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…