A Focus on Listeria monocytogenes

On this page... Listeriosis Isolation and Detection Methods Microbiological Detection Methods Listeria monocytogenes infection Sources and Growth Requirements General Listeria Facts Government Interests Listeria monocytogenes Research Areas Resources Listeria monocytogenes (Lm) is the causative bacteria of listeriosis, a very dangerous and often deadly disease. The CDC reports approximately 2,500 cases a year, 500 of which are fatal. This is more than Salmonella and Botulism. Due to the ubiquitous nature of the bacteria, it is among the most highly researched and investigated foodborne pathogens in the United States, Canada, and many other countries. Federal and state governments, universities, and industries are actively conducting research. Effectively controlling Listeria monocytogenes is challenging and requires intensive management and resources. Although, the risk of developing listeriosis is low, the consequences are devastating for both consumers and processors. Research over the past decade has concentrated on testing and detection methods, control methods, and risk assessments. In 1998, one of the largest outbreaks of Listeria monocytogenes occurred with a large hot dog manufacturer. The result was 15 adult deaths, six stillbirths, and over one million pounds of product recalled. Listeria monocytogenes Research Areas Government Research Studying the prevalence of Listeria monocytogenes in beef processing plants Asessing the potential of a bio- preservative applied to frankfurter casings for control of L. monocytogenes Conducting a comprehensive review of published data and models related to growth and thermal inactivation of L. monocytogenes in ready-to-eat meat and poultry products. Generating a complete genome sequence and plasmids for Listeria monocytogenes Scott A strain, serotype 4b. Evaluate the dose-response of infection following an intragastric inoculation of L. monocytogenes Identifying virulence markers in food isolates of L. monocytogenes groups 1/2a and 1/2c. Determinating the effectiveness of irradiation and packaging treatments to control L. Monocytogenes Determining product implications due to stress-induced chronic infection of turkeys with L. monocytogenes. Studying post-cooking intervention to eliminate L. monocytogenes from packaged and refrigerated RTE poultry products. Tracking L. monocytogenes in a commercial poultry further processing plant to determine the environmental sources of cintamination and if season has an effect on recovery. Academia and Organization Research Distinguishing illness causing strains from non-illness causing strains of L. monocytogenes Determining the relevance of animal models used for the determination of lethal and minimal dosage, modes of infectivity and pathogenesis of Listeria. Evaluating the appropriateness of an absolute zero tolerance for Listeria levels. Determining the likelyhood of contracting listeriosis by eating different types of contaminated food products. Testing the potential biofilm formation of different isolates on varying surfaces in processing facilities. Risk assessment of the prevelence of L. monocytogenes within the food supply, including cooking and handling procedures by the consumer. Evaluating production intervention strategies, education and GMPs Evaluating more sensitive detection methods for low level healthy or injured cell detection of L. monocytogenes Controlling L. monocytogenes in RTE meats using Cetyl Pyridinium Chloride (CPC) Evaluating radiation dosage and treatment methods for packaged RTE meats Studying aerosols and the transmission of L. monocytogenes in processing facilities Studying potentials for genotypes to adapt to environmentals and survive, better surface attachment to surfaces in processing plants. Evaluating the effectiveness of varying sanitation procedures and products in the contolling of L. monocytogenes Industry Research Developing more accurate detection methods Developing agars and quicker rapid detection methods Back to top Government Interests Within the United States, the food industry and government agencies, including the Food and Drug Administration (FDA) and the Department of Agriculture (USDA), have taken steps to reduce contamination. Initiatives to address L. monocytogenes include both long and short term plans. The USDA and FDA are working to establish performance standards for processing facilities of ready-to-eat (RTE) products. They monitor the plants and food regularly. If it is determined that a food is contaminated, monitoring and inspections are intensified. If needed, a product may be recalled to protect consumers. The National Center for Infectious Diseases (NCID) is studying the impact of prevention and education to determine their effects on outbreaks and occurrence. NCID is working with the health departments in outbreak investigations. The Foodborne Diseases Active Surveillance Network (FoodNet) is the US Centers for Disease Control and Prevention (CDC) component of the Food Safety Initiative to identify, control, and prevent foodborne disease hazards at the state level. Listeria is one of the foodborne pathogens under population-based active surveillance for laboratory confirmed cases. Back to top General Listeria Facts Listeria monocytogenes was detected over 90 yeas ago in rabbits and pigs, but it wasn't recognized as an important foodborne pathogen until 1981. The fatality rate is 33 percent. This genus Listeria is composed of seven species of gram positive, non-spore forming facultative anaerobes. Of the seven species: L. seeligeri, L. innocua, L. welshimeri, L. grayi, L. murrayi, L. ivanoii, L. monocytogenes . Only the latter two species are pathogenic to animals and humans. L. monocytogenes is the only species causing human illness. The genus is characterized by its catalase activity, its lack of hydrogen sulphide production and its production of acid from glucose. Listeria ssp. are gram positive, facultative anaerobic, non-spore forming coccoid rods. The following serotypes are exhibited within the species: ½a, ½b, ½c, 3a, 3b, 3c, 4a, 4ab, 4b, 4c, 4d, 4e, and 7. Ninety-two percent of the isolates among humans and animals, include: ½a, ½b, and 4b. There is a significant geographical variation, and 4b is identified as the predominate serotype in Canada, USA, and Europe. 50 percent of the sporadic Listeria infection outbreaks have been determined as serotype 4b. Back to top Sources and Growth Requirements It is the organism’s ability to grow and reproduce in a wide variety of conditions that make it such a dangerous foodborne pathogen. Listeria bacteria are found almost everywhere in the environment, including: soil and ground water, mammals, raw and pasteurized milk, soft cheeses, raw or undercooked seafood, coleslaw, lettuce, and ready-to-eat (RTE) meat and poultry products such as hotdogs and lunch meats. Contamination of cooked meat products most frequently occurs at processing facilities when products and/or food contact surfaces are contaminated. This can occur between cooking and packaging steps. Contamination can also result via other means, such as the introduction from an employee’s apron or shoes, dirty equipment, or floors. The risk for finding L. monocytogenes in RTE products is greatly increased when contact surfaces are contaminated. The microorganism adheres to contact surfaces in food-processing plants and develops into a bacterial biofilm. Micro-colonies protected by glycocalyx adhere to contact surfaces. When colonies grow and run together forming the biofilm, the environment is very conducive to its survival. Biofilms are highly resistant to antibiotics. The increased danger with Listeria forming a biofilm is the nature of the organism. The unusual growth and survival properties add to the complexity and difficulty of controlling and removing it from contact surfaces and contamination within food processing facilities. Listeria monocytogenes can grow in cool, damp environments such as those found in any process area, coolers, or on slaughter floors.The incomplete removal of meat and fat from equipment and improper sanitation can allow biofilms to develop. The most common reservoirs are drains and hoses that lay or drag across the floors in processing facilities. Temperature Listeria monocytogenes has the capability to grow at refrigerated temperatures. L. monocytogenes can grow and reproduce at temperatures from 1°C (33°F) to 50°C (122°F), with the optimum reproduction temperature between 30°C (86°F) and 37°C (98.6°F). Freezing does not eliminate the organism. Listeria is killed by pasteurization, and heating procedures used to prepare ready-to-eat processed meats should be sufficient to kill the bacterium; they do not survive heating to 60C for 30 minutes. pH and Salt Listeria monocytogenes can also survive and grow in foods having moderate to low acidity and salt levels, as well as flourish in moist foods. Many foods are prepared with salt concentrations and pH levels that are optimum for the growth and reproduction of L. monocytogenes. The pH influence on the growth of Lm depends on both the absolute value and the acid type, ranging from 4.0-9.5. The organism is capable of growth in a salt concentration of up to 10 percent, and can survive for a year in a concentration of up to 16 percent. Atmosphere and Water Activity Growth of L. monocytogenes continues in an aerobic environment, but can be enhanced under decreased oxygen levels when carbon dioxide is present. Growth can also flourish where water activity is .90 -.97. Flagella To further increase the danger of the bacteria, L. monocytogenes has the ability to adapt even further to its environment. Studies have determined that temperature and pH can also have an effect on the growth of flagella. Flagellar expression in Lm was seen to grow in mediums with pH values as low as 5 and as high as 11. There is less specific information available as to temperatures in which this occurs, but it has been determined as a factor. Back to top Isolation and Detection Methods Listeria was originally the target for the detection processes, but that has shifted more towards the specific species of L. monocytogenes due to the pathogenicity in humans. Generic Listeria is still a valuable because it is a strong presumptive for the Lm. For example, a study showed that in fresh chicken wings 42 percent tested positive for Listeria , but only 12.5 percent tested positive for L. monocytogenes. Back to top Microbiological Detection Methods The USDA and FDA have developed methods for isolation and detection of L. monocytogenes. They utilize a standard enrichment procedure and the CAMP test. The preferred standard methodology Sample 25g of presumed contaminated food are pre-enriched for Listeria ssp at 30° C for 4 h in buffered Listeria enrichment broth (BLEB). At four hours of incubation, the selective agents acriflavin, sodium nalidixate, or antifungal, cycloheximide are added. Incubation is continued at 30° C for a 48 h. The enrichment culture is streaked at 24 and 48 h on one of the prescribed differential selective-agars in order to isolate Listeria species. The Christie-Atkins-Munch-Peterson (CAMP) test Used to confirm species, a Streak of â-hemolytic Staphylococcus aureus and Rhodococcus equi culture are made in parallel and diametrically opposite to each other on a sheep blood agar plate. Several test cultures are streaked parallel to one another, but at right angles to and between the S. aureus and R. equi streaks. After incubation at 35° C for 24-48 h, plates are examined for hemolysis. L. monocytogenes and L. seeligeri hemolytic reactions are seen in the zone influenced by the S. aureus streak. The other species remain non-hemolytic. Rapid Detection Methods As an alternative to the lengthy standard method, prescribed rapid detection kits may be conditionally used. Assumed Listeria isolates on selective agars from standard or screen positive enrichments are purified on non-selective agar and confirmed by conventional identification tests or by a battery of such tests in kit form. Isolates may be rapidly confirmed as L. monocytogenes (or not) by using specific test kits. Enzyme-linked immunosorbent assays (ELISA) DNA probes Polymerase chain reaction Ligase chain reaction Back to top Listeria monocytogenes infection The infection is acquired by the ingestion of contaminated food. The bacterium attaches to the intestinal mucosa. Once the bacteria crossed the intestinal barrier, Lm cells can be seen in underlying layers of the connective tissue. Most Listerial cells are destroyed by phagocytosis in the liver, but the surviving cells multiply in the hepatocytes. Surface proteins are necessary for entry into the host cells, and once inside, the Listeria monocytogenes cell is encased in a membrane vesicle. Once freed from this membrane, the rapid multiplication begins. The cells move with the formation of a comet like tail that is formed with the use of the host cells Actin and listerial cells surface proteins. This movement enables them to get to the host cells membrane and form protrusions extending to adjacent cells. This protrusion is internalized by the neighboring cell and thus continues the infection process. Back to top Listeriosis Listeria monocytogenes has been linked to some of the most deadly outbreaks of foodborne illnesses in the United States. Cases have been reported as both outbreaks and sporadic cases. The disease can affect anyone, but is most dangerous to the high risk group including: pregnant women, newborns, and adults with compromised immune systems (T-cell suppression). On average, there are 0.7 cases of listeriosis per 100,000 people, but the disease is seen three times higher in the elderly (>70) and 17 times higher in pregnant women. In healthy individuals, the disease can take the form of mild to substantial flu-like symptoms, including: Fever Fatigue Nausea Cramps Vomiting Diarrhea More severe complications can include: Encephalitis Septicemia Mononucleosis-like syndrome Pneumonia Endocarditis Aortic aneurysm Hepatitis Urethritis L. monocytogenes in pregnant women can lead to an intrauterine infection, resulting in stillbirths and miscarriages. Newborns can develop meningitis after birth via transplacental transmission. The onset time for serious complications of listeriosis can be anywhere from a couple of days to three weeks. Mortality of untreated infections is among the highest of all foodborne illnesses, 70 percent. The infective dose of L. monocytogenes is not yet known, it is related to variables, including: Strain variance Susceptibility of the victim Type food from which is was consumed The United States, Canada, and Europe have a zero tolerance level for Lm in RTE meats because of these variables. A blood or spinal fluid test (to cultivate the bacteria) must be performed to determine if you have listeriosis. Most antibiotics including ampicillin and penicillin are effective in the treatment of listeriosis. For people allergic to penicillin, Trimethoprim sulphamethoxazole is often used as treatment. Back to top Resources Listeria monocytogenes USDA/FSIS. June 2003 Listeriosis CDC/National Center for Infectious Diseases/Division of Mycotic Diseases.January 2003 Pubmed – Listeria Search National Library of Medicine, Pubmed What is Listeria monocytogenes? Iowa State University Extension – Food Safety Project USDA Issues Directive to reduce Listeria monocytogenes in ready-to-eat Meat and Poultry Products at Scientific Summit USDA. November 2002 Completed Listeria monocytogenes Research Projects American Meat Institute Foundation Literature Survey of the Various Techniques used in Listeria Intervention(PDF Format) University of Wisconsin–Madison, Food Research Institute Back to top

This document was created by Kristina Brandriff. Users are encouraged to provide feedback and comments. This document was created in Aug 2003; Updated in Dec 2005