Prevalence,antibiotic resistance and genetic diversity of Listeria monocytogenes isolated from retail ready-to-eat foods in China

Listeria monocytogenes is a major foodborne pathogen that is well known as high mortality rate upon infected. This study aimed to investigate the prevalence of L. monocytogenes isolates from retail ready-to-eat (RTE) foods in China and characterize the isolates of L. monocytogenes by antibiotic resistance, serotyping, ERIC-PCR and REP-PCR subtyping analyses. From September 2012 to January 2014, a total of 364 retail RTE foods were obtained. Using the qualitative and quantitative methods, 25 samples (6.87%) were positive for L. monocytogenes. The identity of isolates of L. monocytogenes was confirmed by PCR. All 80 isolates in this survey were sensitive to penicillin and mezlocillin, the highest resistance is clindamycin (51.25%), followed by cephalothin (23.75%) and ampicillin (12.5%). Twenty-seven isolates were susceptible to all 14 tested antibiotics; seventeen isolates were resistant to more than two antibiotics, including six multiresistent strains resist to more than 10 antibiotics. L. monocytogenes isolates belonged to serovar types 1/2a (3a), 4b (4d, 4e), 1/2b (3b, 7) and 1/2c (3c). 29 L. monocytogenes isolates were selected by serotyping. At the relative similarity coefficient of 0.80, it grouped 29 isolates and 5 reference strains into 2 clusters and 3 singletons, 4 clusters and 1 singleton by ERIC-PCR and REP-PCR, respectively. Our study reflects the potential risk of L. monocytogenes infection in China. We also provide a comprehensive surveillance on its incidence on the RTE foods of L. monocytogenes and ensure more accurate treatment of human listeriosis with effective antibiotics.     

Prevalence and characterization of Listeria monocytogenes isolated from retail-level ready-to-eat foods in South China

Listeria monocytogenes is an important food-borne pathogen causing meningitis, meningoencephalitis and abortion. To assess the potential risk to consumer health, the presence of L. monocytogenes was investigated using qualitative and quantitative methods. Ten (6.33%) of 158 retail RTE food samples were positive for L. monocytogenes and the contamination levels were less than 10 MPN/g,while none of 65 dairy products was positive for L. monocytogenes. The 37 strains were grouped into five clusters and two singletons, five clusters and two singletons, and three clusters and one singleton by enterobacterial repetitive intergenic consensus-polymerase chain reaction (ERIC-PCR) and RAPD fingerprint respectively, at similarity coefficient of 80%. The susceptibility test showed that 83.8% were susceptible to 15 antimicrobials; two were penicillin-resistant, and one was multidrug-resistant to kanamycin, tetracycline, sulfamethoxazole, rifampin, gentamycin, penicillin, and ampicillin. Virulent L. monocytogenes that possess partial antimicrobial resistance, and serotypes frequently associated with listeriosis were recovered from RTE foods. Consumers may, therefore, be exposed to potential risks of L. monocytogenes infection in South China. This study contributed to the prevalence and contamination levels of L. monocytogenes in RTE foods in South China for the first time, providing baseline information for Chinese regulatory authorities to formulate a regulatory framework for controlling L. monocytogenes to improve the microbiological safety of RTE foods.     

Antimicrobial resistance profiles of Listeria monocytogenes isolated from ready-to-eat products in Poland in 2007–2011

The aim of the study was to characterize strains of Listeria monocytogenes isolated from ready to eat (RTE) products collected as part of official food control and monitoring in Poland. A total of 105 L. monocytogenes isolates from RTE products: 54- cakes and 51 – delicatessen products were examined. The presence L. monocytogenes in cakes and delicatessen products was 0.4% and 0.7% respectively suggesting the level of contamination of RTE products with L. monocytogenes is very low.     

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.     

Prevalence and antimicrobial resistance patterns of Listeria species isolated from poultry products marketed in Iran

In the present study, a total of 402 poultry product samples composed of raw, ready-to-cook (RTC) and ready-to-eat (RTE) products were examined for the presence of Listeria spp. The total contamination rate with Listeria spp. in poultry products was 33.3% with a higher rate of contamination in warm seasons than in cold seasons. The most species recovered was Listeria innocua (46.3%); the remaining isolates were Listeria monocytogenes (38.8%), Listeria ivanovii (9.7%) and Listeria seeligeri (5.22%). L. monocytogenes was detected in 14.1%, 12.2% and 11.4% of raw, RTC and RTE poultry products, respectively. Serotype 4b (44.9%) was the predominant serotype of L. monocytogenes isolates followed by 1/2a (40.8%), 1/2b (10.2%) and 1/2c (4.08%). Considering seasonal variability, 1/2a was the most prevalent serotype in warm seasons, while 4b was predominant in cold seasons. The Listeria spp. particularly L. monocytogenes isolates were highly resistant to ampicillin, penicillin, fluroquinolones and tetracycline. The results indicate that high prevalence of Listeria spp. especially L. monocytogenes in poultry products, and resistance of the isolates to the antimicrobials commonly used to treat human listeriosis could be a potential health hazard for consumers. In addition, prevalence of L. monocytogenes serotype 4b that involved in the majority of foodborne outbreaks of human listeriosis is a public health concern.     

Presence of Listeria monocytogenes in Chilean food matrices

ObjectivesTo compare Listeria monocytogenes strains obtained from food matrices with those from environmental samples in the same food processing plant.Methods and resultsBetween 2008 and 2012 the presence of L. monocytogenes was evaluated in 2647 food samples. A total of 448 work surfaces and 92 equipment's were also evaluated from 6 plants which produce ready-to-eat (RTE) foods in Santiago, Chile. An additional selected sample of hand and nails samples was also obtained from 13 food handlers working in a sausage elaboration plant.As a whole L. monocytogenes was present in 265 (10%) food samples and 22 (4%) environmental samples. The foods with highest recovery were red meats 14/60 (23%), poultry 223/1196 (19%), the remaining samples accounted a total of 27/1391 (2%). The environmental samples positive for L. monocytogenes were obtained from two food plants both the cheese 8/8 (100%) and from a fresh peaches exporter 3/3 (100%). Finally L. monocytogenes was isolated from 5/13 (38%) food handlers studied.ConclusionsThe study confirms the presence of L. monocytogenes in different matrices, especially in meat and RTE products. Analyses conducted on work surfaces revealed that contamination comes mostly from both raw materials and surfaces in indirect contact with foods.Significance and impact of studyThe study reinforces the need for companies to apply regulations related to food quality and safety systems (HACCP, Hazard Analysis & Critical Control Points) to prevent L. monocytogenes contamination from food processing plants.     

Occurrence and antibiotic resistance profiles of Listeria monocytogenes isolated from seafood products and market and processing environments in Iran

This study was designed to determine the occurrence of Listeria monocytogenes in popular seafood products and their market and processing environments. The frequency of L. monocytogenes contamination was found to be 4.83% in raw and 14.5% in RTE seafood products. In raw products, the prevalence of L. monocytogenes was significantly higher (P < 0.05) in freshwater fish (11.4%) than in seawater fish (1.80%) and shrimp (1.69%). Cold-smoked fish had the highest frequency of L. monocytogenes contamination among the RTE products. The microbial load of L. monocytogenes in seafood products was in the range of <0.3 to 1100 MPN/g; and did not exceed 100 MPN/g in most of the examined samples. The incidence of L. monocytogenes in environmental and personnel samples was 17.1% and 16.2% in markets, and 21.3% and 18.2% in processing plants, respectively. It was found that contamination of processed fish fillets and shrimp flesh with L. monocytogenes mainly originated from the processing environments, rather than the raw materials. In addition, the implemented cleaning procedures were insufficient to eliminate L. monocytogenes from the market and processing environments. Serological examinations revealed that serotype 1/2a (45.7%) was the predominant serotype of L. monocytogenes followed by 4b (40.3%), 1/2c (5.39%), 1/2b (4.68%), and 4c (3.96%). Regarding seasonal variability, 1/2a was the dominant serotype during warm seasons, whereas 4b was the most prevalent serotype during cold seasons. The isolates of L. monocytogenes were highly resistant to penicillin, ampicillin, tetracycline, and vancomycin. The results indicate that prevalence of L. monocytogenes serotypes 1/2a and 4b, which are associated with foodborne outbreaks of human listeriosis; and their resistance to commonly used antibiotics for treatment of human listeriosis could be a public health concern.     

Cook-serve method in mass catering establishments: Is it still appropriate to ensure a high level of microbiological quality and safety?

The purpose of this study was to evaluate the hygienic quality of three traditional “cook-serve” catering establishments in Lombardy, Italy (with an output ranging from 800 to 1700 meals a day), by monitoring the microbiological quality and safety of foods ready for consumption (n = 727), tap water (n = 32), microfiltered drinking water (n = 28), food contact surfaces (n = 280), and food handlers (n = 76). The food contact surfaces showed unacceptable contamination in 17.1% of samples. The hygienic level of the washed hands of food workers was very high, since only 1.3% of samples showed unacceptable contamination. Food sample analyses highlighted a percentage of samples that did not conform to microbial reference standards: the presence of Listeria monocytogenes was found in 11.5% of “soft cheese” samples. Staphylococcus aureus non-conforming percentages ranged from 2.2% for “first and second courses” to 34.6% for “soft cheeses”; Escherichia coli, from 2.7% for “raw vegetables” to 7.7% for “soft cheeses”; total coliforms from 6.7% for “first and second courses” to 76.4% for “raw vegetables”. The results of the water analysis indicated a high frequency of contamination with Pseudomonas aeruginosa (21.4% for microfiltered drinking water and 21.9% for tap water samples). In conclusion, the results suggest that more effort should be made in the application of HACCP principles. In addition in order to ensure a higher level of microbiological safety of meals, various changes in the timing of food preparation and holding temperatures are needed.     

A survey of Listeria monocytogenes strains,isolated from ready-to-eat foods in Israel over a period of 10 years, 1998–2007

Between 1998 and 2007 the food-control laboratories in Israel tested 10,413 samples from five groups of RTE foods: salads/dips, dairy, fish, poultry and meat. A total of 1260 isolates of Listeria monocytogenes were identified and submitted to the reference listeria laboratory. The average isolation rate was 12.1% with the highest prevalence being registered in poultry (27%). The serological results showed the prevalence of Lineage I, followed by Lineages II and III. The report revealed a high isolation rate of L. monocytogenes in certain RTE foods in spite of the existing zero tolerance policy.     

Prevalence and characterization of Listeria monocytogenes isolated from retail food in Henan,China

Listeria monocytogenes, an important foodborne pathogen, is the causal agent of listeriosis. In this study, a total of 954 food samples originating from raw meat, cooked meat products, seafood, and vegetables purchased from supermarkets and open-air markets in Henan province, China, were analyzed for the presence of L. monocytogenes. All L. monocytogenes isolates were subjected to serotyping, pulsed-field gel electrophoresis (PFGE), and antimicrobial resistance. The overall percentage of L. monocytogenes prevalence was 6.2% (n = 59) with the highest rate of 7.4% for cooked meat products followed by raw meat (6.7%). The isolates belonged to five serotypes (1/2a, 1/2b, 1/2c, 4b, and 4c), with serotype 1/2a being predominant (55.9%). PFGE revealed a low genetic diversity among the isolates, irrespective of their sources, suggesting that dominant clones are widespread in different food products in Henan. Resistance to cefotaxime (30.5%) and ciprofloxacin (13.5%) was most often, whereas resistance to tetracycline, trimethoprim/sulfamethoxazole, and erythromycin was observed less frequently. The presence of L. monocytogenes in food products and antimicrobial resistance among the isolates represents a potential public health risk. Our results indicate that effective hygienic measures and bacteriological controls are necessary in China to reduce the contamination of retail food samples by L. monocytogenes.     

Antibacterial activity of cranberry juice concentrate on freshness and sensory quality of ready to eat (RTE) foods

Ready-to-eat (RTE) foods are not further treated before consumption in such a way that may significantly reduce the microbial load, therefore the risk of foodborne disease must be considered. In this regard, the use of natural antimicrobial compounds is an interesting method to be considered. On this topic, the antibacterial activity of cranberry juice concentrate (CJC) have been evaluated in vitro and in situ against 3 foodborne pathogenic bacteria. Results showed a high antimicrobial effect with a noticeable inhibition capacity against Escherichia coli O157:H7, Listeria monocytogenes, Salmonella typhimurium. Acid sensitivity studies of bacteria indicated that at the same pH level (pH = 2.4) in presence of organic acid solution (citric and quinic acids), cranberry juice concentrate showed greater antibacterial effects than the acids due to their phenolic compounds. In situ studies showed 2.5, 1.8 and 5 log reduction of E. coli, L. monocytogenes and S. typhimurium, respectively in presence of cranberry juice concentrate, on pre-cut red peppers after 7 days of storage at 4 °C. A total inhibition of L. monocytogenes on fresh cranberry fruits in primary day of storage, was observed. Cranberries treated with CJC also showed a 3 log reduction of S. typhimurium after 4 days of storage at 4 °C. The results suggest that CJC can be an effective preservation, source of natural antibacterial, to protect the RTE foods from foodborne pathogens contamination without effecting on sensorial properties of treated samples and allow to maintain the freshness, sensory and the nutritional quality of RTE foods.     

Prevention of travel-related foodborne diseases: Microbiological risk assessment of food handlers and ready-to-eat foods in northern Italy airport restaurants

The focus of this study was to assess the hygienic standards of 44 foodservice facilities located in three Italian International Airports (with an output ranging from 100 to 800 meals a day), by monitoring the microbiological quality and safety of foods ready for consumption (n = 773), food contact surfaces (n = 302), and food handlers (n = 287). The hygienic standard of surfaces was sufficiently high. Only 7.9% of surfaces did not conform with advisory standards in terms of total coliforms, and 2.6% were found to be contaminated with Enterococcus spp. at ≥1.0 log₁₀ CFU/cm². The hygienic standard of washed and disinfected hands of food workers was not adequately high: the total bacterial count and coagulase positive Staphylococci exceeded the satisfactory limit in 8.4% and 3.5% of cases, respectively. The microbial analysis of foods examined showed an absence of Listeria monocytogenes and Salmonella spp. Food sample analyses highlighted a percentage of samples that did not conform to microbial reference standards: Staphylococcus aureus non-conforming percentages ranged from 2.3% for “fully cooked food” to 9.2% for “raw fruit and vegetables”; Escherichia coli, from 0.0% for “raw fruit and vegetables” to 6.1% for “cooked and uncooked foods”; total coliforms from 14.3% for “fully cooked food” to 79.8% for “cooked and uncooked food”. In conclusion, the results suggest that more effort is needed in the application of HACCP principles. In order to prevent travel-related foodborne infections, various changes in the timing of food preparation and holding temperatures are needed, together with further training of food handlers.     

Listeria monocytogenes in ready-to-eat vacuum and modified atmosphere packaged meat and fish products of Estonian origin at retail level

The prevalence, counts and genetic diversity of Listeria monocytogenes in ready-to-eat (RTE) vacuum and modified atmosphere packaged meat and fish products was studied in Estonia. Within two consecutive years 370 RTE food samples were collected at retail level from which 11% were found to be positive for L. monocytogenes. Contamination was higher among RTE fish products (17%) than in RTE meat products (6%). Generally, the counts of L. monocytogenes in positive products remained under ten colony forming units (CFU) per gram of product. Only 1.6% of the RTE meat and fish products contained L. monocytogenes in range of 10–100 CFU/g and 0.3% more than 100 CFU/g at the end of shelf-life. The food category containing highest L. monocytogenes prevalence was RTE lightly salted fish products with the prevalence of 32%. Only one (0.3%) RTE food sample exceeded the 100 CFU/g food safety criterion set out in the EU Regulation 2073/2005. Pulsed-field gel electrophoresis (PFGE) characterization of the isolates showed an overall similarity higher than 70%, and nine clusters based on 100% similarity were revealed. PFGE genotyping revealed that the few predominant pulsotypes were associated with particular food plants.     

Successful management of Listeria spp. in an avocado processing facility

Listeria monocytogenes can grow and multiply in various food matrices and cause severe human illness. Apart from the influence on consumer health, L. monocytogenes contamination of ready-to-eat (RTE) food products causes major economic losses due to product recalls. Control of foodborne pathogens in RTE food products is a challenge, specifically in foods that cannot undergo a heat-treatment during processing. The aim of this study was to develop control strategies for the management of L. monocytogenes in an avocado processing facility, additional to a quality control system. An in-house monitoring system (IMS) was established to test specifically for Listeria spp. in the final products and processing environment, including floors, equipment, work areas and personnel. Guacamole and environmental samples were collected and tested on-site for Listeria with the ISO 11290-1 method. Based on the prevalence of Listeria, the facility introduced new strategies in processing to counter cross contamination. Results from the 2014 guacamole production season showed almost complete eradication of Listeria spp. in final products (0.17%, n = 1170) and the processing facility (0.79%, n = 1520). This is a major achievement since the highest incidence of Listeria spp. over a period of five years was measured at 11.39% (n = 948) in the final product during the 2013 season and 13.44% (n = 1927) in the processing facility in 2011. These results indicate that successful management of Listeria spp. in an avocado processing facility can be accomplished with in-house monitoring of the listerial population and subsequent adjustments to the processing system.     

Prevalence of Listeria spp. and antibiotic susceptibility of Listeria monocytogenes isolated from raw chicken and ready-to-eat chicken products in Jordan

Listeria monocytogenes is the causal agent of listeriosis, a disease that can be serious and is often fatal in susceptible individuals. The objective of the study was to determine the prevalence of Listeria spp. in raw chicken and ready-to-eat (RTE) chicken products in Amman, Jordan and the antimicrobial resistance of L. monocytogenes isolates. A total of 280 raw chicken and RTE chicken products (chicken-shawirma, chicken-burger, chicken-sausage and mortadella) were collected from Amman abattoir and local retail markets in Amman city. Listeria spp. were isolated by the conventional International Organization for Standardization (ISO) method and L. monocytogenes identified by biochemical and Polymerase Chain Reaction (PCR). Results of conventional method showed that out of total 280 samples, 141 (50%) were found to be contaminated with Listeria spp. [L. monocytogenes (18.2%), Listeria ivanovi (26.1%), Listeria grayi (3.5%), Listeria seeligeri (1.8), Listeria welshimeri (0.7%)]. The PCR confirmed all L. monocytogenes isolates (51 isolates: 15 from raw dressed broiler chicken, 23 from chicken-burger, 9 from chicken-sausage, and 4 from chicken-shawirma). Five of the tested L. monocytogenes isolates were resistance to two antibiotics (tilimicosin and tetracycline) among the ten tested antibiotics as determined by microbroth dilution method. The results presented in this study indicate the potential risk of contamination of RTE chicken products with L. monocytogenes.     

Prevalence,genetic diversity and antimicrobial resistance of Listeria monocytogenes isolated from ready-to-eat meat products in Nanjing,China

The prevalence of Listeria monocytogenes were investigated in a total of 628 ready-to-eat (RTE) meat products collected from different supermarkets and open-air markets in Nanjing, China. All isolates were further examined for the serogroup, virulence marker gene, genotype and antibiotic resistance. Thirty-three out of 628 samples (5.3%) were positive confirmed by the bacteriological method and PCR including 7.2% (17/236) of sauce pickled products, 4.2% (11/260) of cured products and 5.6% (5/90) of smoked and roasted products. Fifteen isolates (45.5%) belonged to serogroup 1/2a, 3a; 16 (48.5%) belonged to serogroup 1/2b, 3b and 2 (6.1%) belonged to serogroup 1/2c, 3c. All of them were positive for the virulence marker genes-iap, inlA, inlC, inlJ and lmo2672. Thirty-three isolates were grouped into 11 sequence types (STs) by multilocus sequence typing (MLST). The results of the antimicrobial susceptibility revealed that the isolates were sensitive to most of the antimicrobials used in the study except trimethoprim–sulfamethoxazole (100%), chloramphenicol (33.3%), ciprofloxacin (30.3%) and tetracycline (12.1%). Our findings indicated high prevalence of L. monocytogenes especially in sauce pickled products and from open-air markets, high prevalence of serogroups 1/2a, 3a and 1/2b, 3b that involved in the majority of foodborne outbreaks could be a public health concern. In addition, resistance of the isolates to the antimicrobials was also a potential health hazard for consumers.     

Effect of high pressure processing in combination with Weissella viridescens as a protective culture against Listeria monocytogenes in ready-to-eat salads of different pH

This study explored the effect of HPP (400 MPa/1 min) and a Weissella viridescens protective culture, alone or in conjunction, against Listeria monocytogenes in ready-to-eat (RTE) salads with different pH values (4.32 and 5.59) during storage at 4 and 12 °C. HPP was able to reduce the counts of the pathogen after treatment achieving approximately a 4.0 and 1.5 log CFU/g reduction in the low and higher pH RTE salad, respectively. However, L. monocytogenes was able to recover and grow during subsequent storage. W. viridescens grew in both RTE salads at both storage temperatures, with HPP resulting in only a small immediate reduction of W. viridescens ranging from 0.50 to 1.2 log CFU/g depending on the pH of the RTE salad. For the lower pH RTE salad, the protective culture was able to gradually reduce the L. monocytogenes counts during storage whereas for the higher pH RTE salad in some cases it delayed growth significantly or exerted a bacteriostatic effect. exerted a bacteriostatic effect. The results revealed that the increased storage temperature led to an increase in the inactivation/inhibition of L. monocytogenes in the presence of W. viridescens. The combination of HPP and W. viridescens is a promising strategy to control L. monocytogenes and can increase safety even when a break in the chill chain occurs.     

Prevalence of foodborne pathogens in open markets and supermarkets in Thailand

This study was conducted in Thailand (Bangkok and Pathum Thani provinces), from June 2006 to July 2007, in order to assess the prevalence of Listeria monocytogenes, Escherichia coli O157, Salmonella, Shigella and Vibrio parahaemolyticus in foods. Retail raw meats and seafood, including chicken (n = 109), pork (n = 80), beef (n = 108), shrimp (n = 43) and oysters (n = 48), from open markets and supermarkets were analyzed. Salmonella was found in 22 of 61 (36%) open market samples (48% of chicken, none of pork and beef, and 53% of shrimp) and in 12 of 75 (16%) samples from supermarkets (57%, 12%, 24%, 0% respectively). However, a small number of L. monocytogenes were isolated, where 6 of 217 (3%) were samples from open markets (6% of chicken and 3% of pork) and 17 of 171 (10%) were from supermarkets (3% of beef, 4% of chicken, and 32% of pork). In both markets, L. monocytogenes was not detected from shrimps, neither from oysters. E. coli O157, Shigella and tdh-positive V. parahaemolyticus were not isolated in this collection. Several Salmonella and L. monocytogenes isolates were multidrug-resistant. Both markets would need better assessment, since multidrug-resistant strains have been isolated and they may lead to therapeutic failure.     

Antimicrobial activity of clove (Syzgium aromaticum) oil in inhibiting Listeria monocytogenes on chicken frankfurters

The ability of Listeria monocytogenes to survive and grow at refrigeration temperature in some ready to eat (RTE) poultry products is a public health concern. The inhibitory effect of clove oil (1% and 2%, v/w) applied to the surface of RTE chicken frankfurters was determined on seven strains of L. monocytogenes inoculated at low (10²–10³ cfu/g) or high cell numbers (10⁴–10⁶ cfu/g), and stored at 5 °C for 2 weeks or at 15 °C for 1 week. All strains of L. monocytogenes survived and grew on control frankfurters at 5 °C and 15 °C but growth was inhibited under both storage conditions in the presence of either 1% or 2% clove oil. Depending on the sensory considerations, the addition of clove oil to frankfurters may be an effective strategy to control L. monocytogenes in chicken frankfurters.     

Growth capacity of Listeria monocytogenes in ingredients of ready-to-eat salads

Raw minimally processed vegetables and fruits as well as boiled pasta and potato are the main ingredients of ready-to-eat (RTE) salads. These ingredients are diverse in their ability to support growth of Listeria monocytogenes. The objective of this study was to determine to what extent different ingredients can contribute to growth of L. monocytogenes in RTE salads. This was done by assessing the growth capacity of L. monocytogenes in ingredients of RTE salads by means of challenge tests and determination of the effect of product characteristics and presence of competitive flora. The majority of the tested products did not support or hardly supported growth of L. monocytogenes. The calculated maximum increase of 3.4 log CFU/g was exceeded in Galia melon, potato and pasta products. In RTE dinner salads, growth of L. monocytogenes was supported by the carbohydrate component. Galia melon, which is often used as ingredient of fruit salads, can contribute to growth of L. monocytogenes in these products. Growth inhibition, or differences in relative increase, could not always be explained by a_w, pH or presence of competitive flora. The effect of competitive flora, especially lactic acid bacteria, on growth inhibition was observed in non-pasteurized potato, white cabbage and mango.