Medical Fun Facts Podcast

MFF0012: Listeria monocytogenes

On Friday I came across a patient referral asking us to look for Listeria monocytogenes in a patient’s stool specimen.

L. monocytogenes is what I would call cute with an interesting personality. Down the microscope, the bacteria look very neat and uniform, almost like schoolboys in private school uniforms on a parade ground. In cerebrospinal fluid, it produces a nice pleocytosis and like the name suggests, despite being able to cause a subacute bacterial meningitis, rather than seeing a preponderance of polymorphonuclear cells we see a reversed differential with more mononuclear cells.

Listeriosis can be a devastating disease if you get infected. It chiefly causes bacteræmia and bacterial meningitis. It’s a disease which affects the extremes of age and those whose immune systems have been compromised. If you’re pregnant you need to be very wary. It can cause in utero fetal death. I saw most infections when I was training in patients who were on immunosuppressive therapy because they had received transplanted kidneys.

L. monocytogenes is peculiar and like Yersinia enterocolitica can readily grow at refrigerator temperatures. These bacteria happily reproduce at 1 °C so cold meats, soft cheeses and salad bars are ‘hot spots’ for L. monocytogenes. This is why women who are pregnant should be especially wary of cold meats, salad bars and soft cheeses. We’ve had problems with L. monocytogenes in cheese in Australia recently. It’s not only a problem for the women and their unborn children, but also for primary producers and manufacturers and distributors of contaminated products.

One day I’ll do a show on raw milk products too. I cannot believe there are people who willingly and knowingly put the lives of others in danger.

So let’s go back to this referral. The patient had very sadly experienced an in utero fetal death. In this situation, we normally get the fetus plus placental specimens. For some reason, this time we received stool. I’m guessing the referring practitioner may have assumed the source of the L. monocytogenes was food and so in their logic, they thought stool would be a good place to look.

Can I just say that stool is not a great specimen? Your faeces contain millions of bacteria and viruses and potentially eucaryotic parasites for every gram of stool. The vast majority of microorganisms are normal flora, they are commensals. When you’re carrying enteric bacterial pathogens like SalmonellaShigella and Campylobacter, medical laboratory scientists have developed over decades sophisticated methods for isolating these bacteria.

Nowadays, we have nucleic acid amplification methods like multiplex polymerase chain reaction which can provide us with signals for a multitude of bacteria, viruses and parasites. We’re now in the era of culture-independent diagnosis.

Looking for L. monocytogenes in a stool specimen is not rewarding, it is rarely fruitful. It is a needle in a haystack stuff. We did try and one thing you can try is setting up the specimen at 1 °C and hope for the best. We didn’t grow L. monocytogenes. In the end, I had to issue a comment that stool is not an optimal specimen and that the best way to yield L. monocytogenes is by sending us some placental tissue for culture and also to obtain histopathological examination of placental tissue.

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