Category Archives: Listeria welshimeri

Research – Distribution of Listeria spp. on Carcasses of Regularly Slaughtered Swine for Italian Dry Cured Ham

Journal of Food Protection


In recent years, the role of Listeria monocytogenes as a foodborne pathogen in public health has increased. Its presence poses a risk for humans, especially in ready-to-eat foods, such as ham. Understanding the presence and distribution of Listeria spp. on swine carcasses meant for Italian dry-cured hams can be a useful tool to improve food safety. This study assessed the distribution of Listeria spp. (as a marker of contamination with L. monocytogenes) on slaughtered pig carcasses intended for the production of high-quality, Italian, dry-cured ham and examined the roles of the site sampled on the carcass, farming cycle (open versus closed), farm-to-slaughterhouse distance, and time spent in lairage. Samples were collected from swine carcasses (n = 150) before refrigeration, from three different carcass locations (head, shoulder, and thigh), and assessed for the presence of listeriae. A total of 115 carcasses were contaminated with Listeria spp. in at least one location. In all, 178 listeriae were isolated and identified: 130 Listeria innocua, 28 Listeria welshimeri, 17 Listeria ivanovii, and 3 L. monocytogenes. Listeriae were detected on 62.7% of heads, on 25.3% of shoulders, and on 30.7% of thighs, with significant differences between heads versus shoulders and thighs. Animals reared in closed-cycle farms were more contaminated (P < 0.05) than were animals from open-cycle farms (90 versus 71.8%). The distance between farms and slaughterhouse was not related to the contamination rate. Carcasses of swine that stayed in lairage before slaughtering for more than 10 h showed a higher degree of contamination (90%) and were positive for Listeria spp. in more sample sites (55%) compared with those held for less than 2 h (73% of carcasses and 33.3% of samples). Our results show that heads should be detached from carcasses immediately after slaughter for meat-safety purposes and the amount of time animals stay in lairage should be limited. These results will be useful for a more-valid implementation of good manufacturing processes for slaughtering.

  • Swine carcasses are often contaminated with Listeria spp.

  • Heads are more contaminated than shoulders and thighs.

  • Lairage time higher than 10 h is a risk factor for Listeria spp. contamination.

  • Closed-cycle farms presented greater carcass contamination.

Research – Utilizing kitchen steamers to inactivate Listeria monocytogenes and Salmonella enterica on whole cantaloupe melons

Wiley Online



Cantaloupes contaminated with Listeria and Salmonella have caused large scale of foodborne illness outbreaks in recent years. Prior research has shown that heat treatment can be used to reduce microbial contaminates on fruit surfaces. However, no study has evaluated the use of kitchen steamers for inactivating foodborne pathogens on cantaloupes. The purpose of this study was to test the sanitization efficacy of three models of electrical kitchen steamers (steamer‐A, ‐B, and ‐C) for eliminating foodborne pathogens on cantaloupe surfaces. Fresh cantaloupes were spot‐inoculated with Listeria monocytogens or Salmonella enterica and kept at 4 or 22 °C for a day before steam treatment for ≤100 s. Test results show that the surface of nonrefrigerated cantaloupes can be decontaminated (with ≥5.0 log reduction) within 60 s using any of the tested steamers. For refrigerated fruits (4 °C), however, only two steamers (steamer‐B and ‐C) decontaminated the cantaloupes within 60 s and they were unable to render the pathogens undetected in enriched samples. Salmonella was more susceptible than Listeria to steam treatment. In conclusion, electrical kitchen steamers can be utilized by consumers to sanitize whole cantaloupes prior to consumption. However, fruit temperature and steamer selection can influence the heating time required for surface sanitization.

Practical applications

Melons with netted rind surfaces, such as cantaloupes, likely have caused more outbreaks because their surface shelters pathogens from being eliminated by conventional washing and sanitization. As whole melons marketed at retail stores typically are not treated due to shelf‐life considerations, the safety of untreated melons can be of concern to consumers, especially among at‐risk populations. This study focused on the evaluation of the use of commercially available kitchen steamers in eliminating foodborne pathogens on cantaloupe surfaces. This study showed that kitchen steamers can be utilized to sanitize whole cantaloupes and also at household level, be a highly beneficial for the average concerned consumer to reduce the risk of pathogens on produce before consumption.

Research – Multispecies biofilms between Listeria monocytogenes and Listeria innocua with resident microbiota isolated from apple juice processing equipment

Wiley Online


When testing conditions that prevail in fruit juice industry it was found that Listeria monocytogenes ATCC 7644 and a wild strain of Listeria innocua were highly hydrophobic, expressed swimming and twitching, co‐aggregated with yeast cells and produced exopolysaccharide, all characteristics that would favor the adhesive process. In mono‐species biofilms, L. innocua adhered on stainless steel at significantly higher counts than L. monocytogenes achieving values of 6.64 ± 0.01 and 5.80 ± 0.21 log CFU/cm2. In dual species biofilms with resident yeasts, L. innocua cells counts increase significantly in the presence of Candida tropicalis and Candida krusei and decrease significantly in the presence of Rhodotorula mucilaginosa and Candida kefyr. When cocultured with Candida tropicalis, the cell numbers of L. monocytogenes had a significant increase. These results revealed synergic and antagonistic interactions among species.

Practical applications

The study supports the plausibility that interactions between L. monocytogenes and members of resident microbiota, such as C. tropicalis, C. krusei, C. kefyr, and R. mucilaginosa, might play an important role for the survival and dissemination of L. monocytogenes. Apple juice processing conditions was used as simulation of the real condition in fruit juice processing environment and these results will alert to fruit juice industry to adopt the best cleaning and disinfecting practices against Listeria.

USA – CTI Foods LLC Recalls Ready-To-Eat Philly Steak Products Due to Possible Listeria Contamination


WASHINGTON, Sept. 11, 2018 – CTI Foods LLC, an Owingsville, Ky., establishment, is recalling approximately 6,720 pounds of ready-to-eat (RTE) Philly Beef Steak products that may be adulterated with Listeria monocytogenes, the U.S. Department of Agriculture’s Food Safety and Inspection Service (FSIS) announced today.

The RTE Philly Beef Steak products were produced on Aug. 9, 2018. The following products are subject to recall: [View Labels (PDF only)]

  • 672 cardboard cases labeled Classic Sysco having a NET WT of 10 lbs. The cardboard cases contain four 2.5 lb. bags of product.  Both the box and the bags are labeled “FULLY COOKED PHILLY BEEF STEAK SLICED Caramel Color Added” with a package code of 4887097.

The products subject to recall bear establishment number “EST. 19085” inside the USDA mark of inspection. These items were shipped to a food service warehouse in Hamilton, Ohio and were further distributed to food service locations.

The problem was discovered on Sept. 7, 2018, when the establishment received results from their laboratory that four finished product samples from their Aug. 9, 2018 production were potentially positive for Listeria monocytogenes.

There have been no confirmed reports of adverse reactions due to consumption of these products. Anyone concerned about an injury or illness should contact a healthcare provider.

Research – A Comparative Study of the Susceptibility of Listeria Species to Sanitizer Treatments When Grown under Planktonic and Biofilm Conditions

Journal of Food Protection

Listeria species are ubiquitous in nature and can adapt to survive in a variety of niches, including food processing environments. Listeria species that colonize these environments may also have the potential to persist. Food safety strategies designed to manage these niches include regular cleaning and disinfection with proven sanitizers containing biocide-active compounds. Typically, these sanitizers are effective against bacteria growing under planktonic conditions, but their efficacy may be compromised when bacteria are contained in biofilms. The susceptibility of persistent Listeria isolates, i.e., those capable of forming biofilms, to a selection of sanitizers was investigated. A quaternary ammonium compound–based sanitizer was the biocide most effective against planktonic bacteria, with a MIC of 0.0015 to 0.006%. In contrast, ethanol-based sanitizers were the least effective. Although, no triclosan tolerance was observed for planktonic Listeria isolates, triclosan was the only biocide that resulted in a significant biomass reduction. Differences between Listeria species were observed; L. monocytogenes and L. welshimeri biofilms were more tolerant to quaternary ammonium compound–based sanitizers than were L. innocua biofilms. These findings extend our understanding of the application of commonly used sanitizers in the food industry and the efficacy of these sanitizers against Listeria species and their associated biofilms.

Information – FSA – Listeria Fact Sheet

FSA Listeria kswfoodworld food safety food poisoning

Listeria is a harmful bacteria. Listeria thrives in ready-to-eat foods that have already been cooked when purchased.

Listeria monocytogenes (listeria) is a bacterium that causes an illness called Listeriosis. Cases of foodborne illness from listeria are rare, but can involve serious symptoms and even death in certain groups of people. These include:

  • people with weakened immune systems, such as those suffering from cancer
  • pregnant women and their unborn babies
  • newborn babies
  • elderly people

Reasearch – Viable-but-Nonculturable Listeria monocytogenes and Salmonella enterica Serovar Thompson Induced by Chlorine Stress Remain Infectious



The microbiological safety of fresh produce is monitored almost exclusively by culture-based detection methods. However, bacterial food-borne pathogens are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses such as chlorine, which is commonly used for fresh produce decontamination. Here, complete VBNC induction of green fluorescent protein-tagged Listeria monocytogenes and Salmonella enterica serovar Thompson was achieved by exposure to 12 and 3 ppm chlorine, respectively. The pathogens were subjected to chlorine washing following incubation on spinach leaves. Culture data revealed that total viable L. monocytogenes and Salmonella Thompson populations became VBNC by 50 and 100 ppm chlorine, respectively, while enumeration by direct viable counting found that chlorine caused a <1-log reduction in viability. The pathogenicity of chlorine-induced VBNC L. monocytogenes and Salmonella Thompson was assessed by using Caenorhabditis elegans. Ingestion of VBNC pathogens by C. elegans resulted in a significant life span reduction (P = 0.0064 and P < 0.0001), and no significant difference between the life span reductions caused by the VBNC and culturable L. monocytogenes treatments was observed. L. monocytogenes was visualized beyond the nematode intestinal lumen, indicating resuscitation and cell invasion. These data emphasize the risk that VBNC food-borne pathogens could pose to public health should they continue to go undetected.

IMPORTANCE Many bacteria are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses. VBNC cells cannot be detected by standard laboratory culture techniques, presenting a problem for the food industry, which uses these techniques to detect pathogen contaminants. This study found that chlorine, a sanitizer commonly used for fresh produce, induces a VBNC state in the food-borne pathogens Listeria monocytogenes and Salmonella enterica. It was also found that chlorine is ineffective at killing total populations of the pathogens. A life span reduction was observed in Caenorhabditis elegans that ingested these VBNC pathogens, with VBNC L. monocytogenes as infectious as its culturable counterpart. These data show that VBNC food-borne pathogens can both be generated and avoid detection by industrial practices while potentially retaining the ability to cause disease.