Microbe of the Week: Listeria monocytogenes in the Food Industry

By97043pwpadmin

April 7, 2025 | Microbe of the Week

The fridge-raiding ninja of the foodborne pathogen world.

If you think cold food is safe just by nature of being cold, Listeria monocytogenes sees you.[3]

This microscopic mayhem-causer is a foodborne member of the order Bacillales, which contains several foodborne pathogens, and has a taste for cold, salty conditions. Unlike most bacteria that chill out (literally) in the fridge, Listeria just chugs along at speed under refrigerator temps—as low as 32°F (0°C).[7] That means it can slowly multiply on deli meats, soft cheeses, smoked fish, and unpasteurized dairy products while they sit in your fridge looking so tasty and delicious.

So what is Listeria monocytogenes?

Listeria monocytogenes is a rod-shaped, Gram-positive, facultative anaerobe[3]—basically, a microscopic hot dog with an attitude. Because it doesn’t require oxygen, it threatens vacuum-sealed and Modified Atmosphere Packaging (MAP) products alike.

It causes listeriosis, a relatively rare but potentially severe disease, especially dangerous for pregnant individuals, newborns, older adults, and immunocompromised people.[7] These individuals are advised to cook deli meats and smoked fish before eating, and to avoid soft cheeses and unpasteurized dairy.

Listeriosis can lead to sepsis, meningitis, and—during pregnancy—miscarriage or stillbirth. Mortality rates for severe infections can approach 20%.[7] For food scientists, this makes Listeria one of the most serious threats we track in ready-to-eat (RTE) and refrigerated products.

Why it’s a food safety nightmare

Most bacteria are slowed or stopped by refrigeration. But Listeria? It’s psychrotrophic, meaning it grows even in the cold.[6] It also tolerates salt and low-moisture conditions and can persist on stainless steel and plastic surfaces common in food manufacturing environments.[1]

Once Listeria establishes itself, eradication can be a Herculean task. It’s not just hiding—it’s thriving.

Listeria and Biofilms

Listeria forms biofilms—slimy, protective microbial communities on surfaces like stainless steel or conveyor belts. These shield bacteria from sanitizers, heat, and even pressure washing.[7] A surface can appear clean while still harboring enough Listeria to contaminate your entire product run.

Effective cleaning requires validated sanitation steps, regular environmental monitoring, and aggressive hygienic design.

Favorite Hangouts

  • Sliced or prepackaged deli meats
  • Soft cheeses (e.g., brie, queso fresco, feta)
  • Cold-smoked fish
  • Refrigerated pâtés or meat spreads
  • Raw (unpasteurized) milk and dairy[2]
  • Factory surfaces with poor sanitation

(*Author’s note: raw milk is banned in many U.S. states for a reason. Spoiler alert for a future post: it’s not just Listeria.)

How does it cause disease?

Once consumed, Listeria crosses the intestinal barrier and enters the bloodstream. Using proteins called internalins, it sneaks into host cells, then hijacks the cell’s actin filaments to rocket around intracellularly—like a microscopic Tony Hawk.[3]

Because it moves directly between cells, it avoids triggering immune responses. It spreads silently, often reaching:

  • The brain – causing meningitis
  • The blood – causing life-threatening sepsis
  • The fetus – potentially causing stillbirth or neonatal infection[7]

Unlike E. coli or Salmonella, Listeria infections may not cause immediate vomiting or diarrhea. But they’re more likely to be fatal if untreated.

How do we stop it?

Fighting Listeria requires a multi-hurdle approach:

  • Heat – Kill it by cooking food to 165°F (74°C)[2]
  • Acid – It struggles to grow below pH 4.4[1]
  • Water Activity – It prefers aw above 0.92[1]
  • Sanitation – Biofilms mean surface disinfection must be precise and aggressive
  • Formulation – Some CPG products use clean-label antimicrobials to inhibit it[5]

TL;DR

Listeria monocytogenes is a stealthy cold-loving pathogen that thrives where others die: salty, refrigerated, and low-oxygen foods. It’s especially dangerous for the immunocompromised, elderly, and pregnant.

Just because Grandpa likes cold cuts doesn’t mean he should have to share his immune system with a bacteria that hijacks human cells like it’s doing parkour.

Clean your fridge, cook your deli meat, and if that sandwich meat smells even a little weird—just let it go.

Sources

  1. Montville, Thomas J., Karl R. Matthews, and Kalmia E. Kniel. Food Microbiology: An Introduction. 3rd ed. Washington, D.C.: ASM Press, 2012.
  2. Damodaran, Srinivasan, Kirk L. Parkin, and Owen R. Fennema. Fennema’s Food Chemistry. 4th ed. Boca Raton: CRC Press, 2017.
  3. Toldrá, Fidel, ed. Handbook of Meat and Meat Processing. 2nd ed. Boca Raton: CRC Press, 2010.
  4. OpenStax. Microbiology. Houston: OpenStax, 2017. https://openstax.org/books/microbiology/pages/1-introduction
  5. Warriss, P. D. Meat Science: An Introductory Text. Wallingford: CABI Publishing, 2000.
  6. Berk, Zeki. Food Process Engineering and Technology. 4th ed. Amsterdam: Elsevier, 2018.
  7. Lawrie, R. A., and D. A. Ledward. Lawrie’s Meat Science. 7th ed. Boca Raton: Woodhead Publishing, 2006.