| The genus Listeria are a group of Gram-positive,nonsporing and short-rods bacteria.It currently contains 17 species,of which two,Listeria monocytogenes and Listeria ivanovii,are pathogenic.L.monocytogenes,as the major etiological agent of listeriosis,has received increasing attention due to the potential for widespread outbreaks in developed countries.The bacterium has been isolated from a wide variety of sources including foods,animals and environments for its ability to adapt in diverse ecological niches.Human diseases caused by L.monocytogenes varied from mild to severe.Healthy individuals present self-limiting febrile gastroenteritis after large bacteria inoculum orally administered.Severe invasive infections,which manifested as bacteremia with or without cerebral nervous system(CNS)infections,were occurred in immunocompromised individuals.For women in pregnancy,the complications may include abortion,preterm delivery or stillbirth.Listeriosis is an uncommon but severe infection with a high case mortality rate that can exceed 30%.A large number of food products have to recall due to the occurrence of listeriosis outbreaks.The adverse economic effect and the significant disease burden led to the implementation of national surveillance system for infections in developed countries.L.monocytogenes consists of four lineages with distinct but overlapping ecological niche.Most L.monocytogenes strains belong to lineage Ⅰ and Ⅱ.Lineage Ⅰ harbor the serotypes more commonly associated with sporadic human clinical cases and listeriosis outbreaks.While lineage Ⅱ strains are predominant in food products and food-processing environments.Lineage Ⅲ and Ⅳ strains are rare and commonly isolated from animal sources.In China,except for a few sporadic human clinical case reports,no epidemiological information about clinical L.monocytogenes strains has been reported due to lack of surveillance on listeriosis.In this study,28 L.monocytogenes strains,which were collected from patients in China from four regions and isolated from CNS infections,bacteremia and maternal-fetal infections during 2007 to 2012,were serotyped and characterized using pulsed-field gel electrophoresis(PFGE)and multi-locus sequence typing(MLST).ST87 and ST3,both of which belong to serotype 1/2b,predominately cause the human listeriosis in China.The occurrence of Listeria spp.from food products and food processing environments have been well studied.However,there is a lack of published information on the occurrence and antimicrobial susceptibility of Listeria spp.in natural environments in China.In this study,we investigated the occurrence and antimicrobial susceptibility of Listeria spp.isolated from 341 intestinal fecal sample of wild rodents found in natural environments in China.All L.monocytogenes strains were further characterized by serotyping,PFGE and MLST.Almost half of the isolates(5 out of 11)belonged to serotype 1/2b and ST87.This section determined that Listeria is carried in the intestinal tracts of wild rodents from multiple regions at a low rate in China,filling an epidemiological data gap on Listeria on natural environments of China.The ST87 subgroup of L.monocytogenes is among the most commonly isolated in China from food products,natural environments and sporadic clinical cases.In this study,we provided a comparative genomics analysis of L.monocytogenes ST87 subgroup.A panel of 71 strains were chosen to represent diverse sources(including raw meat,poultry,aquatic product,cooked food,environment and patient),different geographical locations(including Sichuan,Beijing,Shanghai,Zhejiang,Fujian,Henan,Hubei,Anhui and Guangdong)and different isolation year(during 2000 to 2015).The ST87 genomes present a highly conserved core genome with the majority of variations located in mobile genetic elements such as prophages and plasmid.Throughout the whole genome of ST87 strains,at least seven hotspots could harbor different prophages.All strains bear a CRISRP/Cas system,which represent alternative bacteriophage defense mechanisms for this subgroup L.monocytogenes.32.4%strains(23/71)harbored a conserved plasmid of about 90Kb,which contained heavy metal resistance genes and a toxin/anti-toxin(TA)system.These genetic elements may contribute to the diverse range of niches colonized by ST87 strains.The plasmid shared a high degree of similarity to the pLM5578,which is from strain 1/2a 08-5578.However,none of clinical strains harbored this plasmid.All the ST87 strains harbored the newly described LIPI-4 pathogenicity island,but not contained LIPI-3 which associated with a subset of lineage I strains.Eleven clades can be discerned from the SNP-based core genome tree and the remainder of the taxonomy had a high degree of uncertainty.Some clades contained identical geographic source strains.While the others contained more than one geographic sources strains.The clinical strains spread across multiple clades or branches.A 10-ORF genomic island containing a restriction-modification(R-M)system were identified unique to ST87 strains.A mutant harboring an in-frame deletion of C-5 cytosine-specific DNA methylase modulated some metabolism genes expression,indicating a state of stress.While a mutant harboring the entire genomic island deletion relieved the stress state and restored as the wild-type strain.The transcriptional data seggusted that the genomic island has no effect on the ST87 strains except for the effect of R-M system.However,the ST87-specific genomic island could be the molecular target for rapid identification of the prevalent subgroup. |
