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.
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.
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.
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.