Have you ever heard that it’s not good to leave cooked rice at room temperature and then reheat it? You might have wondered why, since we often talk about food safety only in relation to animal products. However, leaving rice out at room temperature is bad because it can cause reheated rice syndrome (also known as fried rice syndrome). This doesn't mean you can’t eat leftover rice; you just need to store it correctly, which means putting it in the fridge as soon as it is cool. (For information on how to properly reheat your rice, check out The Kitchn.com.) Now, why are we talking about reheated rice syndrome? It’s because … you guessed it, reheated rice syndrome is caused by a microbe! Specifically, the bacterium Bacillus cereus.
What is Bacillus cereus? Bacillus cereus is a Gram-positive bacterium typically found in soil. Because it lives in the soil, it can end up on various foods, including starchy foods like rice, dairy products, meat, and vegetables grown in soil. Bacillus cereus is generally known as a pathogen that causes food poisoning; however, certain strains of the bacterium can act as probiotics, aiding digestion in humans and animals. Depending on the strain of B. cereus, the bacterium is either aerobic (consumes oxygen) or facultatively anaerobic (can consume oxygen if present but doesn’t need to).
B. cereus growing on a blood agar plate. (Image Source)
Why does heat not kill Bacillus cereus?
At this point, you are probably wondering why reheating the food to a high temperature doesn’t just kill all the bacteria. The secret to that is the power of the SPORE. Bacterial spores are a bacterium’s way of protecting itself from harsh environments. Spores are the most dormant form of bacteria, which means they don’t need as many resources to stay alive. It’s kind of like how some animals hibernate through the winter to survive. The spore form is produced by the bacteria and remains in the spore form until conditions improve, such as when it finds itself in your digestive tract.
Bacillus cereus and other Gram-positive bacteria form endospores, which are spores created inside the mother cell and have thick walls that protect against environmental stresses. These spores live on the rice (and other foods) and can survive the high temperatures of reheating. Once the spores are in your digestive tract, they can infect the cells lining the digestive tract and begin producing toxins.
Detailed illustration of Bacillus cereus bacteria spores on rice grains, showing growth stages with labeled elements on a bright background. (Image Source)
What is an enterotoxin?
The specific type of toxins Bacillus cereus produces are enterotoxins. Enterotoxins are protein toxins that are released by a microorganism and are heat-resistant and target the intestines. Depending on the strain, these toxins cause either diarrhea or vomiting.
There are 3 main diarrheal enterotoxins produced by the diarrhea-causing Bacillus cereus strain: hemolysin BL (Hbl), non-hemolytic enterotoxin (Nhe), and cytotoxin K (CytK). All of these toxins are synthesized by the ribosome and act by forming pores (transport holes) in the host cell membrane. Specifically, these toxins create pores that cause potassium to suddenly leave the cell, leading to rapid cell death. The dying host cells would be your intestinal lining cells, causing digestive tract symptoms.
There is 1 emetic toxin, Cereulide, which is not produced by the ribosome and damages the mitochondrial membrane by promoting potassium movement from the outer membrane to the inner membrane. In the most severe cases, Cereulide can cause damage to other organs, including the liver, in some rare cases, leading to liver failure and even death.
Pore-forming toxin (PFT) attaches to a receptor in the cell membrane and creates a pore through which potassium ions can flow. (Image Source)
Extracellular vesicles
Imagine if bacteria had their own version of Amazon Prime, a system that ships packages directly to target cells, packed with exactly the right cargo to cause maximum damage. That's essentially what extracellular vesicles (EVs) are, and they’re way cooler than you might think.
Extracellular vesicles are nanoscale spheres wrapped in a lipid bilayer membrane, kind of like an even smaller cell. Bacteria across all domains of life release these as a sophisticated communication and transport system, enabling what researchers call “cell-to-cell communication” and targeted delivery of virulence factors. Think of them as molecular delivery trucks used to ship cargo between cells, only instead of fun online purchases people have made, they’re loaded with virulence factors and toxins.
Here’s where it really gets interesting: by packaging toxins inside EVs, bacteria can gain some serious advantages. First: the membrane protects the cargo from harsh environments like your stomach acid. Second: EVs can carry multiple different toxins simultaneously, allowing them to work together in a coordinated attack. Third: EVs can deliver their contents directly into host cells, bypassing many immune defenses. Think of the EVs as Trojan horses, disguising the toxins so they don’t appear to be threats. It’s bacterial warfare at its finest, and B. cereus has mastered the art.
How does B. cereus deliver its toxin?
Here’s the puzzle that stumped researchers for years: B. cereus produces a deadly three-part toxin called Nhe, consisting of three separate protein components: NheA, NheB, and NheC. All three of these pieces have to come together at the right time and place to punch holes in your intestinal cells, causing diarrhea and vomiting associated with food poisoning. Unfortunately, previous studies have only examined purified versions of these toxins in test tubes. That's like studying how a car works by looking at all the parts laid out on the garage floor; you're missing the crucial fact of how everything is assembled and works together in real use. We needed to look at how the bacterium naturally delivers all three components together to the human cell.
Enter the Buchacher et al. research team, who asked exactly this question. They had a hunch, what if B. cereus leaves nothing about the delivery up to chance? What if it uses EVs as pre-packaged delivery vehicles, loading all three Nhe components onto the same molecular truck, shipping them off together to target cells like a cellular express delivery system? Their study set out to test this hypothesis using a combination of advanced proteomics (to identify what proteins are inside the EVs), super-resolution microscopy (to see where the toxins actually go), and cell culture experiments (to prove the EVs can, in fact, kill human intestinal cells).
What is inside the extracellular vesicles?
With the help of a special microscope, you, just like the researchers, can visualize exactly what is inside the extracellular vesicles. Figure 3 in Buchacher et al. contains multiple visualizations. Let's focus first on the images of part B, which uses 3D-SIM microscopy to view what is showing up in the EVs. In the first image, shown in green, is the presence of the NheB toxin component. In the second image, shown in red, is the presence of the NheC toxin component. The third image shows both overlaid on top of each other. These images demonstrate that both enterotoxins NheB and NheC are present in the EVs.
The other key takeaway is from Parts C and D of this figure, which explore how the enterotoxins reach the intestinal (Caco2) cells. To study this, they treated Caco2 cells with B. cereus EVs and viewed the toxins with microscopy. The toxins can be observed on intestinal cells, individually and in combination, suggesting that treatment with EVs leads to the localization of toxins (NheA and NheC) on intestinal cells.
Fig 3. (Image Source)
Extracellular vesicles as a toxin delivery service
So in the end, what did Buchacher’s team discover? Their findings were both fascinating and terrifying. They found that B. cereus does, as they suspected, pack all three Nhe components into EVs, along with an additional enzyme called sphingomyelinase (SMase). But here’s where the real breakthrough comes: the finding that solved a mystery—NheC is found only inside EVs, not floating freely in the surrounding medium. For years, researchers couldn’t figure out how intestinal cells could possibly be exposed to enough NheC to trigger the full toxic effect. It seemed to be in such low concentrations that it shouldn’t have been causing the amount of harm it actually caused. Turns out, the bacteria weren’t freely releasing it at all; they were only sending it via express delivery in the EVs, hiding it from the body’s immune system until it had already entered the cell.
Even better (or worse, depending on whose side you're on), SMase and the Nhe components work together synergistically. SMase damages cell membranes by breaking down lipids, essentially softening the target cell, making it easier for the Nhe toxin to do its job. Together, they cause far more damage than either could alone. SMase is almost like the delivery man, showing up nicely to get you to open the door for the toxin hidden in the package. This shows that the bacteria aren’t just randomly spewing toxins in the human body and hoping for the best; they’re strategic, packaging their weapons in carefully planned combinations for maximum effect. B. cereus has perfected its own express delivery system of destruction for the human intestinal system.
Future implications
Now you know the truth: B. cereus isn’t leaving toxin delivery to chance; it's weaponizing EVs with precision. That leftover rice sitting on your stovetop? It’s the perfect breeding ground for the coordinated attack. Understanding the frightening reality of this delivery system shows why you really do need to be careful to put away your food in a timely manner. Those food safety guidelines aren’t arbitrary; they’re your defense against this bacterium’s sophisticated system to make you sick. This mechanistic understanding also opens the door to exciting new research possibilities—researchers are now exploring ways to block this EV formation and neutralize this threat. Store your food wisely and stop the delivery truck before it reaches your cells.
About the Authors:
Maya Goldwasser ‘26 is a gender studies major and chemistry minor. She has completed a 5-college certificate in Reproductive Health, Rights, and Justice. She is from St. Louis, Missouri, and after graduation, she will be starting medical school this summer at Saint Louis University. She hopes to work in pediatrics or OBGYN in the future. In her free time, she enjoys cooking and baking (including using her sourdough starter, which she gram-stained and observed under the microscope during lab). She also enjoys playing board games and hiking with friends and family.
Magdalena Wennberg ‘26 is a philosophy and biology double major from upstate NY. After graduation she will be working on veterinary school applications and hopes to eventually work with both small animals and livestock. Some of her favorite things include coffee, animals (especially cows), the beach, and anything pink and sparkly!
No comments:
Post a Comment