Monocytogenes Serogroup Research Articles

Surveillance of stewed meat products in Shanghai, China: Importance of controlling processing environment contamination for Listeria monocytogenes

Human listeriosis is a foodborne disease caused by Listeria monocytogenes contamination. Most cases occur due to consumption of ready-to-eat (RTE) foods. Stewed meat in seasoning, a type of pre-packaged ready-to-eat (RTE) food, is a popular traditional product in China. We used whole genome sequencing to analyze core genome multilocus sequence typing (cgMLST), serogroups and multilocus typing sequence typing (ST) of 67 L. monocytogenes isolates. The L. monocytogenes serogroup IIb-ST5 was predominantly isolated from different sources and linked to stewed meat in seasoning of Brand A, B, and C with <7 allelic differences by cgMLST, suggesting that this kind of food may be a high-risk food causing sporadic cases of listeriosis. Furthermore, L. monocytogenes isolates from different batches of stewed meat in seasoning products processed in the same processing plant had <5 five allelic differences, confirming that contamination occurred during manufacture. Additionally, 16 L. monocytogenes isolates from Plant A were the same clone (0 allelic difference), which confirmed the strong ability of L. monocytogenes to survive in the processing environment. Thus, increased and frequent monitoring, as well as constant technical support for the control of L. monocytogenes in food processing plants is necessary.

Distribution of Listeria monocytogenes serogroups in food of animal origin produced in Poland during 2013–2022

Listeria monocytogenes is an important food-borne pathogen that causes severe disease in humans. Many different foods products may be vehicles of these pathogenic strains. The aims of this study were to identified serogroups L. monocytogenes isolates by using PCR and analyze the distribution of serogroups in different food products of animal origin. L. monocytogenes isolates from ready-to-eat (RTE) meat products (n = 1,571), non-meat RTE food (n = 237), and non-RTE products (n = 189) were collected during years 2013-2020. A total of 899 (45.0%) isolates, irrespective of the origin, were classified to serogroup IIa. The remaining 1,098 strains belonged either to serogroups IVb (478; 23.9%), IIc (347; 17.4%) or IIb (273; 13.7%), respectively. In the predominant group products, i.e. RTE food of meat origin, serogroup IIa (702; 44.7%) was the most common followed by IVb (334; 21.3%), IIc (300; 19.1%), and IIb (235; 14.9%), respectively. The statistical differences were observed between the prevalence of L. monocytogenes serogroup IIb in sausages and cold cuts (P &lt; 0.0001), cold cuts and delicatessen food (P &lt; 0.001), as well as strains of serogroup IVb from sausages and cold cuts (P &lt; 0.001). The prevalence of serogroup IIa was statistically higher among strains of milk and milk products origin than RTE fish and fish products (P &lt; 0.05). The differences (P &lt; 0.0001) were also observed in the distribution of L. monocytogenes of serogroup IIa from mixed food and milk as well as fish products. Additionally, the occurrence of serogroup IVb in mixed food differed compared to other food categories, i.e. milk and fish products (P &lt; 0.0001). The obtained results confirmed the presence of L. monocytogenes serogroups IIa and IVb in food of animal origin, including RTE products, that are responsible for listeriosis in humans and suggested that these food products may pose a potential health problem, particularly to sensitive groups of people.

Validated differentiation of Listeria monocytogenes serogroups by FTIR spectroscopy using an Artificial Neural Network based classifier in an accredited official food control laboratory

Listeria monocytogenes is a well-known human pathogen, and especially the young, the elderly, otherwise immunocompromised individuals or pregnant women might suffer severe health consequences from listeriosis. Up to date, Fourier-Transform Infrared (FTIR) spectroscopical methods have been established for decades as a valuable means to differentiate between microbiological specimens at different taxonomical levels. In recent years, machine-based learning methods using Artificial Neural Networks (ANN) have highly advanced the discriminatory power of distinguishing spectrally closely related units such as serogroups of a given species. The present report describes the classification performance evaluation of a manufacturer (Bruker Daltonics, Bremen, Germany) - provided L. monocytogenes serogroup classifier by means of a formalized external validation carried out in a single laboratory. N = 630 absorption spectra from n = 94 food L. monocytogenes isolates pertaining to n = 11 serotypes / n = 3 serogroups were recorded on the IR Biotyper (Bruker Daltonics) and subsequently typed by the given classifier. The quantitative evaluation of inclusivity and exclusivity was performed following the principles of the Guidelines for Validating Species Identifications Using MALDI-TOF-MS issued by the German Federal Office of Consumer Protection (BVL) for a targeted identification. The FTIR classifier allocated all n = 486 spectra from n = 71 serogroup 1/2 and 4 isolates correctly to their respective serogroups, resulting in a true-positive rate of 100%. All remaining n = 144 spectra from n = 23 isolates of serogroup 3 were correctly allocated to an arbitrarily combined class entity of serogroups 3 and 7, likewise yielding both inclusivity and exclusivity rates of 100%. Consequently, in our official food control laboratory, this validated IR Biotyper method has been integrated into the accredited workflow for L. monocytogenes analysis in food samples according to ISO 11290, followed by MALDI-TOF MS confirmation on the species level to subsequent serogrouping and pre-selection by FTIR spectroscopy for Whole Genome Sequencing (WGS). This study confirmed that FTIR spectroscopy in combination with Artificial Neural Networks proves to be a reliable and thus valuable tool for the differentiation of the most common serogroups from Listeria monocytogenes. The application of FTIR spectroscopy saves valuable resources with respect to labor and time and thus facilitates outbreak analyses of the clinically relevant severe food-borne disease listeriosis where potentially a high number of isolates are involved.

Identification of Listeria species and Multilocus Variable-Number Tandem Repeat Analysis (MLVA) Typing of Listeria innocua and Listeria monocytogenes Isolates from Cattle Farms and Beef and Beef-Based Products from Retail Outlets in Mpumalanga and North West Provinces, South Africa.

In this study, Listeria isolates (214) were characterized as follows: L. innocua (77.10%), L. monocytogenes (11.21%), L. welshimeri (5.61%), L. grayi (1.40%), L. seeligeri (0.93%), and L. species (3.73%) that were not identified at the species level, from beef and beef based products from retail and farms in Mpumalanga and North West provinces of South Africa. MLVA was further used to type Listeria innocua isolates (165) and Listeria monocytogenes isolates (24). The L. monocytogenes isolates were also serogrouped using PCR. The MLVA protocol for L. monocytogenes typing included six tandem repeat primer sets, and the MLVA protocol for L. innocua included the use of three tandem repeats primer sets. The L. monocytogenes serogroups were determined as follows: 4b-4d-4e (IVb) (37.50%), 1/2a-3a (IIa) (29.16%), 1/2b-3b (IIb) (12.50%), 1/2c-3c (IIc) (8.33%), and IVb-1 (4.16%). MLVA could cluster isolates belonging to each specie, L. monocytogenes, and L. innocua isolates, into MLVA-related strains. There were 34 and 10 MLVA types obtained from the MLVA typing of L. innocua and L. monocytogenes, respectively. MLVA clustered the L. monocytogenes isolates irrespective of sample category, serogroups, and geographical origin. Similarly, the L. innocua isolates clustered irrespective of meat category and geographical origin. MLVA was able to cluster isolates based on MLVA relatedness. The clustering of isolates from farms and retailers indicates transmission of Listeria spp. MLVA is an affordable, simple, and discriminatory method that can be used routinely to type L. monocytogenes and L. innocua isolates.

Frozen Vegetable Processing Plants Can Harbour Diverse Listeria monocytogenes Populations: Identification of Critical Operations by WGS

Frozen vegetables have emerged as a concern due to their association with foodborne outbreaks such as the multi-country outbreak of Listeria monocytogenes serogroup IVb linked to frozen corn. The capacity of L. monocytogenes to colonize food-processing environments is well-known, making the bacteria a real problem for consumers. However, the significance of the processing environment in the contamination of frozen foods is not well established. This study aimed to identify potential contamination niches of L. monocytogenes in a frozen processing plant and characterize the recovered isolates. A frozen vegetable processing plant was monitored before cleaning activities. A total of 78 points were sampled, including frozen vegetables. Environmental samples belonged to food-contact surfaces (FCS); and non-food-contact surfaces (n-FCS). Positive L. monocytogenes samples were found in FCS (n = 4), n-FCS (n = 9), and the final product (n = 1). A whole-genome sequencing (WGS) analysis revealed two clusters belonging to serotypes 1/2a-3a and 1/2b-3b). The genetic characterization revealed the presence of four different sequence types previously detected in the food industry. The isolate obtained from the final product was the same as one isolate found in n-FCS. A multi-virulence-locus sequence typing (MVLST) analysis showed four different virulence types (VT). The results obtained highlight the relevant role that n-FCS such as floors and drains can play in spreading L. monocytogenes contamination to the final product.

Genetic Diversity and Relationships of Listeria monocytogenes Serogroup IIa Isolated in Poland.

In the present study, 100 L. monocytogenes isolates of serogroup IIa from food and food production environments in Poland were characterized towards the presence of virulence, resistance, and stress response genes using whole-genome sequencing (WGS). The strains were also molecularly typed and compared with multi-locus sequence typing (MLST) and core genome MLST analyses. The present isolates were grouped into 6 sublineages (SLs), with the most prevalent SL155 (33 isolates), SL121 (32 isolates), and SL8 (28 isolates) and classified into six clonal complexes, with the most prevalent CC155 (33 strains), CC121 (32 isolates), and CC8 (28 strains). Furthermore, the strains were grouped to eight sequence types, with the most prevalent ST155 (33 strains), ST121 (30 isolates), and ST8 (28; strains) followed by 60 cgMLST types (CTs). WGS data showed the presence of several virulence genes or putative molecular markers playing a role in pathogenesis of listeriosis and involved in survival of L. monocytogenes in adverse environmental conditions. Some of the present strains were molecularly closely related to L. monocytogenes previously isolated in Poland. The results of the study showed that food and food production environments may be a source of L. monocytogenes of serogroup IIa with pathogenic potential.

Diversity and Antibiotic Resistance Profiles of Listeria monocytogenes Serogroups in Different Food Products from the Transylvania Region of Central Romania

This study used molecular serogrouping to assess the presence and antimicrobial susceptibility profile of Listeria monocytogenes isolates from food products of different animal origin, collected from a county situated in the historical region of Transylvania in central Romania. Seventeen (7.7%) of 221 screened samples were positive for L. monocytogenes; these included 8 (6.2%) of 130 ready-to-eat products (i.e., sausages, ham, and smoked specialties), 6 (12.8%) of 47 raw meat samples (i.e., minced pork, pork organs, and snails), and 3 (6.8%) of 44 dairy samples (i.e., assortment of cheeses). The identified L. monocytogenes serogroups were 1/2a-3a (47.1%), 4b-4d-4e (29.4%), 1/2c-3c (11.8%), and 4a-4c (11.8%). All isolates were resistant to benzylpenicillin and fusidic acid. Resistance was also detected toward oxacillin (88.2%), fosfomycin (82.4%), clindamycin (76.5%), imipenem (52.9%), ciprofloxacin (41.2%), rifampin (41.2%), trimethoprim-sulfamethoxazole (29.4%), and tetracycline (29.4%). On the other hand, all isolates proved susceptible to gentamicin, moxifloxacin, teicoplanin, vancomycin, tigecycline, erythromycin, and linezolid. All tested strains exhibited multidrug resistance, resulting in the expression of a total of 12 resistance profiles. These findings extend the understanding of the spread of an important pathogen in Romanian food products, highlighting a substantial public health issue and medical concern, especially for consumers with a compromised health status.

Whole-Genome Sequencing-Based Characterization of a Listeria monocytogenes Strain from an Aborted Water Buffalo in Southern Italy.

Listeria monocytogenes is a Gram-positive pathogen causing life-threatening infections both in humans and animals. In livestock farms, it can persist for a long time and primarily causes uterine infections and encephalitis in farmed animals. Whole genome sequencing (WGS) is currently becoming the best method for molecular typing of this pathogen due to its high discriminatory power and efficiency of characterization. This study describes the WGS-based characterization of an L. monocytogenes strain from an aborted water buffalo fetus in southern Italy. The strain under study was classified as molecular serogroup IVb, phylogenetic lineage I, MLST sequence type 6, Clonal Complex 6, and cgMLST type CT3331, sublineage 6. Molecular analysis indicated the presence of 61 virulence genes and 4 antibiotic resistance genes. Phylogenetic analysis, including all the publicly available European L. monocytogenes serogroup IVb isolates, indicated that our strain clusterized with all the other CC6 strains and that different CCs were variably distributed within countries and isolation sources. This study contributes to the current understanding of the genetic diversity of L. monocytogenes from animal sources and highlights how the WGS strategy can provide insights into the pathogenic potential of this microorganism, acting as an important tool for epidemiological studies.

Listeria monocytogenes Serogroup 1/2 Strains Have a Competitive Growth Advantage over Serotype 4b during Refrigerated Storage of an Artificially Contaminated Ready-To-Eat Pork Meat Product

Listeria monocytogenes is the bacterial causative agent of listeriosis, a life-threatening disease for humans, mainly transmitted through contaminated food. Human clinical isolates of the pathogen are frequently identified as serotype 4b strains; interestingly, however, serotype 4b (lineage I) is normally underrepresented among the food isolates in which serotype 1/2a (lineage II) is usually prevalent. The present study aimed to assess in situ dominance dynamics for the most commonly detected serotypes of L. monocytogenes implicated in foodborne listeriosis cases. A four-strain mixture comprised of L. monocytogenes serogroup 1/2 (i.e., serotypes 1/2a, 1/2b, and 1/2c) and serotype 4b food isolates was inoculated on a sliced ready-to-eat pork meat product, and dominance rates for the pathogenic strains were estimated based on serotype recoveries by utilizing multiplex polymerase chain reaction (mPCR), during storage of the product at 4 °C and 10 °C. The cumulative mPCR results showed that serotype 4b decreased at both storage temperatures, with the most abrupt decrease being noticed during storage at 10 °C. Irrespective of the storage temperature applied, L. monocytogenes strains of serogroup 1/2 predominated at the end of the meat product’s storage period. Conclusively, the preliminary findings of this research suggested a competitive growth advantage of L. monocytogenes serogroup 1/2 strains over serotype 4b during the refrigerated shelf-life of foods, thus advancing our knowledge on the pathogen’s behavior and contributing toward elucidating the manifested underrepresentation of serotype 4b in favor of serogroup 1/2 strains among the food isolates of the pathogen, particularly those recovered during detection and/or enumeration of L. monocytogenes in meat and products thereof.

ANALYSIS OF CIRCULATING STRAINS, CAUSING INVASIVE LISTERIOSIS IN BULGARIA FOR TEN YEARS, 2010-2019

Listeriosis is a zoonosis with multiple mechanisms of infection and multiple organ symptoms, severe course and high lethality. An increasing incidence of listeriosis has been reported in several European countries in recent years. A limited range of Listeria strains is responsible for most blasts occurring in different countries. The aim of the study is to monitor the spread of the Listeria strains causing invasive listeriosis in Bulgaria for the period 2010-2019 and to analyze the etiological structure of the infection in different hospitals in the country. A total of 56 strains Listeria from 17 hospitals were investigated for confirmation of species and serogroup. The materials were isolated from haemocultures, fetuses water, cerebrospinal fluids and anal and throat secretions and two tests were used. Confirmed strains isolated from the clinical samples for the study period belonged to 4 serogroups of Listeria. Listeria monocytogenes serogroup I was detected in 28 (50%) of the samples, Listeria monocytogenes serogroup II – in 43%, Listeria innocua –in 5% and Listeria welshimeri – in 2% of the samples. The strains isolated from cerebrospinal fluid predominated (52%) and those isolated from haemocultures were 36%. Nineteen of the isolated strains from cerebrospinal fluid belonged to serotype 4b (19/29). Listeria monocytogenes serogroup I was detected in 70% from haemoculture. Listeria innocua and Listeria welshimeri were detected from cerebrospinal fluid. Serotype 1/2a was found in six hospitals and serotype 4b - in five hospitals. The majority of isolated strains was from newborns -12/56 (21.43%). Serotype 1/2a was detected in 12 hospitals and serotype 4b in 11 hospitals. The largest variety of strains was found in Plovdiv, UMBAL “Sv. Georgi”. The data confirmed a steady trend in the spread of certain listeria serotypes in each hospital over the years. The focus in the future should be on screening at-risk groups, mainly women of childbearing and it is recommended to look for the risk of listeriosis.

Occurrence of Listeria and Escherichia coli in frozen fruit and vegetables collected from retail and catering premises in England 2018–2019

Frozen vegetables have previously been associated with outbreaks of listeriosis in both the USA and Europe. An outbreak of Listeria monocytogenes serogroup 4 caused 53 cases in five European countries between 2015 and 2018. Whole genome sequencing (WGS) indicated that frozen sweet corn from a producer in Hungary was the source of illness. However, limited data is available on the prevalence of Listeria in frozen produce. A study of frozen fruit and vegetables from catering and retail premises in England was therefore carried out to assess their microbiological quality with respect to Listeria and Escherichia coli.Between December 2018 and April 2019, 1050 frozen fruit and vegetable samples were collected. Of these, 99% were of a satisfactory or borderline microbiological quality. Eleven samples (1%) contained ≥100 cfu/g of Escherichia coli (considered unsatisfactory in products labelled as ready-to-eat). Listeria monocytogenes or other Listeria species were detected in six samples (2%) of fruit compared to 167 samples (24%) of vegetables and six samples (26%) of fruit and vegetable mixes, but none at a level of ≥100 cfu/g. Characterisation by WGS of 74 L. monocytogenes isolates identified ten genetic clusters indicating a common source. For 8 of the 10 clusters, the isolates came from homogenous food types: four were sweet corn, and there was one cluster each for beans, peas, peppers and broccoli. There were five genetic associations between isolates from frozen vegetables and from clinical cases of listeriosis, including two cultures from frozen beans that were indistinguishable from the 2015–2018 sweet corn outbreak strain.This study indicates that L. monocytogenes was present in 10% of frozen vegetables and even though products are generally not ready-to-eat and are intended to be cooked prior to consumption, these have the potential to cause illness. Clear cooking and handling instructions are therefore required on these products to ensure that the health of consumers is not put at risk, and appropriate Good Manufacturing Practice measures should be followed by all fruit and vegetable freezing plants in order to reduce contamination with Listeria during processing.

Listeria monocytogenes From Farm to Fork in a Brazilian Pork Production Chain

Listeria monocytogenes contamination was assessed in different steps of a pork production chain. Ten lots of pigs were sampled at termination barns, at slaughter (after bleeding, after buckling, after evisceration, and after final washing), at processing (knives, deboning tables, and employees' hands), and of end products (ribs, shoulder, ham, and sausage). All samples (n = 670) were subjected to L. monocytogenes detection, and the obtained isolates (n = 18, identified as Listeria spp.) were characterized by their biochemical characteristics, serogroups, virulence genes, pulsed-field gel electrophoresis profiles, antibiotic resistances (ampicillin, penicillin, gentamicin, and sulfamethoxazole-trimethoprim), and adhesion abilities. The results revealed the low occurrence of Listeria spp. in the evaluated pork production chain. However, four tested sausage samples (40%) were positive for Listeria spp., with L. monocytogenes identified in two (20%) of these samples. Ten isolates were identified as L. monocytogenes (eight from serogroup 1/2a or 3a and two from serogroup 4b, 4d, or 4e): all isolates were also positive for the virulence-related genes hlyA, iap, plcA, actA, inlA, inlB, inlC, and inlJ and susceptible to the tested antibiotics. One sausage sample was contaminated by both serogroups 1/2a or 3a and 4b, 4d, or 4e. Isolates from serogroup 1/2a or 3a obtained during visits 5 and 6 presented distinct genetic profiles by pulsed-field gel electrophoresis, indicating that contamination may come from different sources. The adhesion potential exhibited by Listeria spp. isolates (n = 18) ranged from weak (serogroup 4b, 4d, or 4e) to moderate (L. innocua and L. monocytogenes serogroup 1/2a or 3a). Despite the low occurrence of L. monocytogenes, pathogenic serogroups were detected in sausages, demanding control measures by the industry.

Modelling the fate and serogroup variability of persistent Listeria monocytogenes strains on grated cheese at different storage temperatures

Processed cheese from cow's milk is one of the most consumed dairy products worldwide. Since this product is defined as ready-to-eat, foodborne pathogens such as Listeria monocytogenes can represent a health concern for susceptible populations. In this study, the individual and combined kinetic behaviour of four L. monocytogenes serogroups (namely, 1/2a, 1/2b, 1/2c and 4b) persistently isolated from a Spanish cheesemaking factory was modelled on grated cheese at different isothermal conditions (4 and 12 °C) during a 120-days period. The serogroup variability was characterized over the storage time by the isolation and identification of the different serogroups in the cocktail containing the four strains. This processed cheese did not support the growth of L. monocytogenes during storage. Survival patterns described by the log-linear type model indicated a high variability of L. monocytogenes serotypes at the tested temperatures: L. monocytogenes serogroup 4b showed a more rapid decrease rate at 4 °C than at 12 °C, while the opposite trend was found for the rest of serogroups and the L. monocytogenes cocktail containing all the strains. Survival rate of L. monocytogenes serogroup 1/2c at 4 °C was 0.007 log CFU/d being the most resistant serotype while at 12 °C, serogroup 1/2a showed the lowest survival rate (0.011 log CFU/d), thus showing a prolonged survival at this temperature. This study highlights the potential implications of L. monocytogenes contamination in processed cheese and shows that serogroup variability should be considered when evaluating survival patterns in contaminated products. Finally, the predictive models developed here could be useful to assist food operators to set specific storage conditions and formulations to avoid L. monocytogenes growth and survival in grated cheeses.

Multi‐country outbreak of Listeria monocytogenes serogroup IVb, multi‐locus sequence type 6, infections linked to frozen corn and possibly to other frozen vegetables – first update

An outbreak of invasive Listeria monocytogenes infections confirmed by whole-genome sequencing (WGS) and linked to frozen corn and possibly to other frozen vegetables has been ongoing in Austria, Denmark, Finland, Sweden and the United Kingdom since 2015. As of 15 June 2018, 47 cases were reported and nine patients died. WGS analysis of 29 non-human L. monocytogenes isolates found them to be closely related to the multi-country human cluster of L. monocytogenes serogroup IVb, multi-locus sequence type 6. The non-human isolates were obtained from frozen corn, frozen vegetable mixes including corn, frozen spinach products, frozen green beans and two environmental samples. The strong microbiological link between the human and the non-human isolates is indicative of a common source related to frozen corn and other frozen vegetable mixes persisting in the food chain. Traceability information for the contaminated products pointed to the source of contamination in a Hungarian freezing plant. It is possible that frozen vegetables other than corn processed in this plant, could also be a vehicle of human infection. The information available confirms contamination within the Hungarian plant, but thorough sampling and testing are needed to identify the source of contamination at the plant concerned. On 29 June 2018, the Hungarian Competent Authority banned the marketing of all frozen vegetable products produced by the Hungarian plant, and ordered their immediate withdrawal and recall. This restrictive measure is likely to significantly reduce the risk of human infections and contain the outbreak. Any potentially contaminated frozen vegetables could still represent a possible risk to consumers until completely withdrawn and recalled. New invasive listeriosis cases may also be identified due to the long incubation period, the long shelf-lives of these products, and potential consumption of frozen vegetable products bought by consumers before the recalls and eaten without being properly cooked.

Multi‐country outbreak of Listeria monocytogenes serogroup IVb, multi‐locus sequence type 6, infections probably linked to frozen corn

An outbreak of invasive Listeria monocytogenes (L. monocytogenes) infections defined by whole‐genome sequencing (WGS) and probably linked to frozen corn has been ongoing in five EU Member States (Austria, Denmark, Finland, Sweden and the United Kingdom) since 2015. As of 8 March 2018, 32 cases have been reported and six patients have died due to or with the infection. WGS analysis of six non‐human L. monocytogenes isolates detected from 2016 to January 2018 in Austria, Finland, France and Sweden found these isolates closely related to the multi‐country cluster of L. monocytogenes serogroup IVb, multi‐locus sequence type 6 (ST6). The non‐human isolates were detected in two different samples from mixed frozen vegetables; three samples from frozen corn, and one sample from a surface where various vegetables could have been processed. The only common food item in all non‐human samples was corn. The WGS analysis provides a strong microbiological link between the human and the non‐human isolates and is suggestive of a potential contaminated food source related to frozen corn persisting in the food chain at least since 2016. Traceability information for the three frozen corn samples pointed to frozen corn products packed in Poland and processed/produced in Hungary. Two additional non‐human strains isolated in Austria from frozen vegetable mixes with corn as an ingredient were traced back to the same common origin in Hungary. Further investigations are needed to verify the point of contamination in the food chain. Consumption of frozen corn has been confirmed by two patients, one in Finland and one in Sweden. In addition, a Danish patient reported consumption of mixed frozen vegetables, which could have included corn. The Finnish patient confirmed consumption of frozen corn of one suspected brand, supporting an epidemiological link between the outbreak cases and frozen corn. However, no traceability and microbiological information was available for the corn consumed by the Finnish and the Swedish patients. Food business operators in Estonia, Finland, Poland and Sweden have withdrawn and recalled the implicated frozen corn products from the market. These measures are likely to significantly reduce the risk of human infections in these countries. However, new invasive listeriosis cases may be identified due to the long incubation period (1‐70 days), long shelf‐lives of frozen corn products and potential consumption of frozen corn bought by the customers before the recalls and eaten without being properly cooked. Furthermore, until the root source of contamination is established and control measures implemented, new cases may occur.

Factors influencing the time between onset of illness and specimen collection in the diagnosis of non-pregnancy associated listeriosis in England and Wales.

BackgroundListeriosis is an opportunistic bacterial infection caused by Listeria monocytogenes and predominantly affects people who are immunocompromised. Due to its severity and the population at risk, prompt clinical diagnosis and treatment of listeriosis is essential. A major step to making a clinical diagnosis is the collection of the appropriate specimen(s) for testing. This study explores factors that may influence the time between onset of illness and collection of specimen in order to inform clinical policy and develop necessary interventions.MethodsEnhanced surveillance data on non-pregnancy associated listeriosis in England and Wales between 2004 and 2013 were collected and analysed. The difference in days between onset of symptoms and collection of specimen was calculated and factors influencing the time difference were identified using a gamma regression model.ResultsThe median number of days between onset of symptoms and collection of specimen was two days with 27.1 % of cases reporting one day between onset of symptoms and collection of specimen and 18.8 % of cases reporting more than seven days before collection of specimen. The median number of days between onset of symptoms and collection of specimen was shorter for cases infected with Listeria monocytogenes serogroup 1/2b (one day) and cases with an underlying condition (one day) compared with cases infected with serotype 4 (two days) and cases without underlying conditions (two days).ConclusionsOur study has shown that Listeria monocytogenes serotype and the presence of an underlying condition may influence the time between onset of symptoms and collection of specimen.Electronic supplementary materialThe online version of this article (doi:10.1186/s12879-016-1638-4) contains supplementary material, which is available to authorized users.

Molecular characterization and antimicrobial susceptibility of Listeria monocytogenes isolated from foods and humans

A total of 120 Listeria monocytogenes isolates (107 from foods and 13 from humans) in Shanghai, China, were characterized by serogroup typing and virulence genes detection with PCR, antimicrobial susceptibility testing, and molecular subtyping using pulsed-field Gel Electrophoresis (PFGE). Isolates belonged to three L. monocytogenes serogroups, 1/2c, 3c (n = 47, 39.1%) was the most prevalent, followed by 1/2a, 3a (n = 44, 36.7%) and 1/2b, 3b, 7 (n = 29, 24.2%). Although the isolates were all susceptible to most antibiotics tested, 13 isolates were resistant to ceftriaxone and seven to tetracycline. None of the isolates were resistant to multiple antibiotics. The tet (M) genes were detected among tetracycline-resistant isolates. The isolates all harbored virulence genes hly, prfA, plcA, plcB, inlA, inlB, inlI; 116 isolates (96.7%) were positive for inlC; 78 (65.0%) for inlG; 119 (99.2%) for inlJ; 8 (6.67%) for llsX. A total of 74 distinct patterns were generated for the 119 isolates tested using ApaI and AscI with Pulsed Field Gel Electrophoresis (PFGE). Three clinical isolates shared the same PFGE patterns with retail meet (chicken, duck, and beef) isolates. These findings indicated that food of animal origin likely serve as a major vehicle in transmitting human listeriosis in Shanghai. Active surveillance for L. monocytogenes is needed for better understanding the epidemiology of the pathogen, and for effective control and prevention of human listeriosis.

Characterization of Listeria monocytogenes from three countries and antibiotic resistance differences among countries and Listeria monocytogenes serogroups.

A total of 104 Listeria monocytogenes isolates from 330 fish samples from three countries were characterized by multiplex PCR for serogrouping and virulence markers determination and tested for antibiotics resistance. A 53·8% of the isolates belonged to serogroup 1/2a, 3a; 32% belonged to 1/2b, 3b, 7; 14·4% belonged to 4b, 4d, 4e and 1% belonged to 1/2c, 3c. All isolates exhibited resistance to at least one antibiotic but the resistance rates varied among countries. The isolates exhibited high resistance to penicillin, rifampicin, clindamycin, erythromycin and tetracycline, but low resistance to amoxicillin-clavulanic acid, streptomycin, sulfamethoxazole-trimethoprim, gentamicin, chloramphenicol and kanamycin. When comparing countries, the resistance rate for rifampicin, clindamycin, erythromycin, tetracycline, amoxicillin-clavulanic acid varied among countries. When comparing serogroup, 1/2a, 3a exhibited the highest resistance to clindamycin, erythromycin, tetracycline and vancomycin while serogroup 4b, 4d, 4e exhibited the highest resistance to amoxicillin-clavulanic acid. All isolates carried inlA, inlC, inlJ and lmo2672. Listeriolysin S was carried by 42 and 30% of 4b and 1/2b isolates respectively. Significance and impact of the study: This is one of few studies to correlate antibiotic resistance with Listeria monocytogenes serogroups. The study also compared the antibiotic resistance and serogroups of L. monocytogenes isolates from three countries in one single study. The findings of this study will be helpful in improving data on the antibiotics resistance of L. monocytogenes in developing countries and enriches the epidemiological and public health studies of L. monocytogenes.

Antilisterial activity of bacteriocinogenic Pediococcus acidilactici HA6111-2 and Lactobacillus plantarum ESB 202 grown under pH and osmotic stress conditions

Bacteriocin producing lactic acid bacteria (LAB) cultures can be used as biopreservatives in fermented food products; thus the food industry is interested in stable cultures that produce bacteriocins consistently. Inhibtion of Listeria spp. by bacteriocinogenic Pediococcus acidilactici and Lactobacillus plantarum (both isolated from fermented meats) was investigated under conditions of stress induced by low pH and high salt concentrations. Listeria monocytogenes serogroup IIb (from cheese), L. monocytogenes serogroup IVb (from cheese), L. monocytogenes serogroup IIb (from ground beef) and Listeria innocua NCTC 11288 were used as target strains. P. acidilactici and Lb. plantarum demonstrated antilisterial activity under the stress conditions investigated (pH 3.5; pH 8.5; 7.5% NaCl). However, activity was dependent on the stress conditions applied and on the target organism. L. monocytogenes serogroup IIb (from ground beef) and L. innocua C 11288 were, respectively the most sensitive and the most resistant to the cell-free supernatants produced by the LAB investigated.

Characterization of Listeria monocytogenes isolated from retail foods

Listeria monocytogenes isolates recovered from retail ready-to-eat (RTE) meats, raw chickens and fresh produce were characterized by serogroup identification using PCR, genotyping using pulsed-field gel electrophoresis (PFGE) and antimicrobial susceptibility testing. Five L. monocytogenes serogroups were identified. Of the 167 isolates 68 (41%) belonged to serogroup 1/2b, 3b; 53 (32%) belonged to serogroup 4b, 4d, 4e; 43 (26%) belonged to serogroup 1/2a, 3a; 2 (1.2%) belonged to serogroup 1/2c, 3c; and 1 (0.6%) belonged to serogroup 4a, 4c. PFGE generated 120 patterns which correlated well with PCR serogrouping. Most L. monocytogenes isolates were resistant to sulfonamide (73%) and some were resistant to tetracycline (8.4%) and ciprofloxacin (1.8%). Tetracycline resistance was conjugatively transferable and the tet(M) gene was identified in 14 tetracycline-resistant isolates as well as their transconjugants. These findings indicate that L. monocytogenes present in food were diverse, and that resistance to one or more antibiotics among these isolates was common. In addition, the presence of potential serotype 4b in all food categories is of public health concern, as serotype 4b has been the serotype most frequently associated with human listeriosis.