AssessmentsBacillus cereus (Food poisoning)
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 26-year-old female presents to the emergency department with a sudden onset of extreme nausea, vomiting, and mild watery diarrhea. She ate multiple plates of fried rice at a Chinese buffet a few hours ago. Stool analysis is still pending. Which of the following is most likely the offending organism?
So Bacillus Cereus refers to a rod-shaped bacteria that looks like a wax-candle.
Most often, this bacteria causes food poisoning, but it can also cause more serious infections, mainly in immunocompromised individuals.
B. cereus has a thick peptidoglycan cell wall, which takes in purple dye when Gram stained - so this is a gram-positive bacteria.
Also, it is a motile bacteria, as it has thread-like flagella that can help it swim in a liquid media, or swarm on a solid surface such as culture media.
It’s also aerobic, meaning it requires oxygen for growth.
B. cereus is catalase positive, meaning it produces catalase enzyme.
This can be tested by adding a few drops of hydrogen peroxide to a colony of bacteria, and catalase makes hydrogen peroxide dissociate into water and oxygen, making the mixture foam.
It is also a beta-hemolytic bacteria, because when cultivated on a medium called blood agar, B. cereus colonies cause beta-hemolysis, also called complete hemolysis.
That’s because B. cereus makes a toxin called beta-hemolysin, that causes complete lysis of the hemoglobin in the red blood cells, making the blood agar change color from red to transparent yellow around the colonies
Finally, Bacillus Cereus is a spore-forming bacteria, so it can undergo endosporulation when it feels threatened by the environment, like when the temperature becomes too high or too low, in case of extreme dryness, or when there’s harmful radiation around.
Endosporulation means that the bacteria starts by replicating its DNA, and then it forms a wall inside the cell, isolating a portion from the rest of the cell - let’s call it the mother cell.
Next, the plasma membrane of the cell surrounds the mother cell and then pinches it off, forming a separate body known as a forespore.
The forespore then invaginates into the mother cell and gets completely engulfed by it.
Inside the dying mother cell, the forespore loses water and accumulates calcium, and at the same time gets wrapped in a super tough cortex from the dying mother cell.
At this point, the endospore is able to resist heat, harsh chemicals, digestive enzymes, and even antibiotics.
Finally, as the mother cell dies off, the endospore is released outside. Surprisingly, an endospore can last over a thousand years out, waiting for favorable conditions to come, and then germinate into the bacterial, or vegetative form, which can then grow and divide.
So these endospores can also contaminate food like rice or pasta, and some of them germinate into vegetative bacterial cells, which then start producing a toxin called cereulide within food.
Cereulide is a toxin known to cause vomiting when ingested, so it causes an emetic type of food-intoxication.
Cereulide is also heat resistant, so even when food is cooked or reheated, the toxin is there to stay.
If someone eats that cereulide-contaminated food and gets them into their gut, cereulide bind to the gastrointestinal mucosal chemoreceptors, and then excessively stimulate the vagus nerve, thus activating an exaggerated vomiting reflex.
Aside from cereulide production, the endospores themselves reach the small intestine, where they germinate into the vegetative form.
Here, they produce other toxins, called enterotoxins because they affect the intestinal epithelium.
There are three types of enterotoxins: hemolysin BL, nonhemolytic enterotoxin, and cytotoxin K.
All of these enterotoxins destroy the intestinal epithelium, triggering secretion of electrolytes, like sodium and potassium ions, and water into the intestinal lumen, resulting in watery diarrhea.