Listeria bacteria can hide in the tissue of Romaine lettuce for 60 days


Every week, I like to write about topics that I am currently learning about in my Environmental Health Microbiology class.  Sometimes it is easier than others since topics for the blog need to be interesting and relevant.  This week, an entry into foodborne illness seems necessary.

When it comes to foodborne illness, there are (4) main sources that take a pathogen from farm to consumers.  These bullet points will explain further.

  • Farm (A hen’s reproductive organs are infected leading to infected eggs)
  • Processing/Distribution (Pathogens from the intestine of a cow can contaminate a final meat product)
  • Retail/Food Service (Poor handling/hygiene) Remember, Typhoid Mary?
  • Consumer (Cross contamination or improper cooking/storage temperatures)

Each source has its own check and balances and mostly, human contamination comes into play whenever there is an outbreak of a certain pathogen.  However, this week’s blog will highlight a newly discovered avenue in pathogenic contamination.

Researchers at Purdue University found out that Listeria monocytogenes can actually live in the tissue of romaine lettuce.  I guess Caesar Salads are off the chopping block for a while.


Luckily, L. monocytogenes is predominantly a mild bacterial infection that normally affects pregnant women, newborns, adults who are 65+, and the immunocompromised.  Even though Listeriosis is generally a mild infection for pregnant women, the unborn fetus could be inflicted with a severe disease, or even more traumatic a miscarriage.  Furthermore, adults 65+ and the immunocompromised could develop severe bloodstream infections, like sepsis or severe brain infections like meningitis or encephalitis.  Now the last two infections mentioned are serious threats and not to be taken lightly.  You might have seen commercials touting a bacterial meningitis vaccine for people over 50.  This is why this new discovery hits home for so many.  What starts off as something mild could turn deadly in a short amount of time.

Normally, post-harvest sanitation processes are enough to kill most foodborne pathogens.  However, L. monocytogenes can gain entry into the lettuce tissue in as little as 30 minutes using cracked seed coats, small tears in root tissue during germination, or just plain ordinary damaged plant tissue as a port of entry.

These bacteria can be killed by heat, but the reason it flourishes so well is that the products it infects are ready to eat and/or raw, like fruits (cantaloupes and apples) and vegetables (celery and sprouts), or deli meat and hot dogs.  In fact, a 2011 outbreak in contaminated cantaloupe was the 2nd most deadly foodborne bacterial outbreak in U.S. history with 33 deaths.  So much for it being a milder infection, right?

After a 2016 recall of contaminated pre-packaged salads, these researchers began to investigate the persistence of L. monocytogenes in romaine lettuce, since it is the fastest crop in terms of growth, export, and consumption.  They found that the bacteria can live up to 60 days or until the time of harvest.

Researchers are now focusing their attention on detection strategies of what may happen to the seeds and/or seedlings of romaine lettuce.  They want to find ways to strengthen pre-harvest control especially in the avenues of contamination.  Sanitizers only treat produce externally, so they are studying ways to minimize exposure of these pathogens in the soil, water, and seeds.  Hopefully, their research will lead to advancements that will prevent another deadly outbreak of Listeriosis.

Disclaimer:  “Any opinions stated in this article belong solely to the author and do not necessarily reflect the opinions of CSUN faculty/ staff. Information contained herein has not been verified by CSUN faculty/staff.


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