As some of the regular readers of this blog may know, our lab has been working on Listeria monocytogenes since 2003, studying how it transports iron from the environment (or from the human body) into the cell. After the recent outbreak involving cantaloupes from Colorado, I thought it would be worth writing about it.

First, how do bacteria contaminate a cantaloupe? Listeria normally exists  in the environment living on dead organic material in the soil (fallen leaves, rotting wood, dead plants … it’s called a saprophyte), and also in many different species of animals from birds to fish.  Indeed, some proportion (around 10%) of healthy humans have listeria in the intestinal tract, without causing any problems. Contamination of foodstuff happens in many ways. For instance, a worker involved in packaging fruits or handling equipment that harvests and packages fruits (or other foodstuff) may contaminate the produce if he or she has poor basic hygiene procedures.  But this source is quite limited in scope, and more likely, contamination results from fields that are treated with infected manure.

Where are the bacteria in the contaminated food? In the case of cantaloupes, bacteria can only infect the outside of the fruit.  Unless the husk or skin of the product is broken, the inside will remain sterile.   However, keep in mind that when  you cut the fruit open with a knife, you may transfer bacteria from the outside to the interior, edible part of the fruit.  One of the interesting characteristics of listeria, which is crucial to its contamination of food, is that unlike most bacteria it grows very well at low temperatures, like in the refrigerator (4 C). Therefore, if food is contaminated by this organism and refrigerated for later consumption, listeria will simply go on happily multiplying.  Another unfortunate characteristic of this nasty pathogen is that it has no odor, and so it doesn’t seem to affect the taste of the food. Some outbreaks in France were linked to chocolate milk, and people were drinking milk heavily contaminated by listeria without any noticeably bad flavor.

Can you get rid of Listeria by washing the fruit?
  Yes.  You can use dish-washing detergent and a soft brush, followed by a good rinse and blotting with a paper towel.  However, if  the fruit was bruised during transport, breaking the natural barrier provided by its skin, then the bacteria gain access to the fruit’s flesh.  Since refrigeration doesn’t slow its multiplication, in this case you are better off discarding any fruit that originated in the region suspected of contamination.

Why is it so hard to pinpoint the source of contamination? Most enteric bacterial pathogens, E.coli O157, Shigella dysenteria , and Salmonella typhimurium for examples, rapidly induce symptoms (within 12 to 24 hours) after ingestion of the contaminated food.   Listeria begs to differ.  It may take a couple of weeks, sometimes more, for symptoms to appear. You can imagine how tricky it becomes for epidemiologists to trace the origin of the outbreak.  Do you remember exactly what you ate 2 weeks ago, and where it was?  If you’re a food blogger  you might have a higher chance of answering yes to this question, but even then, it’s not easy.  ;-)  Also, the listerial incubation time differs from person to person, complicating the issue even further.

How dangerous is listeria anyway? It depends on who you are. Most healthy individuals will not even develop symptoms or become infected.  It takes a huge dose (about a billion) of bacteria to infect a healthy person.  However, the very young, the very old, pregnant women, and immunocompromised individuals (undergoing steroid treatments, HIV-infected) are at much higher high risk, because their immune systems are not up to the challenge. In France, where unpasteurized cheese is considered (as it should be) a delicacy, pregnant women are advised to avoid them because they are a source of listerial contamination.

What makes Listeria so deadly?  Once you eat contaminated food, the bacteria passes to the intestine, where it invades the epithelial cells,  white blood cells, and then reaches the bloodstream. It releases toxins, which make you sick with similar symptoms to those of other enteric pathogens like E.coli and Salmonella: fever, intestinal cramps, diarrhea, general discomfort. But that’s just the beginning. If your immune system can’t contain the bacterial growth, the strain has one more deadly trick up its sleeve: it can cross the delicate (and normally powerful) barrier between the blood and the brain. Once that happens, meningitis occurs, as well as other serious neurological problems like brain abscesses and paralysis.  Again, for the most part these problems don’t happen with healthy individuals, only those at high risk – young kids, aging people, pregnant women, and immunocompromised patients.

Can listerial infections be treated by antibiotics?  That’s the good side of this pathogen.  Most strains are sensitive to antibiotic treatment, so many  weapons are available to deal with it.  However, once the strain crosses the blood-brain barrier, antibiotics have difficulty clearing the infection, resulting in a high mortality & morbidity rate.

Some cool facts about listeria  Like many other species of bacteria, listeria can swim because they have little organelles called “flagella”  that propel them in the direction of food and other attractants.   However, it is not able to make flagella at the body temperature of mammals and humans – 37 C – so in our bodies, they cannot swim.   How do they move from cell to cell?  In a fantastic mechanism, that almost seems like the product of a science fiction movie director:  once inside our cells, the bacteria induces some of our own proteins to gather together  (in biochemical terms they  induce these proteins to “polymerize”) forming structures that act like jet propellers, and literally push each bacterium across the cell, making it go through the membrane and reach the neighbor cell.    In a classic cartoon depiction, here is what it looks like:

In this figure from Wikipedia (which originated from the laboratory of Dan Portnoy at UC Berkeley), we see in the outer edge images from electron microscope of the “real thing.”  In the center,  a cartoon depiction of what is taking place.  The bacterium is represented as a black, rod-shaped structure. InlA and InlB are two genes necessary for the initial invasion of epithelial cells.   Once inside the cell, the bacterium is briefly contained inside a little vesicle.  But listeria escapes this small “prison cell” by digesting the vesicle’s membrane with an enzyme called  listeriolysin (LLO).   It is then free inside the cell, and immediately starts the process of polymerizing actin (through the action of proteins like ActA), that act to propel the bacterium across the cell, allowing it to reach the cell adjacent to it.  And the process goes on, and on, and on…

What are we specifically working on?   For listeria to go on multiplying in our body, it needs iron.  It steals iron from us by several different mechanisms.  We are trying to understand what are the most important sources of iron, and how could we prevent listeria from using it.  By interfering with its iron uptake mechanism, we hope to prevent it from multiplying to a level that will cause disease.  Some of our published work can be found jumping here and here.

I hope you found this small overview helpful…

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23 thoughts on “THE US LISTERIA OUTBREAK, 2011

  1. Thanks for posting this, Sally. Your explanation is very clear and understandable, and you’ve made what could be a dry topic into an interesting one.


    • I think that there should be more openness regarding the experiment that took human waste, tried to ‘clean’ it, and have been using it as fertilizer, for the last several years. Do we KNOW where Listeria comes from? Fecal material. Primarily, human fecal material. Opens up an entire host of thoughts, doesn’t it.


  2. Thank you, Sally, for presenting essential scientific info in such a readable format. I didn’t know that listeria bacteria could blissfully survive and multiply in the refrigerator . Now I will thoroughly wash melons before sticking them in the fridge drawer and not just before slicing them open as I used to. It is scary to read that in most cases contamination originates from fields fertilized with infected manure. Because of the pervasive smell, I vividly remember farmers spreading manure on their fields when I was growing up in Europe. Were there as many listeria outbreaks in the past (even the very distant past as in the Middle Ages for instance) as there are now or did we just not know about it? Is the risk greater today for some reason? How can farmers know that the manure they use is safe to use? We get our produce from a little farm next to a horse stable (we live in horse country) and I know they use the manure. Next time I see the farmer I will ask her about it (and bring her a printout of your article so that she knows where I am coming from!).


    • Hello, MC!
      it is not possible to know if Listeria outbreaks happened in the distant past, because the bacteria itself was identified as a pathogen quite recently, in the last century

      As to the use of manure to fertilize crops, I have a link for you that covers most of the relevant facts. Composted manure is safe, so the main issue is making sure farmers are not fertilizing the crops with raw manure, and that water that drains from places with raw manure don’t reach the plants that will be used as food. But, there’s of course a lot more to it, so if you want to take a look at this link, you can have more topics to talk with the farmers in your area. Manure can be tested not only to analyze the nutritional content as a fertilizer, but also its safety


  3. Thanks so much for this Sally – I found the bits about knife transference and cool temperature endurance particularly helpful – you’re quite right, we tend to think of bacteria multiplying in a warm environment. What interesting work you do!


  4. Thank you, Sally, for the excellent and understandable information. I’ve been washing melons in just the way you describe for a good many years now, but the thought that there could be contamination within is a real concern.
    There are surely plenty of worries to go around now that I think of it. Perhaps I’ll let you and Phil get back to your listeria research and I’ll worry about other things.


  5. Loved this lesson on Listeria, Sally! You must be an awesome teacher, explaining difficult things in a way that makes it easy to understand.

    I had no idea of the multiplication in the refrigerator, very interesting and a bit scary too


  6. That was very well done. Consider contacting one of the OKC news stations or the Daily Oklahoman. Offer to be their local expert. Get some good PR for your lab.


  7. Interesting stuff! I didn’t even realize we had an issue with Listeria (and honestly didn’t even know what it was and how serious it was). That’s awesome advice to make sure to wash everything really good and to avoid bruised fruit.

    Thanks for teaching me something new Sally!


  8. Thanks Sally, I really did not realize about how long listeria takes to incubate. Next time I eat a melon I will be sure to use a scrub brush and examine it carefully, just to be on the safe side. I already use soap and water.


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