BACKGROUND
listeria monocytogenes , a food borne pathogen, is frequently isolated from dairy products and poultry.
it can cause invasive diseases in humans and farm animals, including meningitis, fetal loss, sepsis, and febrile gastroenteritis
<cit> .
although l. monocytogenes  is an uncommon human pathogen, it has a disproportionate share of the food borne disease burden.
for example, there were only  <dig>  illnesses annually in the us but l. monocytogenes  infections account for 4% of all hospitalizations and 28% of all deaths from food borne diseases
<cit> .
a large outbreak occurred in the maritime provinces of canada in  <dig>  which provided the first evidence for transmission of listeriosis by food-borne l. monocytogenes  <cit> .
since then, many outbreaks of listeriosis have been reported: six in the us include two in massachusetts in  <dig> and 2007
<cit> , one in california in 1985
<cit> , one in illinois in 1994
<cit> , a multi-states outbreak in 2002
<cit>  and one most recent outbreak in 2011
<cit> ; one in canada in 2008
<cit>  and five in europe including one each in france in 1992
<cit> , switzerland between  <dig> and 1987
<cit> , sweden in 1995
<cit> , italy in 1997
<cit>  and finland in 1999
<cit> .
l. monocytogenes  is a diverse species and has been typed using a range of subtyping procedures to examine the epidemiology and population genetics.
serotyping is a classic subtyping method with limited discriminatory power.
thirteen serotypes of l. monocytogenes  are recognized.
three serotypes  cause the majority of clinical cases and serotype 4b causes the majority of human epidemics
<cit> .
pulsed field gel electrophoresis  provides higher discrimination than serotyping and is often considered the standard subtyping method for source tracking and epidemiologic investigations
<cit> .
multi-locus sequence typing  based on nucleotide sequences of housekeeping genes has also been shown to be highly discriminatory for l. monocytogenes  <cit> , with an added advantage that it provides unambiguous results comparable among laboratories via the internet.
l. monocytogenes  is well recognized to be divided into  <dig> lineages
<cit> .
in a recent study, wiedmann et al.
discovered a fourth lineages, however, lineages iii and iv were rare
<cit> .
brisse et al.
established a standardized mlst scheme using seven housekeeping genes and used it to characterized a large collection of l. monocytogenes  isolates
<cit> .
an mlst database was also established which allows other researchers to submit new mlst data and facilitates international comparison although the use of unpublished mlst data in the database is restricted.
listeriosis is uncommon in china and there was no report of human outbreaks so far.
this may be partly due to lack of surveillance of clinical listeriosis.
surveillance of l. monocytogenes  in foods has been implemented nationally and l. monocytogenes  has been isolated from foods and food processing environments in china including chicken, pork, fish and vegetables
<cit> .
zhou et al analyzed 38 l. monocytogenes  isolates from food products and sewage samples in china using single gene sequencing of the acta gene while jiang et al.
<cit>  characterized 20 l. monocytogenes  isolates from zhejiang province of china by a non-standardized mlst scheme based on three virulence genes and four housekeeping genes.
neither of these sequence data allows one to make a comparison with the current extensive international mlst data.
in this study, isolates were obtained from different food products through food surveillance from  <dig> provinces or cities across china, and analyzed by serotyping, pfge and mlst to further determine the genetic diversity of chinese l. monocytogenes  isolates and to compare chinese isolates with international isolates from published studies.
methods
l. monocytogenes  isolates
two hundred and twelve isolates of l. monocytogenes  from  <dig> provinces/cities in china were used for this study.
the isolates were from different food products isolated by local food surveillance laboratories between  <dig> and  <dig> .
food surveillance was generally conducted with random sampling from open markets and production plants periodically based on national surveillance guidelines.
our isolates were a random sample of these surveillance isolates and were not known to be linked by transmission chain or food sources.
the isolates were identified by pcr targeting hly fragments specific for l. monocytogenes  and serotyped using antiserum against somatic and flagella antigens according to the instructions of the manufacture .
* food indicates the isolates from unknown food source.
a: anhui b: beijing c: chongqing d: fujian e: guangdong f: guangxi g: henan h: hubei i: jiangsu j: shanghai k: sichuan l: zhejiang.
pulsed-field gel electrophoresis analysis
agarose plugs were prepared and pfge was performed, according to centers for disease control and prevention pulsenet standardized procedure for typing l. monocytogenes .
the pulse-net protocol recommends two restriction enzymes apai and asci for pfge.
only asci was used in this study since asci pfge patterns are more easily analyzed than apai patterns by the bionumerics software and operators
<cit> .
briefly, genomic dna was prepared by mixing 240 μl of standardized cell suspension and 60 μl of 10 mg/ml lysozyme solution, followed by incubation at 37°c for 10 min.
sample plugs were digested with  <dig> u of asci  at 37°c for 3 h. plugs were then loaded on 1% seakem gold agarose gel in 5 × tbe  and electrophoresed on a chef dr iii apparatus , using the following parameters: initial switch time, 4 s; final switch time, 40 s; run time, 22 h; angle, 120°; gradient, 6 v/cm; temperature, 14°c; ramping factor, linear.
gels were stained with ethidium bromide and visualized on a uv transilluminator.
salmonella enterica serovar braenderup strain h <dig> restricted with xbai was used for molecular weight determinations in all pfge gels.
similarities between restriction endonuclease digestion profiles were analyzed by using unweighted pair group method with arithmetic mean  of bionumerics software .
multi-locus sequence typing and phylogenetic analysis
the mlst scheme available at
http://www.pasteur.fr/recherche/genopole/pf8/mlst/lmono.html was used.
the nucleotide sequences of internal fragment of the following genes, acbz , bgla , cat , dape , dat , ldh , and lhka , were obtained by pcr using published primers  with the exception of primers for lhka.
a new pair of primers for lhka  were designed to amplify the lhka gene from the isolates which had no amplification products when the published primers were used.
a 50-μl reaction was composed as follows:  <dig>  μl of 10 × pfu buffer with  <dig>  mm mgcl <dig>  125 μm each of deoxynucleoside triphosphate mix,  <dig>  μm forward and reverse primers,  <dig> u of pfu dna polymerase, and 2u of rtaq dna polymerase.
the pcr amplification conditions were as follow: 94°c for 4 min and  <dig> cycles of 94°c for 30 s, 52°c for 30s, and 72°c for 2 min, followed by one cycle of 72°c for 10 min and hold indefinitely at 4°c.
the purified pcr products were sent for sequencing commercially.
for each isolate, the allele combination at the  <dig> loci defines an allelic profile or sequence type .
minimum spanning tree  analysis was used to infer relationships among the isolates and was done using bionumerics .
neighbor-joining tree of the seven concatenated housekeeping gene sequences was constructed using mega  <dig>
<cit> .
a clonal complex  is defined based on eburst algorithm with member sts differing by only one of the  <dig> mlst genes
<cit> .
RESULTS
serotyping
the  <dig> isolates used in this study were typed into seven of the  <dig> known serotypes: 1/2a, 1/2b, 1/2c, 3a, 3b, 4b and 4c.
the most frequent serotypes are 1/2c, 1/2a and 1/2b with a frequency of  <dig> %,  <dig> % and  <dig> % respectively.
the remaining  <dig> serotypes account for only  <dig> % of the total isolates.
pulsed-field gel electrophoresis
pfge analysis divided the  <dig> isolates into  <dig> pulse types .
ptgx6a <dig>  was predominant and accounts for  <dig> % of the isolates, followed by gx6a <dig>  , and gx6a <dig>  .
thirty two pts  were represented by only a single isolate.
a upgma dendrogram was constructed for the  <dig> pts based on presence or absence of bands.
the pts are divided into  <dig> clusters.
cluster i contained all serotype 1/2c isolates, the majority of serotype 1/2a isolates.
cluster ii contained all serotype 4b and 1/2b isolates and the remaining serotype 1/2a isolates.
cluster iii contained only two pts  belonging to serotype 4c and are very divergent .
multi-locus sequence typing
the  <dig> isolates were divided into  <dig> sequence types , among which  <dig> sts have previously reported in other countries,  <dig> sts  were novel.
the most common sts are st <dig> , all of which are serotype 1/2c, st <dig>  with all isolates belonging to 1/2a, and st <dig>  with all except one being 1/2b isolates and the exception being a 3b isolate.
fifteen sts  were represented by only one isolate .
the  <dig> sts were grouped into six clonal complexes and  <dig> singletons according to eburst algorithm .
they were divided into three lineages as defined by wiedmann
<cit> .
lineage i includes two clonal complexes: cc <dig> and cc <dig>  of serotypes1/2b, 3b and 4b, and nine singletons of which seven are serotype 1/2b and two are serotype 4b.
lineage ii includes four clonal complexes: cc <dig>  cc <dig>  cc <dig> and cc <dig>  cc <dig> contains the largest number of sts including st <dig>  st <dig>  st <dig>  st <dig>  st <dig> and st <dig>  all isolates in cc <dig> were serotype 1/2c.
cc <dig> and cc <dig> were serotype 1/2a while cc <dig> includes both serotypes 1/2a and 3a.
the singletons in this lineage were all serotype 1/2a, except for one isolate being serotype 1/2c .
lineage iii contained two isolates, both belonging to st <dig> and serotype 4c.
discussion
correlation among serotype, pulse type and sequence type
in most cases, l. monocytogenes  isolates of the same pt and st belong to the same serotype but there were exceptions.
two isolates  of the same pt  and st  are different serotypes .
among the five isolates of pulse type gx6a <dig>  and st <dig>  four and one were serotype 3a and serotype 1/2a respectively.
the observation indicates that serotype 3a and 1/2a can be easily switched.
additionally there were  <dig> cases of the same pt but different sts.
for example, of  <dig> isolates  with pt gx6a <dig> ,  <dig> isolates were st <dig>   <dig> isolates were st <dig>   <dig> isolates were st <dig>  and one each was st <dig>  st <dig>  st <dig> and st <dig> respectively.
these sts were all grouped into cc <dig> except for st <dig>  which shares  <dig> of the  <dig> alleles with st <dig>  in another case, of the  <dig> isolates of pt gx6a <dig> ,  <dig> were st <dig>  one each was st <dig> and st <dig>  both of which shared only  <dig> alleles with st <dig>  the simpson’s diversity index for pfge is  <dig>  which is only slightly higher than that of mlst .
however the discriminatory power for pfge can be increased by using an additional enzyme apai as recommended by the pulsenet protocol
<cit>  and our study affirms the need to use the additional enzyme for outbreak investigations as discriminatory power of asci is low.
comparison of isolates from china with international isolates
the sts from this study were compared with  <dig> sts from an analysis of  <dig> global isolates from the study of rogon et al.
<cit>  and chenal-francisque et al <cit> , we found that  <dig> of the  <dig> sts in china shared the same sequence types with isolates from patients in other countries, including maternal-fetal infections, central nervous system infections and bacteriemia patients .
seven sts containing nearly half or more than half of the isolates from rogon et al.
<cit>  including st <dig> , st <dig> , st <dig> , st <dig> , st <dig> , st <dig>  and st <dig>  caused maternal-fetal infections.
in addition, at least  <dig> of these sts have caused outbreaks in europe.
st <dig> caused outbreaks in france in  <dig> and in sweden in  <dig> while st <dig> caused an outbreak in italy in  <dig>  these same sequence types isolated from food sources and in particular st <dig> and st <dig> were the  <dig> most common sts in china.
based on these observations, we conclude that these sts have the potential to cause disease in humans in china.
human listeriosis has been rarely reported in china which may be contributed by poor disease awareness, lack of diagnostic tools and lack of surveillance.
this study also affirms the recent report by chenal-francisque et al.
<cit>  that some clones including epidemic clones are prevalent worldwide and globally distributed.
in that study, however, there are only  <dig> isolates from china to represent eastern asia.
our study adds a broader picture from china to the global clones and substantial genetic diversity of l. monocytogenes  to the global gene pool from china.
the  <dig> novel sts from this study were not found in the study of chenal-francisque et al.
<cit> , although  <dig> novel sts fall into their clonal complexes.
further, prevalent sts in china may be rare elsewhere, for example, the third most prevalent st, st <dig> was seen with a single isolate from colombia in the global set of chenal-francisque et al.
<cit> .
our isolates were from over nine food types and only those from chicken and pork had sufficient numbers for comparison of clonal diversity between food types.
there were  <dig> samples each from chicken and pork.
in both food types, st <dig> was predominant with  <dig> and  <dig> isolates in chicken and pork respectively.
genetic diversity is higher from chicken samples as measured by simpson’s index of diversity with  <dig>  and  <dig>  for chicken and pork respectively.
population structure and recombination of l. monocytogenes
many studies have shown that l. monocytogenes  can be divided into three lineages
<cit> .
lineage i includes isolates of serotypes 4b, 1/2b, 3b, 4d and 4e, containing all food-borne-epidemic isolates as well as isolates from sporadic cases in humans and animals.
lineage ii includes isolates of serotypes 1/2a, 1/2c, 3a and 3c, containing both human and animal isolates, but is seldom associated with food-borne epidemics and predominantly isolated from food products.
lineage iii are mostly serotypes 4a and 4c and is predominantly isolated from animals
<cit> .
all our isolates can be allocated into one of the three lineages.
the majority of our isolates  including the  <dig> isolates of st <dig>  belonged to lineage ii since our isolates were from food sources.
fifty six isolates  belonged to lineage i while only two isolates, both being st <dig> belonged to lineage iii.
we used the counting method used by feil et al.
<cit>  to determine the ratio of recombination to mutation per locus.
a single allelic difference between sts within a clonal complex was attributed to either mutation if the difference was a single base or recombination otherwise.
we found that alleles are three times more likely to change by mutation than by recombination .
this estimate is similar to that  reported by ragon et al.
<cit> .
interestingly, five of the eleven recombination events observed were in the same gene , three in cc <dig>  one in cc <dig> and one in cc <dig>  a possible explanation for the high frequency of recombination in abcz is positive selection.
however ragon et al.
<cit>  showed that the ratio of non-synonymous/synonymous substitution rate  of abcz was  <dig>  suggesting that abcz was not under positive selection.
an alternative explanation is that abcz is linked to a nearby gene that is under positive selection and has undergone recombination by hitch-hiking.
this scenario has been observed to have occurred in genes around the o antigen encoding locus in e. coli and other species
<cit> .
examination of sequences 30 kb up and down stream of abcz based on the genome sequence of isolate egd-e did not identify a gene or gene cluster that is likely to be under positive selection.
CONCLUSIONS
this study analyzed  <dig> isolates from food sources in china by serotyping, pfge and mlst, and showed that the common sts in china are also the prevalent sts in other countries, many of which contain isolates from human infections.
a corollary of this observation is that the sts that have caused outbreaks of human infections in other parts of the world have the potential to cause outbreaks in china.
however, there is hardly any data on human l. monocytogenes  infections in china partly due to the lack of clinical listeriosis surveillance.
a recent report of  <dig> cases of neonatal listeriosis in a beijing hospital of  <dig>  live births in  <dig> highlights that the disease may be more common in china
<cit> .
with the country becoming more effluent, food distribution, storage and consumption patterns have also changed.
since the isolates from food sources as shown in this study clearly have the potential to cause disease, there is a need for surveillance of clinical listeriosis and implementation of prevention strategies to prevent emergence and outbreaks of human l. monocytogenes  infections in china.
the findings also have implications for other countries where there is no surveillance system for l. monocytogenes .
authors’ contributions
yw performed the serotyping and mlst typing work and drafted the manuscript.
az performed strain identification.
rz, dj and zl performed the pfge experiments.
zc and yw participated in the analysis of pfge data.
rl participated in data analysis and revised the manuscript.
yw collected some strains.
jx involved in project design.
cy managed the project and co-wrote the manuscript.
all authors read and approved the final manuscript.
acknowledgments
this work was supported by grants  from the ministry of science and technology, people’s republic of china.
we also thank local food surveillance laboratories of  <dig> provinces/cities cdc in china for providing the l. monocytogenes  isolates, and the institut pasteur for providing the mlst database of l. monocytogenes  in their genotyping of pathogens and public health platform.

 listeria monocytogenes, a food borne pathogen, is frequently isolated from dairy products and poultry. it can cause invasive diseases in humans and farm animals, including meningitis, fetal loss, sepsis, and febrile gastroenteritis. although l. monocytogenes is an uncommon human pathogen, it has a disproportionate share of the food borne disease burden. for example, there were only illnesses annually in the us but l. monocytogenes infections account for 4% of all hospitalizations and 28% of all deaths from food borne diseases. a large outbreak occurred in the maritime provinces of canada in which provided the first evidence for transmission of listeriosis by food-borne l. monocytogenes. since then, many outbreaks of listeriosis have been reported: six in the us include two in massachusetts in and 2007, one in california in 1985, one in illinois in 1994, a multi-states outbreak in 2002 and one most recent outbreak in 2011; one in canada in 2008 and five in europe including one each in france in 1992, switzerland between and 1987, sweden in 1995, italy in 1997 and finland in 1999. l. monocytogenes is a diverse species and has been typed using a range of subtyping procedures to examine the epidemiology and population genetics. serotyping is a classic subtyping method with limited discriminatory power. thirteen serotypes of l. monocytogenes are recognized. three serotypes cause the majority of clinical cases and serotype 4b causes the majority of human epidemics. pulsed field gel electrophoresis provides higher discrimination than serotyping and is often considered the standard subtyping method for source tracking and epidemiologic investigations. multi-locus sequence typing based on nucleotide sequences of housekeeping genes has also been shown to be highly discriminatory for l. monocytogenes, with an added advantage that it provides unambiguous results comparable among laboratories via the internet. l. monocytogenes is well recognized to be divided into lineages. in a recent study, wiedmann et al. discovered a fourth lineages, however, lineages iii and iv were rare. brisse et al. established a standardized mlst scheme using seven housekeeping genes and used it to characterized a large collection of l. monocytogenes isolates. an mlst database was also established which allows other researchers to submit new mlst data and facilitates international comparison although the use of unpublished mlst data in the database is restricted. listeriosis is uncommon in china and there was no report of human outbreaks so far. this may be partly due to lack of surveillance of clinical listeriosis. surveillance of l. monocytogenes in foods has been implemented nationally and l. monocytogenes has been isolated from foods and food processing environments in china including chicken, pork, fish and vegetables. zhou et al analyzed 38 l. monocytogenes isolates from food products and sewage samples in china using single gene sequencing of the acta gene while jiang et al. characterized 20 l. monocytogenes isolates from zhejiang province of china by a non-standardized mlst scheme based on three virulence genes and four housekeeping genes. neither of these sequence data allows one to make a comparison with the current extensive international mlst data. in this study, isolates were obtained from different food products through food surveillance from provinces or cities across china, and analyzed by serotyping, pfge and mlst to further determine the genetic diversity of chinese l. monocytogenes isolates and to compare chinese isolates with international isolates from published studies. methods l. monocytogenes isolates two hundred and twelve isolates of l. monocytogenes from provinces/cities in china were used for this study. the isolates were from different food products isolated by local food surveillance laboratories between and. food surveillance was generally conducted with random sampling from open markets and production plants periodically based on national surveillance guidelines. our isolates were a random sample of these surveillance isolates and were not known to be linked by transmission chain or food sources. the isolates were identified by pcr targeting hly fragments specific for l. monocytogenes and serotyped using antiserum against somatic and flagella antigens according to the instructions of the manufacture. * food indicates the isolates from unknown food source. a: anhui b: beijing c: chongqing d: fujian e: guangdong f: guangxi g: henan h: hubei i: jiangsu j: shanghai k: sichuan l: zhejiang. pulsed-field gel electrophoresis analysis agarose plugs were prepared and pfge was performed, according to centers for disease control and prevention pulsenet standardized procedure for typing l. monocytogenes. the pulse-net protocol recommends two restriction enzymes apai and asci for pfge. only asci was used in this study since asci pfge patterns are more easily analyzed than apai patterns by the bionumerics software and operators. briefly, genomic dna was prepared by mixing 240 μl of standardized cell suspension and 60 μl of 10 mg/ml lysozyme solution, followed by incubation at 37°c for 10 min. sample plugs were digested with u of asci at 37°c for 3 h. plugs were then loaded on 1% seakem gold agarose gel in 5 × tbe and electrophoresed on a chef dr iii apparatus, using the following parameters: initial switch time, 4 s; final switch time, 40 s; run time, 22 h; angle, 120°; gradient, 6 v/cm; temperature, 14°c; ramping factor, linear. gels were stained with ethidium bromide and visualized on a uv transilluminator. salmonella enterica serovar braenderup strain h restricted with xbai was used for molecular weight determinations in all pfge gels. similarities between restriction endonuclease digestion profiles were analyzed by using unweighted pair group method with arithmetic mean of bionumerics software. multi-locus sequence typing and phylogenetic analysis the mlst scheme available at http://www.pasteur.fr/recherche/genopole/pf8/mlst/lmono.html was used. the nucleotide sequences of internal fragment of the following genes, acbz, bgla, cat, dape, dat, ldh, and lhka, were obtained by pcr using published primers with the exception of primers for lhka. a new pair of primers for lhka were designed to amplify the lhka gene from the isolates which had no amplification products when the published primers were used. a 50-μl reaction was composed as follows:  μl of 10 × pfu buffer with  mm mgcl 125 μm each of deoxynucleoside triphosphate mix,  μm forward and reverse primers, u of pfu dna polymerase, and 2u of rtaq dna polymerase. the pcr amplification conditions were as follow: 94°c for 4 min and cycles of 94°c for 30 s, 52°c for 30s, and 72°c for 2 min, followed by one cycle of 72°c for 10 min and hold indefinitely at 4°c. the purified pcr products were sent for sequencing commercially. for each isolate, the allele combination at the loci defines an allelic profile or sequence type. minimum spanning tree analysis was used to infer relationships among the isolates and was done using bionumerics. neighbor-joining tree of the seven concatenated housekeeping gene sequences was constructed using mega. a clonal complex is defined based on eburst algorithm with member sts differing by only one of the mlst genes.  serotyping the isolates used in this study were typed into seven of the known serotypes: 1/2a, 1/2b, 1/2c, 3a, 3b, 4b and 4c. the most frequent serotypes are 1/2c, 1/2a and 1/2b with a frequency of %, % and % respectively. the remaining serotypes account for only % of the total isolates. pulsed-field gel electrophoresis pfge analysis divided the isolates into pulse types. ptgx6a was predominant and accounts for % of the isolates, followed by gx6a, and gx6a. thirty two pts were represented by only a single isolate. a upgma dendrogram was constructed for the pts based on presence or absence of bands. the pts are divided into clusters. cluster i contained all serotype 1/2c isolates, the majority of serotype 1/2a isolates. cluster ii contained all serotype 4b and 1/2b isolates and the remaining serotype 1/2a isolates. cluster iii contained only two pts belonging to serotype 4c and are very divergent. multi-locus sequence typing the isolates were divided into sequence types, among which sts have previously reported in other countries, sts were novel. the most common sts are st, all of which are serotype 1/2c, st with all isolates belonging to 1/2a, and st with all except one being 1/2b isolates and the exception being a 3b isolate. fifteen sts were represented by only one isolate. the sts were grouped into six clonal complexes and singletons according to eburst algorithm. they were divided into three lineages as defined by wiedmann. lineage i includes two clonal complexes: cc and cc of serotypes1/2b, 3b and 4b, and nine singletons of which seven are serotype 1/2b and two are serotype 4b. lineage ii includes four clonal complexes: cc cc cc and cc cc contains the largest number of sts including st st st st st and st all isolates in cc were serotype 1/2c. cc and cc were serotype 1/2a while cc includes both serotypes 1/2a and 3a. the singletons in this lineage were all serotype 1/2a, except for one isolate being serotype 1/2c. lineage iii contained two isolates, both belonging to st and serotype 4c. discussion correlation among serotype, pulse type and sequence type in most cases, l. monocytogenes isolates of the same pt and st belong to the same serotype but there were exceptions. two isolates of the same pt and st are different serotypes. among the five isolates of pulse type gx6a and st four and one were serotype 3a and serotype 1/2a respectively. the observation indicates that serotype 3a and 1/2a can be easily switched. additionally there were cases of the same pt but different sts. for example, of isolates with pt gx6a, isolates were st isolates were st isolates were st and one each was st st st and st respectively. these sts were all grouped into cc except for st which shares of the alleles with st in another case, of the isolates of pt gx6a, were st one each was st and st both of which shared only alleles with st the simpson’s diversity index for pfge is which is only slightly higher than that of mlst. however the discriminatory power for pfge can be increased by using an additional enzyme apai as recommended by the pulsenet protocol and our study affirms the need to use the additional enzyme for outbreak investigations as discriminatory power of asci is low. comparison of isolates from china with international isolates the sts from this study were compared with sts from an analysis of global isolates from the study of rogon et al. and chenal-francisque et al, we found that of the sts in china shared the same sequence types with isolates from patients in other countries, including maternal-fetal infections, central nervous system infections and bacteriemia patients. seven sts containing nearly half or more than half of the isolates from rogon et al. including st, st, st, st, st, st and st caused maternal-fetal infections. in addition, at least of these sts have caused outbreaks in europe. st caused outbreaks in france in and in sweden in while st caused an outbreak in italy in these same sequence types isolated from food sources and in particular st and st were the most common sts in china. based on these observations, we conclude that these sts have the potential to cause disease in humans in china. human listeriosis has been rarely reported in china which may be contributed by poor disease awareness, lack of diagnostic tools and lack of surveillance. this study also affirms the recent report by chenal-francisque et al. that some clones including epidemic clones are prevalent worldwide and globally distributed. in that study, however, there are only isolates from china to represent eastern asia. our study adds a broader picture from china to the global clones and substantial genetic diversity of l. monocytogenes to the global gene pool from china. the novel sts from this study were not found in the study of chenal-francisque et al., although novel sts fall into their clonal complexes. further, prevalent sts in china may be rare elsewhere, for example, the third most prevalent st, st was seen with a single isolate from colombia in the global set of chenal-francisque et al.. our isolates were from over nine food types and only those from chicken and pork had sufficient numbers for comparison of clonal diversity between food types. there were samples each from chicken and pork. in both food types, st was predominant with and isolates in chicken and pork respectively. genetic diversity is higher from chicken samples as measured by simpson’s index of diversity with and for chicken and pork respectively. population structure and recombination of l. monocytogenes many studies have shown that l. monocytogenes can be divided into three lineages. lineage i includes isolates of serotypes 4b, 1/2b, 3b, 4d and 4e, containing all food-borne-epidemic isolates as well as isolates from sporadic cases in humans and animals. lineage ii includes isolates of serotypes 1/2a, 1/2c, 3a and 3c, containing both human and animal isolates, but is seldom associated with food-borne epidemics and predominantly isolated from food products. lineage iii are mostly serotypes 4a and 4c and is predominantly isolated from animals. all our isolates can be allocated into one of the three lineages. the majority of our isolates including the isolates of st belonged to lineage ii since our isolates were from food sources. fifty six isolates belonged to lineage i while only two isolates, both being st belonged to lineage iii. we used the counting method used by feil et al. to determine the ratio of recombination to mutation per locus. a single allelic difference between sts within a clonal complex was attributed to either mutation if the difference was a single base or recombination otherwise. we found that alleles are three times more likely to change by mutation than by recombination. this estimate is similar to that reported by ragon et al.. interestingly, five of the eleven recombination events observed were in the same gene, three in cc one in cc and one in cc a possible explanation for the high frequency of recombination in abcz is positive selection. however ragon et al. showed that the ratio of non-synonymous/synonymous substitution rate of abcz was suggesting that abcz was not under positive selection. an alternative explanation is that abcz is linked to a nearby gene that is under positive selection and has undergone recombination by hitch-hiking. this scenario has been observed to have occurred in genes around the o antigen encoding locus in e. coli and other species. examination of sequences 30 kb up and down stream of abcz based on the genome sequence of isolate egd-e did not identify a gene or gene cluster that is likely to be under positive selection.  this study analyzed isolates from food sources in china by serotyping, pfge and mlst, and showed that the common sts in china are also the prevalent sts in other countries, many of which contain isolates from human infections. a corollary of this observation is that the sts that have caused outbreaks of human infections in other parts of the world have the potential to cause outbreaks in china. however, there is hardly any data on human l. monocytogenes infections in china partly due to the lack of clinical listeriosis surveillance. a recent report of cases of neonatal listeriosis in a beijing hospital of live births in highlights that the disease may be more common in china. with the country becoming more effluent, food distribution, storage and consumption patterns have also changed. since the isolates from food sources as shown in this study clearly have the potential to cause disease, there is a need for surveillance of clinical listeriosis and implementation of prevention strategies to prevent emergence and outbreaks of human l. monocytogenes infections in china. the findings also have implications for other countries where there is no surveillance system for l. monocytogenes. authors’ contributions yw performed the serotyping and mlst typing work and drafted the manuscript. az performed strain identification. rz, dj and zl performed the pfge experiments. zc and yw participated in the analysis of pfge data. rl participated in data analysis and revised the manuscript. yw collected some strains. jx involved in project design. cy managed the project and co-wrote the manuscript. all authors read and approved the final manuscript. acknowledgments this work was supported by grants from the ministry of science and technology, people’s republic of china. we also thank local food surveillance laboratories of provinces/cities cdc in china for providing the l. monocytogenes isolates, and the institut pasteur for providing the mlst database of l. monocytogenes in their genotyping of pathogens and public health platform.
listeria monocytogenes can cause invasive diseases in humans and farm animals and is frequently isolated from dairy products and poultry. two hundred and twelve isolates of l. monocytogenes from food sources from provinces/cities in china were analysed by serotyping, pulsed field gel electrophoresis and multi-locus sequence typing. however little is known of genetic diversity of chinese l. monocytogenes isolates and their relationships with global isolates. this study also enhances our understanding of genetic diversity of l. monocytogenes in china. the most common sts were st, st and st, accounting for % of the isolates. listeriosis is uncommon in china but l. monocytogenes has been isolated from foods and food processing environments in china.
 listeria monocytogenes can cause invasive diseases in humans and farm animals and is frequently isolated from dairy products and poultry. listeriosis is uncommon in china but l. monocytogenes has been isolated from foods and food processing environments in china. however little is known of genetic diversity of chinese l. monocytogenes isolates and their relationships with global isolates.  two hundred and twelve isolates of l. monocytogenes from food sources from provinces/cities in china were analysed by serotyping, pulsed field gel electrophoresis and multi-locus sequence typing. the predominant serotypes are 1/2a, 1/2b and 1/2c accounting for % of the isolates. pfge divided the isolates into pulse types. twenty nine pts were represented by more than one isolates with pt gx6a containing the most number of isolates. mlst differentiated the isolates into sts, among which were novel. the most common sts were st, st and st, accounting for % of the isolates.  sts prevalent in other parts of the world are also prevalent in china including sts which caused maternal fetal infections or outbreaks, suggesting that these sts potentially can also cause severe human infections or outbreaks in china. surveillance of these sts will provide important information for prevention of listeriosis. this study also enhances our understanding of genetic diversity of l. monocytogenes in china. 