Listeriosis: diagnosis and methods

Author: Lisa Pucci

Listeriosis, a rare but severe foodborne infection, is caused by the ubiquitous Gram-positive bacillus, Listeria monocytogenes. This organism is recognized as one of the most significant causes of mortality among foodborne bacterial pathogens globally, primarily due to its high case-fatality and hospitalization rates.

The ubiquitous nature and survival mechanisms

L. Monocytogenes is a facultative intracellular pathogen with a widespread ecological niche, found extensively in soil, water, decaying vegetation and the intestinal tracts of various animals. This saprophytic existence in the environment is compounded by its unique physiological robustness:

• Psychrotrophic capability: A major public health concern stems from its ability to survive and actively multiply at standard refrigeration temperatures, allowing it to reach infectious doses in refrigerated, ready-to-eat (RTE) foods over time.
• Stress adaptation: The bacterium has evolved exceptional mechanisms to tolerate common food preservation and sanitation stresses, including low pH, high salinity, and exposure to disinfectants, posing a persistent challenge in food-processing environments.

Clinical manifestations and high-risk populations

While healthy, immunocompetent individuals typically experience a mild, self-limiting non-invasive listeriosis (febrile gastroenteritis), the disease assumes a severe, systemic form in vulnerable groups.

The most critical populations at risk for invasive listeriosis – where the bacteria disseminate beyond the gastrointestinal tract – include:

• Immunocompromised Individuals (e.g., those with malignancy, HIV, or chronic disease)
• The elderly (aged 65 and above)
• Pregnant women and their newborns

In these susceptible hosts, L. monocytogenes can traverse critical biological barriers, notably the blood-brain barrier (leading to meningoencephalitis) and the placental barrier (resulting in materno-fetal listeriosis). The infection is 13 to 20 times more common in pregnant women than in the general population, accounting for 16–27% of all Listeria infections. Fetal complications are severe, including spontaneous abortion, stillbirth, preterm labor, and neonatal infection.

While most often diagnosed in the third trimester, the infection can occur at any stage, and early gestation cases may be underestimated. The French prospective MONALISA study confirmed the devastating outcomes: 83% of confirmed maternal-neonatal cases had major adverse consequences, including a 24% fetal loss rate and 45% premature delivery rate.

Intracellular pathogenesis and diagnostic challenges

The organism’s virulence is attributed to a sophisticated arsenal of proteins, chief among them listeriolysin O (LLO), a pore-forming cytolysin that enables the bacterium to escape the phagosome upon host cell invasion. Once within the host cell cytosol, Listeria utilizes the ActA protein to hijack the host’s actin polymerization machinery, propelling itself within and between adjacent cells without ever exiting the protective intracellular environment.

The highly variable and often prolonged incubation period (ranging from a few days up to several weeks) and the non-specific, flu-like symptoms in high-risk groups frequently delay clinical suspicion and diagnosis. Definitive diagnosis relies on the isolation and culture of the organism from normally sterile sites, such as blood or cerebrospinal fluid, emphasizing the need for a high index of clinical suspicion when managing severe febrile illness in at-risk patients.

Diagnostic methods

The definitive diagnosis of invasive listeriosis is based on the isolation of the bacterium from a normally sterile site. Crucial clinical specimens for microbiological culture include Blood culture and Cerebrospinal Fluid culture. Standard tests like vaginal or stool cultures are unreliable due to widespread asymptomatic carriage in the general population (1-15%).

Supplementary laboratory methods include Gram staining and Nucleic Acid Amplification Tests (e.g., PCR).

The Serodiagnostic advancement

Given the paramount importance of early diagnosis and therapy, a newer approach integrates serodiagnostic assays for anti-listeriolysin O (LLO) antibodies.

• LLO Serology: Anti-LLO antibodies are reliable indicators of listeric infection and can monitor patients throughout the long incubation period.
• Role: This serological screening test offers a rapid, non-invasive means to identify high-risk individuals and minimize the devastating consequences of pregnancy-related listeriosis.

Prevention as the primary strategy

Since the infection is primarily foodborne and L. monocytogenes can multiply under refrigeration, rigorous food hygiene and prevention remain the most effective strategy for pregnant women. Recommendations emphasize avoiding high-risk foods and implementing strict kitchen hygiene practices.

Foods to avoid (or consume only if cooked)

At-risk individuals must avoid consuming the following foods, unless they have undergone a complete thermal treatment (heating to an internal temperature of at least +74°C):

• Dairy Products: raw milk, soft cheeses, and blue-veined cheeses;
• Meat and Cured Meats: raw or undercooked meats, hot dogs, sliced deli meats (ready-to-eat and lightly cured), pâtés and fresh meat spreads
• Fish Products: refrigerated smoked fish (e.g., smoked salmon), raw fish (sushi, sashimi)
• Ready-to-Eat Vegetables: ready-made salads, unwashed fruits, and vegetables.

References

• McLauchlin J. The relationship between Listeria and listeriosis. Food Control, 1996;187-193.
• Mateus T, Silva J, Maia R, et al. Listeriosis during pregnancy: a public health concern. ISRN Obstetrics and Gynecology, 2013.
• Center for Diseases Control – CDC. 2017 [Online]. Available: https://www.cdc.gov/listeria/prevention/index.html
• Charlier C, Perrodeau E, Leclercq A, et al. Clinical features and prognostic factors of listeriosis: the MONALISA national prospective cohort study. Lancet Infect Dis, 2017;1-10.
• Berche P, Reich KA, Bonnichon M, et al. Detection of anti-listeriolysin O for serodiagnosis of human listeriosis. The Lancet. 1990;335:624-627.
• Pucci L, Massacesi M, Liuzzi G. Clinical management of women with listeriosis risk during pregnancy: a review of national guidelines. Expert Rev Anti Infect Ther. 2018 Jan;16(1):13-21.
• Massacesi M, Oliva N, Pucci L, Brandi G, De Santi M. Listeriolysin O antibodies detection in pregnant women: results from an Italian pilot study. Ann Ig. 2019 Jul-Aug;31(4):326-332.
• Istituto Superiore di Sanità – EpiCentro – L’epidemiologia per la sanità pubblica. Available: https://www.epicentro.iss.it/listeria/

Lisa Pucci holds a PhD in Biomedical Biotechnologies and is Diagnostics and Assays Production Coordinator at Diatheva. With her expertise in biomedical technologies, she plays a key role in ensuring the development and manufacturing of high-quality diagnostic kits.