Listeria: The mystery organism

Issue: June 2017

ByDonald Kaye, MD, MACP

ByMatthew E. Levison, MD, FACP

Listeria monocytogenes is ubiquitous in the environment. It is a foodborne pathogen, but the infecting dose is unknown. Person-to-person transmission, other than from mother to fetus or directly to the newborn at the time of birth, is not believed to occur. With an incubation period of up to 2 months or more, it can be difficult to trace a food source; epid emiology is now being delineated with better investigative tools.

Epidemiology

Listeriosis is a worldwide zoonosis primarily caused by L. monocytogenes, one of seven Listeria spp. L. ivanovii, which infects animals, very rarely causes disease in humans. L. monocytogenes is a non–spore-forming, beta-hemolytic, gram-positive bacillus that is sometimes arranged in short chains. It grows best at 30°C to 37°C (86°F to 98.6°F); but it also grows at refrigerator temperatures (2°C to 3°C or 35.6°F to 37.4°F) and is reported to survive at freezer temperatures. Listeria are motile at room temperature (20°C to 25°C or 68°F to 77°F) by means of one to five flagella distributed over their surface, which are produced at room temperature, but not at 37°C (98.6°F), and cause the organisms to have characteristic “tumbling” motility in vitro.

Virulent Listeria induce their own phagocytosis into host cells; they lyse the phagosome and multiply unrestricted within the host cell cytoplasm. Listeria then move within and from one host cell to another by polymerization of host cell actin at one end of the bacterium; this propels the bacterium through the cytoplasm of the host cell and into neighboring cells without damaging the host cell.

The virulence of this bacterium varies depending on the particular strain. Serotyping of L. monocytogenes strains, based on variations in the somatic (O) and flagellar (H) antigens, has determined that only three (1/2a, 1/2b, and 4b) of the 12 serotypes of L. monocytogenes cause 95% of human cases; serotype 4b is most commonly associated with outbreaks.

L. monocytogenes are widely distributed in the environment, where they can form biofilms, which enable them as a community to attach to solid surfaces and become extremely difficult to remove. They are found in the intestinal tract of many animals, including cattle, sheep and humans. They are found in soil and water, especially near cattle farms, indicating that these animals may contribute to L. monocytogenes dispersal in the environment through their fecal matter. The organisms can also be found in raw milk and foods made from raw milk (see Table).

Listeriosis was first recognized as a foodborne illness in the early 1980s. Pasteurization or cooking at temperatures higher than 150°F (65.6°C) for 30 minutes should kill L. monocytogenes. However, contamination can occur after pasteurization or cooking by contact with contaminated surfaces in food preparation areas. Because the organisms can multiply at refrigerator temperatures and survive in frozen food, it is not surprising that listeriosis is often associated with ingestion of packaged, ready-to-eat (RTE) products (eg, coleslaw; smoked fish; shellfish, such as shrimp; pâté; and deli meats), after having been stored at refrigeration temperatures, especially because its presence in food does not alter taste or smell.

Most human infections that occur after oral ingestion of contaminated food are asymptomatic or result in a self-limited, localized gastrointestinal tract illness with fever and diarrhea. However, invasive disease may occur in some patients. L. monocytogenes survive and multiply within phagocytic host cells, and their location within these cells permit access to the bloodstream. Invasive disease can present as bacteremia without an evident focus or bacteremia with localization in distant organs such as the central nervous system (meningitis, meningoencephalitis, or abscess of the brain or spinal cord), bone (osteomyelitis), joints (monoarticular septic arthritis), or vasculature (endocarditis or endarteritis, such as mycotic aneurysm of the abdominal aorta). Neonatal listeriosis is acquired transplacentally, during delivery through the birth canal, or possibly after delivery via mother’s milk. Human-to-human transmission of Listeria is otherwise not known to occur.

The infective dose — that is, the number of bacteria that must be ingested to cause illness — is not known, but it is likely to vary with the virulence of a particular strain and susceptibility of the host. Ingestion of a large number of Listeria is likely necessary to cause illness in healthy individuals, but only a few bacteria in people at high risk for infection may be sufficient to cause illness. Gastric acidity is a natural barrier for enteric pathogens like Listeria, and elevating gastric pH could possibly reduce resistance to infection with these organisms. Studies in rats have shown that cimetidine, a histamine H2 receptor antagonist that inhibits stomach acid production, significantly lowers the infective dose of virulent L. monocytogenes. Similarly, use of proton pump inhibitors — drugs that also reduce gastric acid production — significantly increased risk for listeriosis in people aged 45 years and older in a population-based, case-control study using Danish health registries.

Listeriosis is rare (fewer than 1,000 cases are reported annually in the United States, more than 95% of which are invasive); however, the disease has a high overall case-fatality rate of 20% to 30% — the highest case-fatality rate among foodborne pathogens. Invasive disease occurs most frequently in persons aged 65 years and older, pregnant women and immunocompromised hosts. Pregnant women account for 10% to 20% of all cases; most cases occur during the third trimester, when cell-mediated immunity is at its lowest. Pregnancy-associated listeriosis is usually a mild illness in the mother, but it is often associated with fetal death, premature delivery or severe neonatal disease. About 20% of all pregnancy-related cases result in fetal loss or neonatal death. About 70% of infections in nonpregnant persons occur in immunocompromised patients, corticosteroid therapy being the most important finding in these patients. Other risk factors include advanced age, recent cancer chemotherapy, Hodgkin lymphoma and lymphatic leukemia. Although relatively uncommon among the infections that complicate HIV/AIDS, listeriosis is 300 times more common in patients with HIV/AIDS than in the general population.

Electron micrograph of a Listeria bacterium in tissue.

Localized skin infections can occur in veterinarians and farmers by direct contact with infected animals, usually following exposure to products of conception. One case has been reported in a gardener following exposure to soil or vegetation.

Surveillance

Listeriosis was added to the list of nationally notifiable diseases in 2000. Between 2000 and 2014, 696 to 896 cases were reported each year. In 2014, for example, 769 cases were reported from 47 states, with an incidence rate of 0.24 per 100,000 population. However, many cases are likely not detected or reported. L. monocytogenes is rarely diagnosed as the cause of gastroenteritis and fever, partly because this organism is not detected by routine stool culture. Early spontaneous abortion or miscarriage associated with listeriosismay also be underdiagnosed. The CDC estimates that in 2011, when approximately 800 cases were reported to the National Notifiable Diseases Surveillance System (NNDSS), 1,600 cases of listeriosis actually occurred in the U.S.

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Although most listeriosis cases are sporadic, the detection of a listeriosis outbreak provides an opportunity to prevent additional illness and death by removing a contaminated vehicle from the food supply, and the investigation of outbreaks often provides information about transmission of L. monocytogenes that can be used to improve food safety. There are several national systems for monitoring listeriosis in the U.S. that have delineated the epidemiology of listeriosis in recent years: PulseNet, the Listeria Initiative, and GenomeTrakr. Each of these systems serves a different purpose.

PulseNet is the national molecular subtyping network for enteric bacterial disease surveillance that was established in 1996. L. monocytogenes isolates, usually from blood, cerebrospinal fluid, tissue from a normally sterile site, or products of conception (eg, amniotic fluid, placental or fetal tissue), are forwarded to state public health laboratories for genotyping, the results of which are submitted to a central database (PulseNet) to identify otherwise unrecognized clusters of listeriosis cases. Initially, pulsed-field gel electrophoresis (PFGE) was used for DNA fingerprinting; then, whole genome sequencing (WGS) was added in the fall of 2013. The genotypes of clinical isolates are then compared with data from state laboratories, the CDC, FDA and the U.S. Department of Agriculture’s Food Safety and Inspection Service, which perform WGS on food and environmental L. monocytogenes isolates.

WGS with single-nucleotide resolution has significantly improved resolving power compared with PFGE. WGS can distinguish strains deemed identical by PFGE (false-positive results), and the newer method can identify strains that are genomically identical but deemed by PFGE as different from one another (false-negative results). False-positive results would indicate an outbreak when there is actually none, and false-negative results would prevent the identification of true outbreaks.

With an incubation period (the time from exposure to onset of illness) of more than 2 months, it can be especially difficult to trace an illness back to the food responsible for it. Therefore, health officials need to interview patients with laboratory-confirmed cases as quickly as possible after Listeria infection is diagnosed to maximize patients’ recollection of the foods they consumed before they got sick. A standardized questionnaire, the CDC Listeria Initiative Case Report Form, is used to record demographic, clinical, laboratory and epidemiologic data. The completed case report forms are then submitted to the CDC for compilation.

When PulseNet identifies a cluster of L. monocytogenes patient isolates with the same genotype, it notifies epidemiologists, who then begin an investigation to look for a common source. They use Listeria Initiative data to rapidly conduct a preliminary case-control analysis comparing responses to food exposures reported on the case report form by patients in the cluster to those responses from patients outside the cluster. The patient information helps to identify foods possibly associated with the cluster. Without the Listeria Initiative database, appropriate comparison data (“controls”) for listeriosis investigations would be difficult to obtain through traditional methods.

Piloted in 2004 and implemented nationwide in 2005, the Listeria Initiative was designed to expedite investigation of and response to clusters and outbreaks. It allows regulators to promptly suspend food production from a facility, to halt distribution of contaminated products and recall contaminated products that had already been distributed.

The number of jurisdictions reporting in the Listeria Initiative increased from 14 in 2005 to 48 in 2014. The proportion of NNDSS cases of listeriosis reported to the Listeria Initiative increased from 17% in 2005 to 85% in 2014, and the proportion of human PulseNet isolates reported to the Listeria Initiative increased from 28% in 2005 to 87% in 2014.

GenomeTrakr is a national laboratory network that collects information about Listeria isolated from food and the food production environment, using WGS for pathogen identification. The GenomeTrakr network comprises 15 federal labs, 25 state health and university labs, one U.S. hospital lab, two other labs located in the U.S., 20 labs located outside of the U.S., and collaborations with independent academic researchers who collect and share genomic and geographic data on Listeria and other foodborne pathogens. The data are housed at the National Center for Biotechnology Information. When a Listeria isolate from food or the environment is sequenced, GenomeTrakr checks the PulseNet database to identify possible links to Listeria DNA fingerprints from patients.

PFGE and WGS have shown that many sporadic cases were, in fact, part of an otherwise unrecognized outbreak, leading to more outbreaks being identified, often at an earlier time when there are fewer cases. In the 3-year period of 2010-2012, before using WGS, nine outbreaks were resolved with an average of 23.8 cases per outbreak, whereas in the 3-year period of 2013-2015, when WGS was used, 13 outbreaks were resolved with 9.5 cases per outbreak. However, it still took years to resolve several outbreaks. Improved control measures starting in the 1990s greatly reduced the prevalence of L. monocytogenes in many types of food, particularly in RTE meats and meat products. However, in recent outbreaks, PFGE and, increasingly, WGS have facilitated recognition of foods not traditionally associated with listeriosis, such as celery, cantaloupe, bean sprouts, stone fruits, caramel apples, and ice cream. Contamination in food preparation areas, worker behavior, lack of adequate workplace sanitation, and inadequate temperature control are often at issue.

Listeriosis is a difficult disease to study, but progress has been made in developing the tools to better understand its epidemiology.

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For more information:
Donald Kaye, MD, MACP, is a professor of medicine at Drexel University College of Medicine, associate editor of the International Society for Infectious Diseases’ ProMED-mail, section editor of news for Clinical Infectious Diseases and an Infectious Disease News Editorial Board member.
Matthew E. Levison, MD, FACP, is a ProMED-mail associate editor and bacterial disease moderator, professor of public health, Drexel University School of Public Health, and adjunct professor of medicine and former chief of the division of infectious diseases, Drexel University College of Medicine.

Disclosures: Kaye and Levison report no relevant financial disclosures.