"Listeria sandwiches" in New Zealand--and potentially, in your own fridge

by Tara Smith     Department: Health & Life Sciences

Listeria monocytogenes is a nasty bacterium. Typically infection is acquired via the food we eat, and even with proper food handling, it's a difficult infection to completely prevent. Unlike most bacteria, Listeria can grow throughout a wide range of temperatures: from right around freezing all the way up to 100 degrees F. Therefore, while refrigeration slows or halts the growth of most food-borne bacteria, Listeria is the veritable Energizer bunny of microbes: it just keeps going and going and going.

While most of us can fight off a Listeria infection, it is particularly dangerous for those with a compromised immune system (including the elderly). It is also a danger to pregnant women and their fetus, as Listeria can cause miscarriage. This is a tragic example of the trade-offs experienced during pregnancy: providing for the developing fetus, but at the same time assuring the health of the mother as well. In the case of Listeria, if the mother becomes infected, the bacterium can replicate in the placenta. Therefore, expulsion of the infected organ--and typically, death of the fetus as a result--appears to be a way the body protects itself.

Listeria, in its natural habitat, is a soil bacterium; humans aren't the normal host. However, the bacterium isn't picky about where it grows, and many human infections are a result of contaminated cold cuts or other ready-to-eat foods. Because Listeria can grow at refrigerator temperatures, even a small amount of contamination can bloom in the fridge, resulting in dangerous levels of the bacterium by the time the food is eaten. This is likely what happened in a New Zealand hospital and in other food-borne outbreaks:

Non-reheated frankfurters and undercooked chicken have been prime culprits. In one study, cases were more likely to have eaten food bought at a deli or have eaten soft cheeses. In about two thirds of the cases it was possible to isolate Listeria from at least one food item in their refrigerators, especially RTE meats, leftovers, cheeses and raw vegetables. Since this is a lot of what is in almost any refrigerator this isn't a lot of help. Most cases are isolated or "sporadic" cases. Only a small fraction occur in large outbreaks, but outbreaks can be large. The mean size is 40 cases. In one large outbreak Mexican-style soft cheese was responsible for 142 cases, with a mortality of 34%.

As Revere notes, it's for this reason that pregnant women are cautioned to avoid these ready-to-eat foods; and safety should trump convenience every time.

Tags: bacteria, infection, Listeria, outbreak