Bacillus cereus (Food poisoning)

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Bacillus cereus (Food poisoning)


Introduction to bacteria

Bacterial structure and functions


Bacillus cereus (Food poisoning)


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Bacillus cereus (Food poisoning)

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USMLE® Step 1 style questions USMLE

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A microbiologist is studying the case report of a young woman who developed sudden-onset emesis and cramping abdominal pain 30 minutes after eating. The patient did not have diarrhea. Which of the following foods is most likely implicated in this case?  

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Bacillus cereus p. , 136

food poisoning p. 175, 727

spore formation p. 129


Bacillus cereus p. , 136

Food poisoning

Bacillus cereus p. , 136


With Bacillus Cereus or just B. Cereus, “bacillus” means little rod, and “cereus” means wax.

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.


Bacillus cereus is a rod-shaped, spore-producing, gram-positive bacterium that's known to cause food poisoning. It can contaminate a variety of foods, including cooked rice, pasta, and potato salad. As a spore-forming bacteria, Bacillus cereus can survive for long periods outside of the body. Symptoms include profuse watery diarrhea, nausea, vomiting, and abdominal cramping. Symptoms usually start within 6 to 15 hours after ingesting contaminated food.


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