In the days before the Thanksgiving, the Centers for Disease Control in Atlanta released a stark warning: Don’t eat the Romaine Lettuce. The agency was investigating a a multi-state outbreak of a strain of bacteria called
Escherichia coli O157:H7, which in addition to causing diarrhea and other unpleasant gastrointestinal symptoms, can also cause acute kidney failure and death.
The suspected carrier of this virulent microbe: Romaine Lettuce.
The internet has been awash in memes highlighting the lethality of poor Romaine lettuce, which is normally lauded for its tastiness in salads and nutrition content. But don’t just blame it on Romaine!
This is actually a interesting ecological saga of microbial romance, ruminant guts, and watershed management intersecting with our salad plates.
Let’s start with E. coli.
E. coli is a type of bacteria that normally lives in the intestines of humans and animals. Most E. coli strains are harmless, and actually provide a protective role in the intestine by preventing disease-causing bacteria from moving in. They also help make Vitamin K. Scientists love using E. coli an a model bacteria in the lab. It is easy to feed and grow!
On an environmental scale, E. coli can get transferred from its home in the intestines to the larger world via … poop.
So how do the bacteria make it from poop into another intestinal home? Water! Humans came up with this ingenious idea of making our poop go away by sending it into flowing water.
We have mitigated the E. coli transfer somewhat by treating our sewage (which I realize is just a fancy word for poop-water) before dumping it out into rivers and lakes. However, leaky septic systems and intense storm overflow events may cause sewage to bypass the treatment process and end up in the water anyway. Animal poop lying around on the ground (or in big lagoons on a cattle feedlot) may get washed into waterways by rain.
Some of this E. coli-laden water makes it to a farm irrigation system, where it waters the the lettuce. And leaves behind a nice film of E. coli bacteria in search of a new intestine to live in.
Most E. coli versions aren’t dangerous. This water-borne bacteria becomes a problem when the kinds of E. coli venturing forth into the water include less-benign, disease-causing versions. These pathogenic strains secrete toxins that can damage tissue cells in the human body. Scientist can track these variants by their serotype designations (i.e. O157:H7) that refer to the arrangement of sugar molecules in the E. coli bacterium cell wall.
So where do the toxins come from? For example, E. coli O157:H7 produces a Shiga toxin that damages intestinal cells to cause bloody diarrhea, and potentially even kidney failure by damaging the tiny kidney capillaries with the debris of broken cells. Shiga toxin was first identified in the bacterium Shigella dysenteriae, a pathogen that causes dysentery (severe diarrhea.) Producing bacterial toxins may confer evolutionary advantages over non-toxin mediated infection by increasing the virulence of the bacteria, as well as increasing transmissibility of bacterial particles via diarrhea output.
But how did the genes for making Shiga toxin make it from the genome of Shigella into its cousin E. coli? Through bacteria sharing genes via bacteriophage, or its less sexy name, Transduction.
Bacteriophages (virues that infect bacteria) are often floating around places like intestines where lots of different kinds of bacteria are hanging out.
In this case, a phage may have transferred a snippet of genetic material containing the toxin-coding region from a Shigella spp. bacterium into an E. coli bacterium, granting a previously benign microbe an infectious advantage. (Tangent: Bacteria can also use this process to transfer genes for antibiotic resistance.
Okay, so now that we have a Shiga-toxin producing strain of E. coli, how did it end up on the Romaine lettuce? Are sick humans just pooping indiscriminately all over our lettuce fields (or within the watersheds that flow into irrigation systems?)
Possibly, however a more likely culprit has a ruminant digestive system and hooves:
Turns out that cattle (and other ruminants such as deer, goats and sheep) are asymptomatic carriers of the Shiga-toxin producing strains of E. coli. The bacteria do not make the cattle sick, possibly because they lack specific cell receptors the bacteria use to attack blood vessels. The cattle are a living reservoirs for these E. coli strains, as well as contributing to literal reservoirs for the bacteria in the form of waste retaining ponds full of cattle poop. That poop-carrying water is just a rainstorm and a watershed away from making its way to the lettuce fields. Voila, you’ve got an epidemic of foodborne illness!
While it’s tempting to reduce the media narrative to “Romaine Lettuce is now deadly,” I think it’s much more interesting to look at the interconnections between all of the players in this story (The Cattle, the Poop, the watersheds, the E. Coli and the bacteriophage). The science is much weirder than you might imagine.