSs are also distinct from those of YT, 16, HPS4,
H38 and H39. Although H47 contained the gene composition of funA-neuA-wzx and
lstAwza-wzb-wzs-iscR, it is still assumed to NT strain by the in silico analysis in this study because
of the continuous NSSSs within its capsule locus.
Comparison of the detection performance of mPCR and GID
The mPCR and GID detection rates were calculated using the data generated from GID, mPCR
and the in silico serotype analysis. Of the 100 isolates we tested, 93 and 73 were identified as
single serotypes by mPCR and GID, respectively. Compared with GID, the mPCR detection rate
for serotyping H. parasuis isolates was 93%, a value higher than that of GID (73%).
Of the typable isolates tested by GID, nine showed discrepant results with mPCR. Taking
the in silico serotype as the standard, the serotypes of seven serotype-discrepant isolates (H12,
H35, H36, 16, YT, HPS4 and HPS6) from mPCR agreed well with the results of the in silico
serotype analysis (Table 4). However, the in silico serotype analysis did not support any of the
results from mPCR or GID for the remaining two serotype-discrepant isolates, H38 and H39
(Table 4). The concordance between the mPCR and GID test was 66% (66/100), including 64
typable isolates and two common NT strains by mPCR and the GID test.
In this research, the results of mPCR and GID analyses produced nearly identical serotype
profiles for the isolates. Considering serotypes 5 and 12 as the same serotype, the serotypes 5, 4, 7
Serotype by mPCR
in silico serotype
8 / 12
and 13 are the most frequently detected serotypes in China, but the prevalence frequency for
each serotype manifested obvious differences between the mPCR and GID tests. The results
showed identical serotype profiles to those from studies in Denmark [
] and Canada [
and an investigation of multinational samples [
]. Furthermore, studies performed in
], Spain [
], USA/Canada [
], Australia [
], China [
] and Brazil  also had
similar results. In all cases, serotype 4, 5 and 13 were the collectively predominant serotypes.
Moreover, compared with previous reports from China, serotype 7 was the dominant serotype
in this work.
High NT isolate rates were reported in all previous described studies, and the two most
prevalent serotypes were 5 and 4. However, if the NT strains are included, they will become
the third dominant H. parasuis isolate group in Germany [
], USA/Canada [
] and Denmark
], and even exceed the number of serotype 4 and 5 isolates in some cases [
compared with serotyping by GID, mPCR substantially reduced the number of NT isolates
from 27% to 7% in this work. The second dominant H. parasuis isolate group was changed
from NT strains in the GID test to serotype 4 in the mPCR test. The number of NT isolates
played a key role in the serotype profile and prevalence order for H. parasuis. It is also worth
noting that a defect in capsule expression can still be existed, even though a entire capsule
locus may be present in the isolates. Consequently, it is very important for NT strains to
identify whether they are capsulated-strains or non-capsulated strains.
Capsule locus analysis of the NT strains from mPCR revealed the presence of a single
deletion or NSSS at the position of a serotype-specific gene adopted by the mPCR for K3, H38, and
H39, and multiple NSSSs at the serotype-specific region within this locus for YT, 16, HPS4,
and H47. It is clear that the deletion of the serotype-specific target and emergence of NSSS
within the capsule locus produced NT isolates in the mPCR test and in the accompanying in
silico serotype analysis. In previous research [
], the deletions and NSSSs identified also
resulted in a lower concordance between the mPCR and in silico serotype analysis. Similarly,
insertions and deletions also caused discrepancies between the phenotypic and genotypic
serotyping of Shigella flexneri [
]. To some extent, finding deletions and NSSSs in H. parasuis
DNA probably indicates unstable serotype-specific regions within its capsule locus.
Overall, the principal serotype profile from mPCR and GID in our research was the same
or similar to profiles of most other previous reports. As a genotypic serotyping method, mPCR
is superior to phenotypic serotyping based on GID. In terms of the improved detection rate
for typable isolates, the stability of the clinical test, and the compatibility of the results between
different laboratories, mPCR will be a valuable alternative to the traditional serotype methods
used for typing field isolates of H. parasuis. Investigation of the capsule expression and capsule
structures are now required for exploring the origins of NT strains. Additionally, efforts should
also be directed in future towards searching for more stable serotype-specific genes to remove
the adverse impact of deletions and NSSSs in the mPCR test.
S1 Fig. Band patterns of the molecular serotyping PCR for all 15 serotypes reference
strains and part of isolates. M denotes Quick-load 100bp DNA Ladder (New England Biolabs
Inc., USA). Lane 1: H2O (blank control). Lane 2: No.4 (serotype 1 reference strain). Lane 3:
qixian. Lane 4: SW140 (serotype 2 reference strain). Lane 5: 211/212. Lane 6: SW114 (serotype
3 reference strain). Lane 7: SW124 (serotype 4 reference strain). Lane 8: H12. Lane 9: H23. Lane
10: H24. Lane 11: H25. Lane 12: H35. Lane 13: H36. Lane 14: H44. Lane 15: Nagasaki (serotype
5 reference strain). Lane 16: W1. Lane 17: ZX. Lane 18: H15. Lane 19: H17. Lane 20: H45. Lane
21: H46. Lane 22: 131 (serotype 6 reference strain). Lane 23: C5 (serotype 8 reference strain).
9 / 12
Lane 24: D74 (serotype 9 reference strain). Lane 25: 174 ((serotype 7 reference strain)). Lane 26:
H19. Lane 27: HE. Lane 28: HM. Lane 29: H555 (serotype 10 reference strain). Lane 30: H49.
Lane 31: H465 (serotype 11 reference strain). Lane 32: HPS6. Lane 33: ST. Lane 34: H425
(serotype 12 reference strain). Lane 35: YZ-12. Lane 36: 84±17975 (serotype 13 reference strain).
Lane 37: YZ-13. Lane 38: 84±22113 (serotype 14 reference strain). Lane 39: FS2. Lane 40: 84±
15995 (serotype 15 reference strain). Lane 41: H38. Lane 42: H39. Lane 43: K3. Lane 44: 16.
Lane 45: HPS4. Lane 46:YT. Lane 47: H47.
S1 Table. Description of H.parasuis reference strains and isolates included in this study.
We would like to thank Ruifu Yang and Yanfeng Yan from the State Key Laboratory of
Pathogen and Biosecurity of Beijing Institute of Microbiology and Epidemiology for their help with
English language in the manuscript and database submission of the H. parasuis genomic data,
respectively. We are also grateful to Patrick Blackall of the Animal Research Institute,
Queensland, Australia and Albert Rovira of the Veterinary Diagnostic Laboratory, University of
Minnesota, Minnesota, USA for kindly contributing 15 H. parasuis serotype reference strains.
Conceptualization: LM XL ZL JL YL.
Funding acquisition: YujC ZL YL.
Investigation: LM LW SC YueC XL YujC JZ CL ZL JL YL.
Project administration: LM.
Resources: YueC CL YujC.
Writing ± original draft: LM.
Writing ± review & editing: LM XL ZL JL YL.
10 / 12
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