Listeria monocytogenes in the Chinese food system: strain characterization through partial actA sequencing and tissue-culture pathogenicity assays

Genomic epidemiology of hypervirulent Listeria monocytogenes CC619: Population structure, phylodynamics and virulence

Listeria monocytogenes is a ubiquitous foodborne pathogen causing human and animal listeriosis with high mortality. Neurological and maternal-neonatal listeriosis outbreaks in Europe and the United States were frequently associated with clonal complexes CC1, CC2 and CC6 harboring Listeria Pathogenicity Island-1 (LIPI-1), as well as CC4 carrying both LIPI-1 and LIPI-4. However, human listeriosis in China was predominantly linked to CC87 and CC619 from serotype 1/2b. To understand the genetic evolution and distribution patterns of CC619, we characterized the epidemic history, population structure, and transmission feature of CC619 strains through analysis of 49,421 L. monocytogenes genomes globally. We found that CC619 was uniquely distributed in China, and closely related with perinatal infection. As CC619 strains were being mainly isolated from livestock and poultry products, we hypothesized that pigs and live chicken were the reservoirs of CC619. Importantly, all CC619 strains not only harbored the intact LIPI-1 and LIPI-4, but these also carried LIPI-3 that could facilitate host colonization and invasion. The deficiency of LIPI-3 or LIPI-4 markedly decreased L. monocytogenes colonization capacity in a model of intragastric infection in the mouse. Altogether, our findings suggest that the hypervirulent CC619 harboring three pathogenicity islands LIPI-1, LIPI-3 and LIPI-4 is a putatively persistent population in various foods, environment, and human population, warranting the further research for deciphering its pathogenicity and strengthening epidemiological surveillance.

Exogenous Iron Induces NADPH Oxidases-Dependent Ferroptosis in the Conidia of Aspergillus flavus

Aspergillus flavus is saprophytic soil fungus that contaminates seed crops with the carcinogenic secondary metabolite aflatoxin, posing a significant threat to humans and animals. Ferrous sulfate is a common iron supplement that is used to the treatment of iron-deficiency anemia. Here, we identified an unexpected inhibitory role of ferrous sulfate on A. flavus. With specific fluorescent dyes, we detected several conidial ferroptosis hallmarks in conidia under the treatment of 1 mM Fe²⁺, including nonapoptosis necrosis, iron-dependent, lipid peroxide accumulation, and ROS burst. However, unlike traditional ferroptosis in mammals, Fe²⁺ triggered conidial ferroptosis in A. flavus was regulated by NADPH oxidase (NOXs) activation instead of Fenton reaction. Transcriptomic and some other bioinformatics analyses showed that NoxA in A. flavus might be a potential target of Fe²⁺, and thus led to the occurrence of conidial ferroptosis. Furthermore, noxA deletion mutant was constructed, and both ROS generation and conidial ferroptosis in ΔnoxA was reduced when exposed to Fe²⁺. Taken together, our study revealed an exogenous Fe²⁺-triggered conidial ferroptosis pathway mediated by NoxA of A. flavus, which greatly contributes to the development of an alternative strategy to control this pathogen.

Effect of Temperatures Used in Food Storage on Duration of Heat Stress Induced Invasiveness of L. monocytogenes

The unpropitious conditions of the food processing environmenttrigger in Listeria monocytogenes stress response mechanisms that may affect the pathogen's virulence. To date, many studies have revealed that acid, osmotic, heat, cold and oxidative stress modify invasiveness of L. monocytogenes. Nonetheless, there is limited data on the duration of the stress effect on bacterial invasiveness. Since most food is stored at low or room temperatures we studied the impact of these temperatures on the duration of heat stress effect on invasiveness of 8 L. monocytogenes strains. Bacteria were heat-treated for 20 min at 54 °C and then incubated at 5 and 20 °C up to 14 days. A decrease in invasiveness over time was observed for bacteria not exposed to heating. It was found that heat shock significantly reduced the invasion capacity of all strains and the effect lasted between 7 and 14 days at both 5 and 20 °C. In conclusion, 20-min heating reduces invasion capacity of all L. monocytogenes strains; however, the stress effect is temporary and lasts between 7 and 14 days in the food storage conditions. The invasiveness of bacteria changes along with the incubation time and is temperature-dependent.

Sequence-Based Characterization of Listeria monocytogenes Strains Isolated from Domestic Retail Meat in the Tokyo Metropolitan Area of Japan

The level of Listeria monocytogenes contamination of domestic retail meat in Tokyo, Japan, was assessed by comparison of isolates from 2004 to 2007 with those isolated before 2003. The overall prevalence of L. monocytogenes among these samples significantly diminished over time (1998-2003, 28.0%; 2004-2007, 17.6%) reflecting a significant decrease in the frequency of contamination of beef. Serotype 1/2a was isolated most frequently, reflecting a change in the predominant serotype in pork from 1/2c to 1/2a. We performed a simple genetic subtyping method based on 3 genes, iap, sigB, and actA, as well as traditional multilocus sequence typing to classify the allele types (ATs). No extensive variation among sequence types was detected. However, increased genetic diversity among the ATs of the 3 genes in the 2004-2007 isolates was evident. We identified AT 26 of the iap gene, which was not previously reported in Japanese isolates, and 6 ATs of the sigB gene, including 4 with nonsense mutations not currently registered in L. monocytogenes DNA databases. sigB is an evolutionally conserved gene that plays a role in the stress response. Our results indicate that the sigB gene may be relatively unstable among L. monocytogenes strains circulating in Japan.

Genetic subtyping of Listeria monocytogenes via multiple-locus sequence typing using iap, sigB and actA

Pulse field gel electrophoresis (PFGE) is widely used for listeriosis surveillance. Although this technique is effective for epidemiology, the data among laboratories are inconsistent. We previously reported a method for Listeria monocytogenes subtyping combined with sequence analysis of partial iap and whole genome restriction fragment length polymorphism (RFLP) using XbaI, ClaI (BanIII) and PstI. However, distinguishing subtypes was challenging, because the output comprised complicated fragment patterns. In this study, we aimed to establish a simple genotyping method that does not depend on visual observation, rather it focuses on multi-locus sequence typing (MLST) using three genes, iap, sigB and actA. Sixty-eight strains of L. monocytogenes including EGD-e as a reference strain were investigated to ensure consistency with previous data on the genetic characterization. All strains were grouped into 29 types by both analyses. Although there are some differences in classification, major clades included the same strains. Simpson's indices of diversity (SID) by MLST and iap-RFLP-based typing were 0.967 (95% confidence interval [CI]: 0.955/0.978) and 0.967 (95% CI: 0.955/0.979), respectively. The discriminatory power of both methods can be considered almost identical. Compared with the results of 38 selected strains, the strains within the MLST clusters in this study coincided with those obtained using PFGE. Thus, the MLST strategy could help differentiate among L. monocytogenes isolates during epidemiological studies.

Invasiveness of Listeria monocytogenes strains isolated from animals in Poland

Animals are important reservoir of Listeria monocytogenes, a pathogen causing serious infections in both humans and livestock. However, data on invasiveness of L. monocytogenes strains of animal origin is very scarce. Ability of 18 L. monocytogenes strains of animal origin to invade HT-29 cells was investigated. Plaque forming assay was used to assess invasiveness and ability of the pathogen to spread in the cell line. Almost 40% of L. monocytogenes strains were weakly invasive. It was shown that strains from serogroup 4b exhibited the highest invasiveness, whereas serogroup 1/2b consisted of strains of invasiveness below 0.0001%. Analysis of translated inlA and inlB gene sequences revealed no premature stop codons. Lineage-specific mutations in low invasive strains were identified within inlA and inlB sequences. Our results demonstrate high incidence of low invasive animal L. monocytogenes strains, which may be at least partly explained by unique point mutations in the InlA and InlB.

Prevalence and Virulence Characterization of Listeria monocytogenes in Chilled Pork in Zhejiang Province, China

Listeria monocytogenes is an important foodborne pathogen that can grow in refrigeration temperature and causes severe human infections. The aims of this work were to estimate the prevalence of L. monocytogenes in chilled pork in Zhejiang, China and to examine the virulence features of the isolates. Of 331 meat samples, 196 were positive for Listeria spp., with L. innocua accounting for 54.4%, L. monocytogenes for 11.5%, and L. welshimeri for 4.2%. The most prevalent L. monocytogenes serotype was 1/2c (60.5%), followed by serotypes 1/2a (28.9%), 1/2b (7.9%), and 4b (2.6%). All L. monocytogenes isolates contained virulence-associated genes examined. Adhesion and invasion ability of serotype 1/2c isolates was much lower than those of other serotypes. Only one isolate was defective in cell-to-cell spread. These findings are important for risk assessment of chilled pork as a source of potential transmission of L. monocytogenes to other food products, particularly to ready-to-eat food products.

Salt stress-induced invasiveness of major Listeria monocytogenes serotypes

Of the 13 serotypes, 4b serotype strains are responsible for the majority of recorded invasive listeriosis outbreaks, although some recent listeriosis outbreaks have been attributed to strains of serotypes 1/2a and 1/2b. Virulence and response to osmotic stress in 41 Listeria monocytogenes strains representing serotypes 1/2a, 1/2b and 4b was investigated. It was found that serotype 4b and 1/2b strains exhibited highest invasion efficiency and formed largest plaques in HT-29 cell monolayer. Invasiveness in response to 10-min exposure to 0·3 mol l⁻¹ NaCl was the highest in serotype 4b strains. We demonstrated that 4b serotype L. monocytogenes strains not only have the greatest pathogenic potential but also are the most invasive in response to salt stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

Listeria monocytogenes 4b serotype strains are responsible for the majority of recorded invasive listeriosis outbreaks. We showed that strains of serotype 4b are not only the most virulent L. monocytogenes strains but also have the best capacity to enhance their invasiveness in response to salt stress. Our results suggest possession of effective stress response mechanisms of 4b serotype strains, which may contribute to the high infection potential of this subpopulation.

Genetic diversity and molecular typing of Listeria monocytogenes in China

BACKGROUND

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.

RESULTS

Two hundred and twelve isolates of L. monocytogenes from food sources from 12 provinces/cities in China were analysed by serotyping, Pulsed Field Gel Electrophoresis (PFGE) and Multi-locus Sequence Typing (MLST). The predominant serotypes are 1/2a, 1/2b and 1/2c accounting for 90.1% of the isolates. PFGE divided the isolates into 61 pulse types (PTs). Twenty nine PTs were represented by more than one isolates with PT GX6A16.0004 containing the most number of isolates. MLST differentiated the isolates into 36 STs, among which 15 were novel. The 3 most common STs were ST9 (29.1%), ST8 (10.7%) and ST87 (9.2%), accounting for 49.0% of the isolates.

CONCLUSIONS

STs prevalent in other parts of the world are also prevalent in China including 7 STs (ST1-ST3, ST5, ST6, ST8, ST9) 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.

Effect of heat exposure on invasiveness of Listeria monocytogenes strains

The effect of a 54°C heat exposure lasting 20 and 60 minutes on the invasiveness of 10 Listeria monocytogenes strains in the exponential and stationary phases was investigated. It was shown that heat exposure significantly reduced the L. monocytogenes count. A maximal reduction of 5 log took place after 60 minutes of heating. No significant differences in survival between exponential and stationary phases were observed for most strains. Twenty minutes of heat exposure resulted in a reduction of the invasiveness of L. monocytogenes, which was more pronounced in the exponential-phase bacteria. Prolongation of heating to 60 minutes was shown to have a significant impact on the invasiveness of five stationary-phase strains. It was demonstrated that heat exposure influences the survival of L. monocytogenes strains as expected. Also, the invasiveness was significantly changed in a time- and growth phase-dependent manner. Low reductions of bacterial counts in milder conditions, that is, 20 minutes of heat, were accompanied by a decrease of the invasiveness of all L. monocytogenes strains. Prolongation of heating time to 60 minutes resulted in significant reduction of bacterial numbers. However, bacteria, especially those in the stationary phase, which survived this treatment can become more invasive.

Prevalence and characterization of antimicrobial resistance of foodborne Listeria monocytogenes isolates in Hebei province of Northern China, 2005-2007

A total of 2177 food samples collected from nine cities in northern China during 2005 to 2007 were screened for the presence of Listeria monocytogenes. All L. monocytogenes isolates were subjected to serotyping, antimicrobial susceptibility, pulsed-field gel electrophoresis (PFGE), as well as PCR screening to identify genes responsible for tetracycline resistance [tet(L), tet(M), tet(K), tet(S) and tet(B)], transposon Tn916, and class 1 integron. Contamination with L. monocytogenes was detected in 4.13% (90/2177) of the total samples representing various food products. The pathogen was mainly isolated from frozen food made of wheat flour or rice products (26/252, 10.32%) and raw meat products (46/733, 6.28%). Besides, 3.31% (10/302) of cooked meat, 1.17% (4/343) of seafood, 0.98% (2/204) of non-fermented bean products and 0.62% (2/323) of vegetables samples were contaminated by this bacterium. The L. monocytogenes isolates belonged to five serotypes (1/2a, 1/2b, 1/2c, 4b, and 3a), with serotype 1/2a being dominant (48.88%). Antimicrobial resistance was most frequently observed for ciprofloxacin (17.8%), tetracycline (15.6%) and streptomycin (12.2%). Overall, resistance was observed against 14 out of 18 antimicrobials tested while multiple resistances occurred among 18.9% (17/90) isolates. Interestingly, two isolates were resistant to more than five antimicrobials. Among 14 tetracycline-resistant isolates, 13 carried tet(M) gene including nine possessing Tn916, and one harbored tet(S) gene. PFGE analysis revealed genetic heterogeneity among individual serotypes as well as scattered occurrence of some genotypes without any clear-cut correlation to source or food type. The widespread distribution of epidemiologically important serotypes (1/2a, 1/2b and 4b) of L. monocytogenes, and their resistance to commonly used antibiotics indicate a potential public health risk. Our data also indicate that L. monocytogenes could act as a reservoir of mobile tet genes along the food chain.

Effect of osmotic stress and culture density on invasiveness of Listeria monocytogenes strains

The effect of osmotic stress on its capacity to invade the human enterocytic cell line HT-29 was studied in the early log through the stationary phase in 10 L. monocytogenes strains representing three genetically independent lineages. The results demonstrate that the transition of the bacteria from the log to the stationary phase results in a stepwise reduction of invasiveness. This effect was heterogeneous in the studied L. monocytogenes population, as the range of invasiveness reduction between the log and stationary phases varied from 10- to 380-fold depending on the strain. Ten-minute exposure to 0.3 M NaCl was sufficient to generate invasiveness alteration. No significant change in invasiveness induction caused by osmotic stress was found between the different points of the log phase (OD₆₀₀ 0.4-1.2), being significantly different in the early log phase (OD₆₀₀ 0.2-0.3) and in the stationary phase after 18 h of culture. The level of internalins and opuCA transcripts in response to osmotic stress did not correlate with invasiveness alteration in most L. monocytogenes strains. Prolongation of stress exposure to 1 h and an increase in NaCl concentration from 0.3 to 1.8 M had no significant effect on a further increase in invasiveness. Short exposure times and low NaCl concentrations were sufficient for the generation of maximal invasiveness response of L. monocytogenes. It appears that although stationary-phase bacteria exhibit lower invasiveness than log-phase bacteria, they have a greater capacity to enhance their pathogenicity in response to stress.

Protective immunity induced by a LLO-deficient Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen able to cause serious disease in human and animals. Listeriolysin O (LLO), a major virulence factor secreted by this bacterium, is a vacuole-specific lysin that facilitates bacterial entrance into the host cytosol. Thus, LLO plays a key role in the translocation and intracellular spread of L. monocytogenes. To study the effect of LLO on virulence and immunopotency, a LLO-deficient L. monocytogenes mutant was constructed using a shuttle vector followed by homologous recombination. The mutant strain had lost hemolytic activity, which resulted in an extremely reduced virulence, 5 logs lower than that of the parent strain, yzuLM4, in BALB/c mice. The number of bacteria detected in the spleens and livers of mice infected with the mutant was greatly reduced, and the bacteria were rapidly eliminated by the host. Kinetics studies in this murine model of infection showed that the invasion ability of the mutant strain was much lower than that of the parent strain. Moreover, immunization with the mutant strain conferred protective immunity against listerial infection. In particular, stimulation with Ag85B240-259, strong specific Th1 type cellular immunity was elicited by vaccination C57BL/6 mice with hly deficient strain delivering Mycobacterium tuberculosis fusion antigen Ag85B-ESAT-6 via intravenous inoculation. These results clearly show that highly attenuated LLO-deficient L. monocytogenes is an attractive vaccine carrier for delivering heterologous antigens.

Different contamination patterns of lineage I and II strains of Listeria monocytogenes in a Spanish broiler abattoir

The purpose of this study was to determine whether genetically similar or diverse strains of Listeria monocytogenes colonize the environment and carcasses in a single Spanish broiler abattoir over time. The study was composed of 5 surveys over a 1.5-yr period and included the monitoring of cleaning and disinfection procedures. Overall, a total of 212 samples were tested for the presence of L. monocytogenes, and 31% of the samples were found to be positive. Listeria monocytogenes was isolated from carcasses and product contact and noncontact sites in the evisceration and carcass classification areas of the abattoir. A total of 132 L. monocytogenes isolates were characterized by PCR-based serotyping and pulsed-field gel electrophoresis (PFGE) restriction analysis with the endonucleases ApaI and AscI. Molecular serotyping showed that L. monocytogenes isolates were of serotypes 1/2a and 1/2b. Isolates of serotype 1/2b (89.4%) were contaminating carcasses as well as environmental product contact and noncontact sites, whereas isolates of serotype 1/2a (10.6%) were recovered only from environmental product noncontact sites. A relatively low genetic diversity was found in this group of L. monocytogenes isolates from the abbatoir; only 14 different PFGE types (A1 to A14) were obtained. Nine pulsotypes belonging to serotype 1/2b (lineage I) were grouped in only one PFGE genetic cluster, whereas 5 pulsotypes belonging to serotype 1/2a (lineage II) were grouped into 4 PFGE genetic clusters. Two genetically related pulsotypes of serotype 1/2b (A1 and A2, 64.4% of the isolates) predominated and persisted in the abattoir. Our study indicated that a few strains of L. monocytogenes lineage I that were genetically very closely related might be specifically adapted to colonizing the evisceration zone of the abattoir and were predominant on carcasses over 1 yr. On the other hand, a genetically diverse group of lineage II strains were present in the abattoir environment, but never contaminated carcasses.

Neonatal listeriosis in Taiwan, 1990-2007

OBJECTIVES

Listeria monocytogenes is an important pathogen in neonates in Western countries, with a fatality rate of 20-30%. There is limited information on neonatal listeriosis in Eastern countries. The purpose of this study was to delineate the occurrence and clinical picture of neonatal listeriosis in Taiwan.

METHODS

A questionnaire-based survey of all of the 17 medical centers in Taiwan was performed, and a literature review of neonatal listeriosis as reported in Taiwan from 1990 to 2007 was made.

RESULTS

A total of 14 cases (10 male, four female) of neonatal listeriosis were identified, including 11 found from the survey of four medical centers and another three collected from the literature review. Three were found to have occurred prior to 2000 and 11 were found to have occurred after 2000. The age of onset was less than 3 days in all cases. L. monocytogenes was identified from blood in 13, cerebrospinal fluid in four, and gastric aspirate in two. Half of the cases (7/14) had involvement of the central nervous system with pleocytosis and hypoglycorrhachia in cerebrospinal fluid, and three of them even developed hydrocephalus. The mortality rate was 29%.

CONCLUSIONS

Our findings suggest that listeriosis may emerge as an important health threat among newborn infants in Taiwan.

Toward an improved laboratory definition of Listeria monocytogenes virulence

Listeria monocytogenes is an opportunistic foodborne pathogen that encompasses a diversity of strains with varied virulence. The ability to rapidly determine the pathogenic potential of L. monocytogenes strains is integral to the control and prevention campaign against listeriosis. Early methods for assessing L. monocytogenes virulence include in vivo bioassays and in vitro cell assays. While in vivo bioassays provide a measurement of all virulence determinants of L. monocytogenes, they are not applied routinely due to their reliance on experimental animals whose costs have become increasingly prohibitive. As a low cost alternative, in vitro cell assays are useful for estimating the virulence of L. monocytogenes strains. However, these assays are often slow, and at times variable. Prior attempts to ascertain L. monocytogenes virulence by targeting virulence-associated proteins and genes have been largely unsuccessful, since many of the assay targets are present in both virulent and avirulent strains. Recent identification of novel virulence-specific genes (particularly internalin gene inlJ) has opened a new avenue for rapid, sensitive, and precise differentiation of virulent L. monocytogenes strains from avirulent strains. The application of DNA sequencing technique also offers an additional tool for assessing L. monocytogenes virulence potential. By providing an update on the laboratory methods that have been reported for the determination of L. monocytogenes pathogenicity, this review discusses future research needs that may help achieve an improved laboratory definition of L. monocytogenes virulence.

Prevalence and Antimicrobial Resistance of Foodborne Pathogens Isolated from Food Products in China

This study was undertaken to investigate the prevalence of Salmonella, Listeria monocytogenes, Staphylococcus aureus, Vibrio parahaemolyticus, and Escherichia coli O157:H7 in Chinese food products. The prevalence of these pathogens was 3.46%, 5.79%, 7%, 0.24%, and 0%, respectively. Raw meats were mainly contaminated with Salmonella (39/365, 10.7%), L. monocytogenes (26/365, 7.1%), and S. aureus (40/365, 11%), while cooked food products were mainly contaminated with L. monocytogenes (45/384, 11.7%) followed by S. aureus (12/384, 3.1%), and raw milk was mainly contaminated with S. aureus (34/209, 16.3%) and Salmonella (4/209, 1.9%). Antimicrobial resistance was evaluated in Salmonella, L. monocytogenes, and S. aureus. Antimicrobial resistance for L. monocytogenes was most frequently observed for cefotaxime (51/72, 70.8%) followed by furazolidone (40/72, 55.6%). Multiple resistance (resistance to >or=2 antibiotics) was observed for 63.9% (46/72) of L. monocytogenes isolates. Resistance of Salmonella was most frequently observed to amoxicillin (11.6%), ticarcilline (11.6%), cephalothin (11.6%), and cefuroxime (11.6%). Multiple resistance was observed for 16.3% (7/43) of the Salmonella isolates. Staphylococcus aureus was resistant to penicillin (93.1%) followed by tetracycline and oxacillin COAG (49.4% and 37.9%, respectively). About 79% (69/87) of S. aureus isolated demonstrated multiple resistance. The data showed that raw meat, cooked food products, and raw milk were most commonly contaminated with foodborne pathogens and many pathogens were resistant to different antibiotics. The study provided useful information for assessment of the possible risk posed to Chinese consumers, which has significant public health impact in China.

Traceback identification of an ingredient (pork dewlap) as the possible source of Listeria monocytogenes serotype 4b contamination in raw chicken products

In surveys conducted on finished product samples from a single poultry processing plant in Spain, Listeria monocytogenes was found in 14 different uncooked products. To track contamination patterns, 77 L. monocytogenes isolates were characterized by PCR-based serotyping, pulsed-field gel electrophoresis (PFGE) restriction analysis, and PCR-based allelic analysis of the virulence gene actA. Serotyping revealed that 12 isolates (15.6%) were of the L. monocytogenes serotype 4b complex (serotype 4b or the closely related serotypes 4d and 4e). A combination of endonucleases AscI and ApaI PFGE patterns yielded 15 different pulsotypes among all 77 tested isolates. All the serotype 4b isolates belonged to one pulsotype. Sequencing of the actA gene confirmed that all serotype 4b isolates corresponded to the same allelic subtype. The subtype was recovered from five product types, but its presence was not correlated with the production line or the date of isolation, suggesting a possible association of this strain with a common ingredient. This traceback investigation established that pork dewlap, an ingredient common to all the products contaminated with this strain, was the most probable source of L. monocytogenes 4b. The same 4b strain was isolated from four samples of pork dewlap from one specific supplier. After replacement of this contaminated ingredient in the fresh products, this strain of L. monocytogenes serotype 4b was not detected. This study confirms the effectiveness of molecular subtyping to control contamination by specific strains of L. monocytogenes and the importance of testing the different ingredients added to the food products.

Interactions between the environmental pathogen Listeria monocytogenes and a free-living protozoan (Acanthamoeba castellanii)

Listeria monocytogenes can cause severe disease in animal hosts, but it has no recognized animal host reservoir. We tested the hypothesis that L. monocytogenes retains virulence traits to survive predation by amoebae and that listeriolysin O plays a crucial role in this process. Co-culturing of L. monocytogenes and Acanthamoeba castellanii demonstrated that L. monocytogenes does not actively kill amoebae, but in the presence of amoebae, high bacterial population densities can be maintained over a period of at least 96 h. A gentamicin protection assay demonstrated that there is no significant difference in the ability to survive predation between serovars (4b versus 1/2a and 1/2c; P = 0.08) and between five species of Listeria (P = 0.14). Three of these species do not harbour the hly gene responsible for listeriolysin O production. A hly knockout strain had poorer survival compared with the parental strain (P = 0.04 at 24 h; P = 0.04 at 48 h; P = 0.02 at 72 h) and electron microscopy was consistent with a wild-type strain being able to escape the phagosome whereas the hly knockout strain did not appear to have this ability. Thus, while there is weak evidence that listeriolysin O can contribute to improved survival after ingestion by amoebae, listeriolysin O does not appear to provide a significant selective advantage under the conditions of this study.

International Life Sciences Institute North America Listeria monocytogenes strain collection: development of standard Listeria monocytogenes strain sets for research and validation studies

Research and development efforts on bacterial foodborne pathogens, including the development of novel detection and subtyping methods, as well as validation studies for intervention strategies can greatly be enhanced through the availability and use of standardized strain collections. These types of strain collections are available for some foodborne pathogens, such as Salmonella and Escherichia coli. We have developed a standard Listeria monocytogenes strain collection that has not been previously available. The strain collection includes (i) a diversity set of 25 isolates chosen to represent a genetically diverse set of L. monocytogenes isolates as well as a single hemolytic Listeria innocua strain and (ii) an outbreak set, which includes 21 human and food isolates from nine major human listeriosis outbreaks that occurred between 1981 and 2002. The diversity set represents all three genetic L. monocytogenes lineages (I, n = 9; II, n = 9; and III, n = 6) as well as nine different serotypes. Molecular subtyping by EcoRI automated ribotyping and pulsed-field gel electrophoresis (PFGE) with AscI and ApaI separated the 25 isolates in the diversity set into 23 ribotypes and 25 PFGE types, confirming that this isolate set represents considerable genetic diversity. Molecular subtyping of isolates in the outbreak set confirmed that human and food isolates were identical by ribotype and PFGE, except for human and food isolates for two outbreaks, which displayed related but distinct PFGE patterns. Subtype and source data for all isolates in this strain collection are available on the Internet and are linked to the PathogenTracker database (www.pathogentracker.com), which allows the addition of new, relevant information on these isolates, including links to publications that have used isolates from this collection. We have thus developed a core L. monocytogenes strain collection, which will provide a resource for L. monocytogenes research and development efforts with centralized Internet-based data curation and integration.

Prevalence and lineages ofListeria monocytogenesin Chinese food products

AIMS

This study was undertaken to investigate the prevalence and lineages of Listeria monocytogenes in different kinds of food products in local Chinese markets.

METHODS AND RESULTS

A total of 2686 food samples and 645 water samples were collected and L. monocytogenes was isolated from 2.28% (76 of 3331) of all samples. The prevalence of L. monocytogenes (14 of 290, 4.83%) in raw meat products was significantly higher than that in other raw food products (P < 0.05). Among 844 ready-to-eat (RTE) food samples, 21 samples were positive for L. monocytogenes. RTE packaged food products from two supermarkets had a prevalence ranging from 0.00% to 25.00%. The prevalence of L. monocytogenes in meat products of freshly slaughtered hogs was 0.95% (four of 420), significantly lower than that in raw meat products in the retail markets (P < 0.05). Ten isolates were recovered from 645 water samples, which were collected after hands washing by shopkeepers or waiters. A total of 38 isolates were randomly selected for lineage classification based on the nucleotide variation of actA gene. Eighty percentage of isolates from RTE food products belonged to Lineage II while only 20% belonged to Lineage I.

CONCLUSIONS

Food products in Chinese markets are contaminated with L. monocytogenes. Raw meat products have the highest contamination rates among all the raw food samples. RTE food products are more likely to be contaminated with Lineage II strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

The data presented here show the main contamination sources of L. monocytogenes in Chinese food products.