Bacillus cereus, Gram stain of bacillus cereus

Gram stain of bacillus cereus

Habitat of Bacillus cereus

  1. Bacillus cereus is isolated from soil, vegetables, milk, cereals, spices, fried rice, boiled poultry and meat, soups, and desserts.
  2. It is also found in mashed potatoes, beef stew, apples, hot chocolate sold in vending machines, and other areas of improper food handling.
  3. In 1887 B. cereus was isolated from the air in a stall in Frankland and Frankland.
  4. It is a saprophytic type of Bacillus.
  5. Vegetative cells and spores are widespread.
  6. They are opportunistic pathogens for immunocompromised patients and sometimes also human pathogens.
  7. It is not considered to be normal human flora, but it can temporarily colonize the skin or the gastrointestinal or respiratory tract.
  8. Endospores show much greater resistance to physical and chemical agents such as heat, cold, dehydration, radiation, disinfection, antibiotics, and other toxins.
  9. It can grow in the presence or absence of oxygen.
  10. It interacts with other microorganisms in the rhizosphere, the region that surrounds the roots of plants.
  11. It is also found in the intestine of the invertebrate microflora and is an arthropod intestinal symbiont, where it shows filamentous growth in mealybugs, cockroaches, and termites.
  12. Alcohol does not kill Bacillus cereus easily; In fact, they are known to colonize distilled spirits and alcohol-soaked wipes and wipes in amounts sufficient to cause infection.
  13. The presence of B. cereus in a patient’s stool is not sufficient to make a diagnosis of B. cereus because the bacteria can be present in normal stool samples; a concentration of 105 bacteria or more per gram of food is considered diagnostic.
  • Bacillus cereus is a rod-shaped, gram-positive rod with square ends.
  • Occasionally, gram-variable or even gram-negative can occur with increasing age.
  • They are in the form of a single rod or appear in short chains.
  • Well-defined connections between chain links are easily visible.
  • The color of the tissue section can appear long and stringy.
  • They are straight or slightly curved.
  • They are not encapsulated.
  • Contains spores with central spores.
  • The spores are oval (ellipsoidal) and do not swell the stem cell or form in the animal’s blood or tissue or in aerobic cultures.
  • It is 1 × 3-4 µm in size.
  • It is mobile and flagellated with pediatric flagella.
  • It is mobile through two types of motility, swimming and swarming.
  • Endospores can survive long periods of exposure to air and other adverse environmental conditions.
  • It is a beta-hemolytic bacterium that causes foodborne diseases.
  • Its virulence factors include aerolysin and phospholipase.

Genome Structure

  1. The circular chromosome is 5,411,809 nt long.
  2. It contains 5,481 genes, 5,234 coding proteins, 147 structural RNA, and 5,366 RNA operons.
  3. The plasmid is 5 to 500 kb in size.

Cultural Characteristics of Bacillus cereus

  • Most of the Bacillus spp. grow easily on nutrient agar or peptone medium.
  • The optimal temperature for growth ranges from 20 ° C to 40 ° C, mainly 37 ° C.
  • B. cereus is mesophilic and can adapt to a wide variety of environmental conditions.
  • On nutrient agar at 37 ° C, it forms large (2-5 mm) gray-white granular colonies with a less wavy edge and less membranous consistency.
  • On 5% sheep blood agar at 37 ° C, B. cereus colonies are large, pinnate, opaque, gray, granular, spreading, opaque colonies with a rough, matt surface and irregular circumference.
  • It is beta-hemolytic on blood agar.
  • Colony sizes are irregular and represent the configuration of the swarm from the site of initial inoculation, possibly due to the motility of B. cereus.
  • In some cases, smooth colonies develop alone or in the middle of rough colonies.
  • When grown outside of the initial inoculum, smooth colonies are surrounded by a uniform zone of beta hemolysis that frames the centrally located colony.
  • MYP agar has been the standard medium for plating B. cereus but has poor selectivity such that the background flora is not inhibited and can mask the presence of B. cereus. B. cereus colonies are usually lecithinase positive and mannitol negative on MYP agar.
  • Bacara is a selective and differential chromogenic agar that promotes the growth and identification of B. cereus but inhibits the growth of the background flora. Bacillus cereus colonies turn orange-pink with a dull halo.
  • Chromogenic agar for the enumeration of the B. cereus group has been suggested as a substitute for MYP. Typical colonies grow as uniform orange-pink colonies surrounded by a precipitation zone.

Pathogenicity and Virulence factors of Bacillus cereus

  1. B. cereus is most commonly associated with food poisoning, but the organism can also cause post-traumatic ophthalmitis, which requires rapid and aggressive local treatment.
  2. The organism is widespread in the environment and is found in most raw foods, especially grains such as rice.
  3. Gastroenteritis caused by B. cereus is mediated by one of two enterotoxins.
  4. The heat-stable and proteolysis-resistant enterotoxin cause the emetic form of the disease. The mechanism of action of heat-stable enterotoxin is unknown.
  5. The emetic form manifests itself in nausea, vomiting, abdominal cramps, and occasionally diarrhea, and is self-limiting, with recovery occurring within 24 hours.
  6. Heat-labile enterotoxin causes the diarrhea form of the disease; Each stimulates the adenylate cyclase-adenosine monophosphate system in intestinal epithelial cells, causing severe watery diarrhea.
  7. The form of diarrhea has an incubation time of 1 to 24 hours and manifests itself as severe diarrhea with abdominal pain and cramps; Fever and vomiting are rare.

B. cereus is the Bacillus species most common in opportunistic infections, including post-traumatic eye infections, endocarditis, and bacteremia. The pathogenesis of B. cereus eye infections is also not fully understood.

At least three toxins are involved; they are necrotic toxin (a heat-labile enterotoxin), cytolysin, and phospholipase C (a strong lecithinase).

  1. The rapid destruction of the eye characteristic of B. cereus infections is likely the result of the interaction of these toxins and other unidentified factors. Bacillus species can temporarily colonize the skin and be found as insignificant contaminants in blood cultures.
  2. However, in the presence of an intravascular foreign body, these organisms may be responsible for persistent bacteremia and signs of sepsis (i.e. fever, chills, hypotension, shock).
  3. Infections from other sites are rare and generally affect intravenous drug users or immunocompromised patients.

Clinical manifestation of Bacillus cereus

Food poisoning

Two forms of food poisoning: vomiting (vomiting form) and diarrheal diseases (diarrhea form).

Emetic form

The emetic form of the disease arises from eating contaminated rice. Most of the bacteria are killed when the rice is first cooked, but the heat-resistant spores survive. If cooked rice is not refrigerated, spores germinate and bacteria can multiply quickly. The heat-stable enterotoxin released is not destroyed when the rice is reheated. The emetic form of the disease is poisoning caused by ingestion of the enterotoxin, not the bacteria. Therefore, the incubation period after eating the contaminated rice is short (1 to 6 hours) and the duration of the illness is also short (less than 24 hours). Symptoms include vomiting, nausea, and abdominal cramps. Usually, there is no fever or diarrhea.

Form of diarrhea

The diarrhea form of B. cereus food poisoning is a real infection that results from ingestion of the bacteria in contaminated meat, vegetables, or sauces. There is a long incubation period during which the organism in the patient’s intestinal tract reproduces and follows the release of the heat-labile enterotoxin. This enterotoxin is responsible for severe watery diarrhea, nausea, and abdominal cramps.

B. Eye Infections

B. cereus eye infections usually occur after penetrating traumatic injury to the eye with an object contaminated with dirt.

These include severe keratitis and endophthalmitis.

C. Other infections

B. cereus has also been implicated in localized infections such as wound infections and systemic infections including endocarditis, catheter-associated bacteremia, central nervous system infections, osteomyelitis, and pneumonia.

The presence of a medical device or the use of intravenous drugs predisposes to these infections.

Laboratory diagnostics of Bacillus cereus

SPECIMENS: Faeces, vomitus, remaining food (if any), eye specimen (corneal swab)


After Gram staining, organisms appear microscopically as large Gram-positive rods in singles, pairs, or serpentines with square ends.

Endospore formation is seen as an unstained round or oval area in the center of the cell. The spores are oval (ellipsoid) and do not swell out of the stem cell.


  1. Growth on 5% sheep blood agar, chocolate agar, routine blood culture media, and nutrient broths.
  2. Growth detectable within 24 hours after incubation in media that have been incubated at 35 ° C, in-room air, or in 5% carbon dioxide (CO2).
  3. Colony character on blood agar: Large, pinnate, spreading, opaque, gray, granular, spreading, and opaque colonies with a rough, matted surface and irregular, beta-hemolytic perimeters.
  4. Bacillus cereus can be isolated from stool with selective media such as MYPA (mannitol, egg yolk, polymyxin, phenol red, and agar), PEMBA (polymyxin, egg yolk, mannitol, bromothymol, blue agar).
  5. These media take advantage of the positive phospholipase C reaction in egg yolk agar, the lack of acid production from mannitol, and the incorporation of pyruvate or polymyxin as selective agents.


Catalase: positive

Oxidase: negative

OF test: fermentative

Indole: negative

Methyl red: positive

Vogues proskauer: positive

Glucose: fermentative, production of acid

Sucrose: fermentative, production of acid

Lactose: no fermentation

Starch hydrolysis: positive

Nitrate reduction: positive

Gelatin hydrolysis: positive

Spore staining: endospore-forming bacteria

Motility: motile


  1. Serologic methods are available for the detection of B. cereus toxin in food and feces.
  2. The micro slide gel diffusion test is generally used as a toxin detection system.


The toxigenic potential of B. cereus isolates, genes encoding emetic-toxin cereulide (ces), and enterotoxins (nhe, hbl and cytK) can be analyzed by multiplex PCR.

Bacillus cereus treatment

People with B. cereus food poisoning only require supportive treatment.

Oral rehydration or, occasionally, intravenous fluid and electrolyte replacement is indicated in severely dehydrated patients. Antibiotics are not indicated.

Patients with the invasive disease require antibiotic therapy. Bacillus cereus is susceptible to clindamycin, erythromycin, vancomycin, aminoglycosides, and tetracycline. It is resistant to penicillin and trimethoprim.

Prevention and control of Bacillus cereus

Diarrheal poisoning and vomiting produced by this organism can be easily prevented through proper food handling procedures.

Meat and vegetables should not be kept at temperatures between 10 and 45 ° C for long periods, and rice that is kept overnight after cooking should be refrigerated and not kept at room temperature.

Prevention of infection in patients after surgery or in those who are immunosuppressed or predisposed to infection depends on good

Bacillus cereus, Gram stain of bacillus cereus

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