Bacillus cereus (Food poisoning)

Definitions & Key takeaways

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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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.
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.
Finally, B. cereus can also contaminate medical devices such as needles and prosthetic devices like central lines and indwelling catheters.
From such devices, bacteria can get in the bloodstream causing bacteremia. From there, they can spread to other organs and cause infection, mostly in immunocompromised patients such as those with an underlying malignancy, or in newborns who have an immature immune system.
If they spread to the heart, they cause endocarditis; if they get to the bones, they cause osteomyelitis; and if they make it all the way to the central nervous system, they may cause meningoencephalitis.
Finally, if B. cereus can also spread to the eyes, causing endophthalmitis, when they get to the interior of the eye, or keratitis, when they invade the cornea.
Symptoms of B cereus food intoxication depend on the type of infection. With the emetic type, there’s nausea, vomiting and abdominal cramps, and all of these classically start 1 to 5 hours after someone has eaten cereulide contaminated-food.
With the diarrhea type, there are moderate to severe abdominal cramps and it typically starts within 6 to 18 hours after ingestion of the endospores.
With bacteremia, there are signs of systemic infection like fever, chills, malaise. Additionally, with endocarditis there may be night sweat and new cardiac murmurs, with osteomyelitis there’s bone pain and swelling, and with meningoencephalitis there is headache, confusion and stiff neck.
Finally, with endophthalmitis there can be a severe eye pain and swelling, and the eye can even bulge out of its socket - which is called proptosis.
With keratitis, there may be impaired vision. Diagnosis of B.
cereus food poisoning is done by isolating bacteria from the vomitus or stool. The presence of B.
cereus is confirmed only if there are more than 10 to the fifth power colony forming units per gram of biological sample, or if more than 10 to the fifth power colony forming units per gram are isolated from possibly contaminated food like rice or pasta.
Another way for testing for B. cereus is to look for its toxins in the food, using assays such as latex agglutination tests, immunochromatography, and testing for the presence of the bacteria’s genetic material using polymerase chain reaction - or PCR.
When a systemic infection is suspected, blood cultures are done to identify the bacteria in the blood. Treatment of B.
cereus infections usually consists of only supportive treatment for foodborne illnesses. With invasive infections like bacteremia, treatment is done by first checking if there is an indwelling medical device suspected to be the source of the infection, and then that device is removed or replaced.
Next, antibiotic therapy is initiated, and vancomycin as the drug of choice, but imipenem and gentamicin can be alternatively used.
Alright, as a quick recap… Bacillus Cereus is a rod shaped, gram-positive, beta-hemolytic, catalase-positive, spore-forming aerobic bacteria.
It produces a toxin called cereulide which causes the emetic form of food intoxication, and three enterotoxins, called hemolysin BL, nonhemolytic enterotoxin, and cytotoxin K which cause the diarrhea form of food-intoxication.
Sometimes this bacteria can contaminate medical devices, and cause bacteremia and a number of infections to immunocompromised people, such as endocarditis, osteomyelitis, meningoencephalitis and eye infections.
Diagnosis of B. cereus infections relies on identifying the bacteria in biological samples, or using assays to test for the presence of its toxins.
Treating B. cereus infections mainly consists of supportive care in case of food-intoxication, but for systemic infections, antibiotics like vancomycin are used.