Mano y L. mono

I’ve tried writing this post a couple times now and it always turns out dull. This time I am including that as a preface so that I can make myself consciously aware of the fact and thus hopefully avoid it… I’ll try to keep the words to a minimum and get to the pictures. If you want to know more, ask in the comments or visit the CDC website on Listeria monocytogenes.

I work as a regulatory microbiologist: I test food products to help you eat safe. It doesn’t always work  as I can’t test everything, or you wouldn’t have food, but I try to get a good assortment.

One of the things I test for is food borne Listeria monocytogenes (L. mono from now on), which is a potentially dangerous bacterium found in all kinds of places, but capable of living on food stuffs; and that is the kind I look for.

I recently went to a training seminar and afterwards received a Proficiency Test sample to ensure I learned everything that I took part in. I used this opportunity to take some pictures of the identification process with a sample I knew would show you the results I wanted.

Before I talk about the steps of my procedure I would like to talk briefly about L. mono, and why it is important that I test for it.

There are six common species of the Listeria genus: monocytogenes, innocua, ivanovii, seeligeri, welshimeri, and grayi. These all grow under the same conditions (which are very wide), so finding one in a location means the others can very well grow there. In fact many facilities that process food won’t even bother testing specifically for L. mono, they’ll just test for any Listeria and assume the worst. Of the six, L. mono and L. ivanovii are pathogenic to animals, and only L. mono can target humans. However they can be difficult to tell apart for many testing methods.

This is L. mono after a Gram stain. (L. mono is a gram positive rod.)
This is L. mono after a Gram stain. (L. mono is a gram positive rod.)

L. mono is responsible for less than 1% of all food borne illnesses in the United States per year, so it sounds like it is a minor player. The thing about L. mono is though, when you get it; it can mess you up. Even though it is not common, of the people that get it 90-95% need to be hospitalized, and roughly 10% of them die anyways. When you compare that to Salmonella, which accounts for about 40 percent of foodborne illness in the U.S., and of which only about a third of those infected needing hospitalization and less than one half of one percent die; you can begin to see how serious L. mono can be. Not only is it so dangerous from a purely infection stance, it only has the nasty habit of causing pregnant women to abort. That is why every lab that works with L. mono is off limits to pregnant women, a single accidental exposure can go very bad, very quickly. L. mono doesn’t mess around and it hates babies.

Ok, so that’s why I do what I do. Here is what I do.


I am sent food products (such as this ham) and I grind them up to make sure that any L. mono has plenty of access to food.


meat bags
Then I take the product, put it in a bag of special growth medium, and incubate it for 24 hours.


After incubation, I take a tiny portion of the sample and run it through a PCR screening test. This checks to see if L. mono specific strands of DNA are in the sample.

The PCR screening process is just the first test. If it returns a positive result, we have to confirm that what we have is L. mono and not some cross reading by the machine. This is done by growing the sample on various selective medias that do specific things when L. mono grows on them.

The first thing we do is take some of the liquid from our enrichment (the sample bags from earlier), and streak it for isolation on an esculin production plate. All Listeria species will produce grey colonies with black halos on this plate so it only lets us know that much.


Hem a
Next we take a single colony from the esculin plate and grow it on blood agar. All Listeria will grow on blood plates.
Hem b
However; L. mono, L. ivanovii, and L. seeligeri will cause hemolysis which can be seen by holding the plate up to a light. Basically they cause the blood cells to bust open and leave a clearing on the plate.


After we get a blood plate growing, we will take a single colony from it that shows typical growth and streak it to a Chromogenic plate. On this plate L. mono and L. Ivanovii will grow blue colonies with a slight lightening of the agar around them. The others grow blue colonies, but without the halo effect.


From the last blood plate we will perform what is called a CAMP test (named with the initials of the guys who discovered the method) in which we put a beta Lysine disk in the middle of a plate, and streak a single colony in a straight line outward from the center.
When we hold the CAMP test plate up like we did with the original plate we will see a strong spot of Hemolysis near the disc in the center. This happens with L. mono but not with L. ivanovii. So now we have a test differentiating L.mono specifically.


API Biochemical
And finally we perform a bio chemical assay test. With this test we put a small amount of sample taken from our blood into several wells on a special strip. The unique combination of these ten chemical reactions can definitively identify down to the exact species of Listeria. In this case L. mono.

At this point the final step for confirmation is to perform a gram stain and look at the organism itself. By now we know we have a pure culture and that it is going to be L. mono, but the visual confirmation is our finality. I put the picture up at the top though, so I don’t think I need to repost it again… just scroll back up.

So, there you have it; L. mono testing, isolation and confirmation. This whole process takes at least 8 days of work. And you got to see it in just the few minutes it took to scroll through this post. Have a safe dinner.

Chris T.

Chris is a microbiologist with a passion for nature. He has a degree in Natural History and spends his time taking pictures of mushrooms, riding his bike, painting, and watching tv.

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  1. Ohhhhhh SO fascinating to me right now! I’m learning about viruses and bacterial infections and THIS is just the kind of thing I wanted to see!

  2. Yeah, I knew you wanted to see some pictures of things. But I knew that the only picture I had of the bacteria itself was the gram stain. I didn’t think that would be a lot of research help for your project. But I’m glad you enjoyed it. 🙂

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