Category: Microbiology

1. Introduction

Listeria monocytogenes is a significant pathogen in food safety due to its ability to cause severe illness, particularly in vulnerable populations such as pregnant women, newborns, elderly individuals, and immunocompromised individuals. It is the causative agent of listeriosis, a potentially fatal disease characterized by meningitis, septicemia, and, in pregnant women, miscarriage or stillbirth. What makes L. monocytogenes particularly concerning in food safety is its resilience to environmental stressors and its ability to grow at refrigeration temperatures, unlike most other foodborne pathogens. Its ubiquity in the environment and ability to colonize food-processing equipment further heighten its importance in food safety. The pathogen’s adaptability and potential to cause outbreaks linked to a wide range of foods, from dairy products to ready-to-eat (RTE) items, necessitate an in-depth understanding of its biology and control strategies (Carpentier & Cerf, 2011).

2. Cell Structure and Morphology

monocytogenes is a Gram-positive, facultative anaerobic bacterium with a characteristic rod-shaped morphology. It typically measures 0.5–2 µm in diameter and 0.5–2 µm in length. The organism is motile at temperatures below 30°C, using a flagellar system for movement. The species is divided into 13 serotypes, but serotypes 1/2a, 1/2b, and 4b are most commonly associated with human listeriosis (Swaminathan & Gerner-Smidt, 2007). L. monocytogenes demonstrates a remarkable ability to survive under adverse conditions, including high salt concentrations, low pH, and a broad range of temperatures. Its ability to form biofilms on food-processing surfaces further complicates its eradication in food safety settings.

3. Sources of contamination and Illness

monocytogenes is widely distributed in the environment and can be found in soil, water, decaying vegetation, and animal feces. Foods commonly implicated in contamination include unpasteurized milk and dairy products, soft cheeses, deli meats, smoked fish, and fresh produce. The bacterium tolerates extreme environmental conditions, including temperatures as low as -0.4°C, pH levels as low as 4.4, and reduced water activity (aw ≥ 0.92). It can enter the body through the consumption of contaminated food, with a minimum infectious dose estimated to be between 100 and 1,000 cells, depending on host susceptibility (Farber & Peterkin, 1991). Once ingested, the bacterium invades epithelial cells in the gut and spreads systemically, potentially causing septicemia, meningitis, or adverse pregnancy outcomes. Major outbreaks include the 2011 cantaloupe outbreak in the U.S., which resulted in 33 deaths and 147 illnesses, highlighting the severe consequences of listeriosis (CDC, 2011).

4. Identification Methods

In microbiology labs, L. monocytogenes can be identified using selective media such as Oxford agar or Polymyxin Acriflavin Lithium-chloride Ceftazidime Esculin Mannitol (PALCAM) agar, both of which inhibit the growth of competing organisms. Colonies on these media typically exhibit a distinctive black or grey coloration due to the hydrolysis of esculin. The bacterium grows well at 37°C but can also grow at lower temperatures, facilitating its isolation. Enrichment procedures often involve incubating samples in broth such as Buffered Listeria Enrichment Broth (BLEB) at cold temperatures to allow selective growth. Molecular methods, such as polymerase chain reaction (PCR), are also employed for rapid and specific detection of L. monocytogenes in food samples (Jadhav et al., 2012).

5. Prevention and Control

Controlling L. monocytogenes in food involves a combination of thermal and non-thermal strategies. Cooking foods to an internal temperature of at least 74°C effectively kills the organism. Refrigeration at temperatures below 4°C can slow but not completely halt its growth, emphasizing the need for additional preservation methods. Natural antimicrobials such as essential oils (e.g., thyme, oregano, and rosemary oils) and antimicrobial peptides like nisin have shown efficacy against L. monocytogenes. Synthetic antimicrobials, such as organic acids and their salts (e.g., sodium lactate), are also widely used. Preventive measures in food processing include strict sanitation protocols, the design of equipment to prevent biofilm formation, and regular monitoring of food products for contamination (Gandhi & Chikindas, 2007).

References

• Carpentier, B., & Cerf, O. (2011). Review—Persistence of Listeria monocytogenes in food industry equipment and premises. International Journal of Food Microbiology, 145(1), 1–8. https://doi.org/10.1016/j.ijfoodmicro.2011.01.017
• Swaminathan, B., & Gerner-Smidt, P. (2007). The epidemiology of human listeriosis. Microbes and Infection, 9(10), 1236–1243. https://doi.org/10.1016/j.micinf.2007.05.011
• Farber, J. M., & Peterkin, P. I. (1991). Listeria monocytogenes, a food-borne pathogen. Microbiological Reviews, 55(3), 476–511.
• Jadhav, S., Bhave, M., & Palombo, E. A. (2012). Methods used for the detection and subtyping of Listeria monocytogenes. Journal of Microbiological Methods, 88(3), 327–341. https://doi.org/10.1016/j.mimet.2011.12.013
• Gandhi, M., & Chikindas, M. L. (2007). Listeria: A foodborne pathogen that knows how to survive. International Journal of Food Microbiology, 113(1), 1–15. https://doi.org/10.1016/j.ijfoodmicro.2006.07.008

About Author

Name : Pratiksha Shrestha

pratiksha.shrestha2001@gmail.com

Ms. Shrestha is currently a Graduate Research Assistant at Louisiana State University, USA and holds a Master’s degree in Food Engineering and Bioprocess Technology from the Asian Institute of Technology (AIT), Thailand. She previously, she worked for the Government of Nepal at the Department of Food Technology and Quality Control (DFTQC), Kathmandu, and also served as a teaching faculty at the College of Applied Food and Dairy Technology (CAFODAT) affiliated with Purbanchal University, Nepal.