Listeria and the Pregnant Lady

Listeria growing on a metal chip (C. Assisi, unpublished)

Listeria monocytogenes is one of my favorite bacteria because it is extraordinarily adaptable and extremely stress tolerant, surviving acidic pH, cold, salt, freezing, and many other stresses with aplomb. Personally, I would give a lot to have similar levels of stress tolerance and adaptability, but LM, as we’ll call it, uses its abilities for nefarious purposes.

LM is found virtually everywhere–stagnant pond water, soil, decaying leaves, the guts of birds and animals, especially ruminants (such as goats, sheep, or cows), and in delis and manufacturing plants. Once in a plant or deli, LM can form colonies embedded in a tough, sticky covering–a structure known as a biofilm–and is then even more resistant to stress. This biofilm will sporadically shed bacteria into the environment, potentially cross-contaminating foods prepared there. For instance, the Blue Bell ice cream outbreak a couple of years ago was eventually traced back to LM which formed biofillms on some equipment, cross-contaminating the ice cream in very low doses (2).

Once in the human body, LM invades the intestinal wall by deploying an entry protein which mimics a normal human protein. The cell takes in the LM and once inside, LM multiplies, rearranges the cell’s interior skeleton and rockets into the neighboring cells.

Listeria monocytogenes life cycle
Listeria monocytogenes life cycle (

In addition to wreaking havoc on the intestinal wall, LM can also enter lymph nodes and from there escape into the bloodstream. From there,  it can cross into cerebrospinal system or the placenta (see below). This is called invasive listeriosis. Cerebrospinal infections and septicemia typically only occur in patients who are elderly or immunocompromised, while placental infections obviously only occur in pregnant women.

Listeriosis. From Lecuit et al., 2007

In order to protect the unborn child, pregnant women have lower immunity to many pathogens (3). Meanwhile, LM is attracted to the placenta, and can cross it and infect the baby. Researchers aren’t entirely sure what the risks are if a women contracts listeriosis early in the pregnancy, but by the third trimester, infections with LM often lead to miscarriage, preterm birth, or the baby being born with a raging infection with LM (which can result in death) (3). The mom, meanwhile, often doesn’t have any symptoms or only mild ones.

So, what can a pregnant woman do to protect her little one?

  1. Since LM is tolerant to salt and many other stresses, the only sure way of destroying it is cooking. Consequently, foods which are not cooked (or recooked) prior to eating have the highest risk and should be avoided. Some examples include–(from the CDC’s website)
    1. deli meats/lunch meats/hot dogs (uncooked)
    2. raw milk
    3. unpasteurized cheeses (especially soft cheeses like queso fresco)
    4. sprouts
    5. pre-cut fruits (at risk for cross-contamination in the store during prep)
    6. smoked seafood
  2. If you’re going to eat deli meat, buy the pre-packaged version, and be sure it has preservatives. These slow down LM’s growth, so you’re less likely to get sick if there is contamination. Pre-packaged deli meats are also less likely to have LM in the first place*.
  3. Eat things promptly. LM can grow in the fridge, so the longer your food is in the fridge, the more LM there will be (if your food is contaminated).
  4. Pay attention to news alerts about food recalls and outbreaks.
  5. Follow the recommendations here about foods to avoid while pregnant.


References and further reading:

  1. Good, layman’s-level description of LM and other foodborne pathogens: BadBugBook_FDA_2ndEd_2013
  2. Blue-Bell_root_cause_assess_TXplant_2016
  3. A bit dense, but a good medical overview of listeriosis in pregnancy: Listeria_Infection_in_Pregnancy_A_Review_of_Litera

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