Bacillus cereus- An Overview
Bacillus cereus (B. cereus) is a type of bacteria that can cause food poisoning and other infections in humans. It is widely distributed in nature and can be found in soil, water, plants, dust, and food products. B. cereus belongs to a group of closely related bacteria that also includes Bacillus anthracis (the cause of anthrax) and Bacillus thuringiensis (a biopesticide).
B. cereus is a gram-positive, rod-shaped, spore-forming bacterium that can grow in aerobic or anaerobic conditions. It can produce toxins that are either heat-stable or heat-labile, depending on the type of food poisoning it causes. B. cereus can also produce enzymes that degrade various biological molecules, such as phospholipase C, lecithinase, hemolysin, and protease.
B. cereus can cause two types of food poisoning: the emetic (vomiting) syndrome and the diarrheal syndrome. The emetic syndrome is caused by ingesting a preformed toxin called cereulide that is produced by B. cereus in certain foods, such as rice, pasta, and pastry. The symptoms include nausea, vomiting, and abdominal cramps within 1 to 6 hours after eating the contaminated food. The diarrheal syndrome is caused by ingesting B. cereus cells or spores that produce a heat-labile enterotoxin in the small intestine. The symptoms include watery diarrhea, abdominal pain, and sometimes fever within 6 to 15 hours after eating the contaminated food.
B. cereus can also cause non-intestinal infections, such as eye infections (especially after traumatic injuries), wound infections, respiratory infections, blood infections, endocarditis (infection of the heart valves), meningitis (infection of the brain and spinal cord), osteomyelitis (infection of the bone), and pneumonia. These infections are more common in people who have a weakened immune system or who have other risk factors, such as intravenous drug use, surgery, trauma, or medical devices.
B. cereus infections are diagnosed by isolating and identifying the bacterium from clinical specimens, such as stool, vomit, blood, eye swabs, or wound swabs. The presence of toxins or genes encoding toxins can also be detected by various methods, such as gel diffusion test or polymerase chain reaction (PCR).
B. cereus infections are usually self-limiting and do not require specific treatment. However, some cases may require supportive care, such as fluid and electrolyte replacement for dehydration or antibiotic therapy for severe or systemic infections. B. cereus is generally susceptible to antibiotics such as clindamycin, erythromycin, vancomycin, aminoglycosides, and tetracycline, but resistant to penicillin and trimethoprim.
B. cereus infections can be prevented by proper food handling and storage practices, such as cooking food thoroughly, refrigerating leftovers promptly, reheating food adequately before consumption, and avoiding cross-contamination between raw and cooked foods. Prevention of non-intestinal infections depends on good hygiene and wound care practices, as well as avoiding exposure to contaminated soil or dust.
Bacillus cereus is a ubiquitous bacterium that can be found in various environments and sources. Some of the common habitats of B. cereus are:
- Soil: Soil has long been considered to be the natural habitat of this species, as it can form spores that are resistant to environmental stress and can germinate when conditions are favorable. B. cereus can interact with other microorganisms and plants in the soil, and can also be transferred to other habitats through soil contamination.
- Food: B. cereus is a common foodborne pathogen that can contaminate various raw and processed foods, such as vegetables, cereals, rice, meat, poultry, milk, cheese, spices, soups, desserts, and beverages. B. cereus can grow rapidly in foods that are improperly stored or handled, especially at temperatures between 10°C and 45°C, and can produce toxins that cause food poisoning. B. cereus can also survive pasteurization and cooking processes due to its spore-forming ability.
- Plants: B. cereus can colonize the rhizosphere, the region surrounding plant roots, and can act as a plant growth-promoting rhizobacterium (PGPR) or a plant pathogen depending on the strain and the plant species. B. cereus can also be isolated from plant surfaces, such as leaves and fruits, and can cause spoilage or decay.
- Animals: B. cereus can be found in the gut microflora of various invertebrates, such as insects, arthropods, nematodes, and sponges, where it can exhibit mutualism or parasitism depending on the host and the strain. B. cereus can also be transiently present on the skin or the gastrointestinal or respiratory tracts of humans and other vertebrates, where it can cause opportunistic infections or be harmless commensals.
- Other materials: B. cereus spores can be isolated from various materials that are exposed to air or soil contamination, such as dust, air filters, medical devices, distilled liquors, alcohol-soaked swabs and pads, cosmetics, and pharmaceutical products. B. cereus can cause contamination or infection problems in these materials due to its spore resistance and toxin production.
Bacillus cereus is a bacterium that has a rod-shaped or bacillus morphology. It has a Gram-positive cell wall, which means that it retains the purple color of the crystal violet stain after the Gram staining procedure. However, some strains may appear Gram-variable or even Gram-negative with age.
Bacillus cereus can be seen as single rods or in short chains with clear-cut junctions between the cells. The rods are straight or slightly curved, with square or rounded ends. They are usually 1×3-4 µm in size.
Bacillus cereus is non-capsulated, meaning that it does not have a polysaccharide layer outside the cell wall. However, it can produce endospores, which are dormant and resistant forms of the bacteria that can survive harsh environmental conditions. The endospores are oval or ellipsoidal and located in the center of the cell. They do not swell or distort the shape of the cell. The endospores are not formed in animal blood or tissues or in aerobic culture.
Bacillus cereus is motile and flagellated, meaning that it can move by using whip-like appendages called flagella. It has peritrichous flagella, which are distributed all over the surface of the cell. It can exhibit two types of motility: swimming and swarming. Swimming motility is the movement of individual cells in liquid media, while swarming motility is the coordinated movement of groups of cells on solid surfaces.
Bacillus cereus is a beta-hemolytic bacterium, meaning that it can lyse red blood cells and produce a clear zone of hemolysis on blood agar plates. It also has various virulence factors, such as phospholipase C, cereulide, sphingomyelinase, metalloproteases, and cytotoxin K, that contribute to its pathogenicity and cause food poisoning and other infections in humans and animals.
Bacillus cereus has a circular chromosome with 5,411,809 nt in length . It contains 5481 genes, 5234 proteins coding, 147 structural RNA, and 5366 RNA operons . The genome of B. cereus is closely related to Bacillus anthracis, the cause of anthrax, and Bacillus thuringiensis, an insect pathogen used as pesticide . However, B. cereus can be distinguished from these species by several genomic features, such as the presence of flagellin gene (although nonfunctional), the absence of pathogenicity island containing anthrax toxin genes, and the variation in plasmid content .
B. cereus can harbor different plasmids that range from 5 to 500 kb in size . Some of these plasmids are involved in the production of toxins or other virulence factors that contribute to the pathogenicity of B. cereus . For example, B. cereus ATCC 10987 contains a single large plasmid (pBc10987), of ∼208 kb, that is similar in gene content and organization to B.anthracis pXO1 but is lacking the pathogenicity-associated island containing the anthrax lethal and edema toxin complex genes . Another example is B. cereus ATCC 14579, which contains three plasmids: pBClin15 (14.6 kb), pCER270 (270 kb), and pBCE4810 (208 kb). The latter two plasmids carry genes encoding enterotoxins (nhe, hbl, and cytK) and emetic toxin (ces) .
The genome structure of B. cereus provides insights into its metabolic capabilities, ecological adaptations, and evolutionary relationships with other Bacillus species. It also helps to identify potential targets for diagnosis, treatment, and prevention of B. cereus infections.
Bacillus cereus is a mesophilic bacterium that can grow in a wide range of environmental conditions. It can grow either aerobically or anaerobically, and it is motile by peritrichous flagella. It forms endospores that are resistant to heat, desiccation, radiation, and disinfectants.
Bacillus cereus can grow on various media, such as nutrient agar, blood agar, chocolate agar, and selective or differential media. The optimum temperature for growth is 37°C, but it can also grow at temperatures ranging from 10°C to 50°C.
On nutrient agar, B. cereus forms large (2-5 mm), grey-white, granular colonies with a less wavy edge and less membranous consistency. On blood agar, it forms large, feathery, spreading colonies that are beta-hemolytic, meaning that they produce a clear zone of lysis around the colonies.
One of the selective and differential media used for B. cereus isolation is mannitol egg yolk polymyxin agar (MYP agar). This medium contains mannitol as a carbohydrate source, egg yolk as a substrate for lecithinase production, polymyxin as a selective agent against gram-negative bacteria, and phenol red as a pH indicator. B. cereus colonies on MYP agar are usually pink and surrounded by a white precipitate zone due to lecithinase activity. They also do not produce acid from mannitol, which differentiates them from other Bacillus species.
Another selective and differential medium used for B. cereus isolation is Bacara agar, which is a chromogenic medium that contains substrates for phospholipase C and beta-glucosidase enzymes. B. cereus colonies on Bacara agar are pink-orange with an opaque halo due to phospholipase C activity. They also produce beta-glucosidase, which cleaves a chromogenic substrate and produces a blue-green color in the medium.
Bacillus cereus can also be identified by biochemical tests, such as catalase, oxidase, indole, methyl red, Voges-Proskauer, starch hydrolysis, nitrate reduction, gelatin hydrolysis, and carbohydrate fermentation tests.
Bacillus cereus is a gram-positive, rod-shaped, spore-forming bacterium that can be identified by various biochemical tests. Some of the common biochemical characteristics of B. cereus are:
- Catalase positive: It produces bubbles when exposed to hydrogen peroxide.
- Citrate positive: It utilizes citrate as a sole carbon source and turns the medium blue.
- Gelatin hydrolysis negative: It does not liquefy gelatin in the presence of gelatinase enzyme.
- Oxidase negative: It does not produce cytochrome oxidase enzyme and does not turn the indicator dark blue or purple.
- Indole negative: It does not produce indole from tryptophan and does not turn the indicator red.
- Methyl red negative: It does not produce mixed acids from glucose fermentation and does not lower the pH of the medium below 4.4.
- Voges-Proskauer positive: It produces acetoin from glucose fermentation and turns the indicator red.
- Nitrate reduction variable: It may or may not reduce nitrate to nitrite or nitrogen gas depending on the strain and conditions.
- Starch hydrolysis positive: It produces amylase enzyme and breaks down starch into glucose, turning the medium clear around the colonies after iodine addition.
- Lecithinase positive: It produces phospholipase C enzyme and hydrolyzes lecithin in egg yolk, causing a zone of opacity around the colonies.
- Hemolysis positive: It produces hemolysins that lyse red blood cells and cause a clear zone around the colonies on blood agar.
Bacillus cereus can also be differentiated from other Bacillus species by its ability to ferment or not ferment various carbohydrates. Some of the common carbohydrate fermentation reactions of B. cereus are:
- Glucose positive: It ferments glucose and produces acid.
- Sucrose variable: It may or may not ferment sucrose depending on the strain and conditions.
- Lactose negative: It does not ferment lactose and does not produce acid or gas.
- Mannitol negative: It does not ferment mannitol and does not produce acid or gas.
- Fructose positive: It ferments fructose and produces acid.
- Maltose positive: It ferments maltose and produces acid.
- Trehalose positive: It ferments trehalose and produces acid.
These biochemical characteristics can help in the identification and confirmation of B. cereus isolates from food, clinical, or environmental samples.
Bacillus cereus is a gram-positive, spore-forming bacterium that can cause food poisoning and various infections in humans and animals. The pathogenicity and virulence of B. cereus depend on the production of several toxins and enzymes that can damage the host cells and tissues, as well as evade the host immune system. Some of the main virulence factors of B. cereus are:
- Enterotoxins: These are proteins that can affect the intestinal epithelial cells and cause diarrhea or vomiting. B. cereus produces two types of enterotoxins: the heat-stable emetic toxin (cereulide) and the heat-labile diarrheal toxins (hemolysin BL or HBL, nonhemolytic enterotoxin or NHE, and cytotoxin K or CytK). Cereulide is a cyclic peptide that is preformed in contaminated food and can induce nausea and vomiting within 1 to 6 hours after ingestion. HBL, NHE, and CytK are tripartite toxins that are produced by the bacteria in the intestine and can cause watery diarrhea, abdominal cramps, and nausea within 8 to 16 hours after ingestion . These toxins act by forming pores in the plasma membrane of the intestinal cells, leading to potassium efflux, water secretion, and cell death.
- Hemolysins: These are proteins that can lyse red blood cells and other host cells by disrupting their membranes. B. cereus produces several hemolysins, such as hemolysin II (Hly-II), hemolysin III (Hly-III), sphingomyelinase (Sph), and cereolysin O (CLO). Hly-II is a pore-forming toxin that can also activate the NLRP3 inflammasome, a multiprotein complex that mediates inflammation and pyroptosis. Hly-III is a cholesterol-dependent cytolysin that can also damage endothelial cells and cause vascular leakage. Sph is a phospholipase that can degrade sphingomyelin, a major component of cell membranes. CLO is a thiol-activated toxin that can also induce apoptosis.
- Phospholipases: These are enzymes that can hydrolyze phospholipids, which are essential for the structure and function of cell membranes. B. cereus produces two phospholipases: phosphatidylinositol-specific phospholipase C (PI-PLC) and phosphatidylcholine-specific phospholipase C (PC-PLC). PI-PLC can cleave phosphatidylinositol, which is involved in signal transduction and membrane anchoring of proteins. PC-PLC can cleave phosphatidylcholine, which is the most abundant phospholipid in cell membranes. These phospholipases can cause membrane disruption, cell lysis, tissue necrosis, and inflammation.
- Other factors: B. cereus can also produce other factors that contribute to its virulence, such as metalloproteases, beta-lactamases, InhA1 protease, NprA protease, S-layer proteins, flagella, capsule, biofilm, and plasmids . Metalloproteases are enzymes that can degrade extracellular matrix proteins and modulate host immune responses. Beta-lactamases are enzymes that can degrade beta-lactam antibiotics and confer resistance to B. cereus. InhA1 protease is an enzyme that can degrade complement proteins and impair host innate immunity. NprA protease is an enzyme that can degrade fibrinogen and interfere with blood clotting. S-layer proteins are surface proteins that can mediate adhesion to host cells and protect B. cereus from phagocytosis. Flagella are appendages that can enable motility and chemotaxis of B. cereus. Capsule is a polysaccharide layer that can prevent dehydration and enhance survival of B. cereus in harsh environments. Biofilm is a complex community of bacteria embedded in a matrix of extracellular polymeric substances that can enhance resistance to antibiotics and host defenses. Plasmids are extrachromosomal DNA elements that can carry genes encoding for toxins, enzymes, antibiotic resistance, or other virulence factors.
In summary, B. cereus possesses a variety of virulence factors that enable it to cause food poisoning and infections in humans and animals. These factors include enterotoxins, hemolysins, phospholipases, and other factors that can damage host cells and tissues, as well as evade host immune system.
Bacillus cereus can cause different types of infections depending on the site of entry and the immune status of the host. The most common clinical manifestations are:
Food poisoning: This is caused by the ingestion of food contaminated with B. cereus toxins. There are two forms of food poisoning: emetic and diarrheal. The emetic form is characterized by nausea, vomiting, and abdominal cramps within 1 to 6 hours after eating contaminated food, especially rice. The diarrheal form is characterized by watery diarrhea, abdominal pain, and cramps within 6 to 15 hours after eating contaminated food, especially meat, vegetables, or sauces. Both forms are usually self-limiting and resolve within 24 hours.
Eye infections: B. cereus can cause severe eye infections, especially after traumatic injuries with soil-contaminated objects. The most common types of eye infections are keratitis and endophthalmitis. Keratitis is the inflammation of the cornea, which can lead to pain, redness, blurred vision, and sensitivity to light. Endophthalmitis is the inflammation of the inner eye structures, which can lead to severe vision loss or blindness. B. cereus eye infections are associated with rapid tissue destruction and poor prognosis .
Other infections: B. cereus can also cause localized or systemic infections in other parts of the body, such as wounds, respiratory tract, blood, heart, brain, bones, and lungs. These infections are more likely to occur in immunocompromised patients or those with medical devices or intravenous drug use. B. cereus can cause wound infections, bacteremia, endocarditis, meningitis, osteomyelitis, and pneumonia .
Bacillus cereus is a gram-positive rod-shaped bacterium that can cause food poisoning and other infections. Laboratory diagnosis of B. cereus is not usually done for food poisoning cases, as the symptoms are self-limiting and clinical. However, laboratory diagnosis may be necessary for severe or invasive infections, or for routine examination of foods to prevent contamination.
The laboratory diagnosis of B. cereus involves the following steps:
- Specimen collection: Depending on the type of infection, different specimens may be collected, such as feces, vomitus, remaining food, eye swab, wound swab, blood culture, or cerebrospinal fluid.
- Direct detection methods: The specimens are examined microscopically after Gram staining to look for large gram-positive rods with square ends, sometimes forming chains or serpentine arrangements. Spore formation may be seen as unstained oval regions within the cells. Spores are oval and do not swell the mother cell. B. cereus may appear gram-variable or gram-negative with age.
- Culture: The specimens are inoculated on various media, such as 5% sheep blood agar, chocolate agar, nutrient agar, or selective media such as MYP agar (mannitol, egg yolk, polymyxin, phenol red, and agar) or Bacara agar (a chromogenic medium). The media are incubated at 35°C in ambient air or 5% CO2 for 24 hours. B. cereus forms large, feathery, spreading, gray-white colonies with irregular edges and beta-hemolysis on blood agar. On MYP agar, B. cereus colonies are usually lecithinase-positive (forming a white precipitate around the colonies) and mannitol-negative (not changing the color of the medium). On Bacara agar, B. cereus colonies turn pink-orange with an opaque halo.
- Biochemical analysis: The colonies are further identified by performing various biochemical tests, such as catalase (positive), oxidase (negative), OF test (fermentative), indole (negative), methyl red (positive), Voges-Proskauer (positive), glucose fermentation (positive), sucrose fermentation (positive), lactose fermentation (negative), starch hydrolysis (positive), nitrate reduction (positive), gelatin hydrolysis (positive), spore staining (positive), and motility (positive).
- Serodiagnosis: Serologic methods are available for the detection of B. cereus toxins in food and feces. A microslide gel diffusion test can be used to detect the presence of enterotoxins or emetic toxins.
- Molecular methods: Molecular methods can be used to analyze the toxigenic potential of B. cereus isolates by detecting the genes encoding emetic toxin (ces) and enterotoxins (nhe, hbl, and cytK) by multiplex PCR.
The treatment of Bacillus cereus depends on the type and severity of the infection.
Most cases of food poisoning caused by B. cereus are mild and self-limiting, requiring only supportive care. This includes drinking plenty of fluids, resting, and taking over-the-counter medications for nausea, vomiting, or diarrhea if needed. Antibiotics are not indicated for food poisoning, as they do not affect the toxins produced by the bacteria .
However, some people may develop more severe or prolonged symptoms, especially if they have a weakened immune system or underlying medical conditions. These people may need hospitalization and intravenous fluids and electrolytes to prevent dehydration and shock. In rare cases, B. cereus food poisoning can lead to complications such as liver failure, kidney failure, or death .
B. cereus eye infections usually occur after a traumatic injury to the eye with a contaminated object. These infections can cause severe damage to the eye and vision loss if not treated promptly and aggressively. The treatment of B. cereus eye infections involves surgical removal of any foreign material from the eye, irrigation of the eye with antibiotics, and systemic antibiotics .
The choice of antibiotics depends on the susceptibility of the isolate and the patient`s allergies. B. cereus is usually susceptible to clindamycin, erythromycin, vancomycin, aminoglycosides, and tetracyclines. It is resistant to penicillin and trimethoprim . Ciprofloxacin has also been reported to be effective in some cases .
B. cereus can also cause various localized or systemic infections, such as wound infections, endocarditis, bacteremia, meningitis, osteomyelitis, and pneumonia. These infections are more common in immunocompromised patients, intravenous drug users, or patients with indwelling or implanted devices. The treatment of these infections requires antibiotic therapy based on the site and severity of infection and the susceptibility of the isolate .
In addition to antibiotics, surgical drainage or removal of infected tissue or devices may be necessary. Patients with systemic infections may also need supportive care such as fluids, oxygen, or vasopressors.
Bacillus cereus is a common cause of food poisoning and other infections that can be prevented by proper food handling and hygiene practices. Some of the prevention and control measures are:
- Cooking food thoroughly and keeping it at safe temperatures (above 60°C or below 5°C) to prevent the growth of bacteria and spores. Rice, pasta, potatoes, and other starchy foods are especially prone to B. cereus contamination and should be cooked well and not left at room temperature for long periods.
- Refrigerating or freezing leftovers as soon as possible and reheating them until steaming hot before eating. Discarding any food that has been left out for more than two hours or that smells or looks spoiled.
- Washing hands with soap and water before and after handling food, especially raw meat, poultry, fish, eggs, and dairy products. Also washing hands after using the toilet, changing diapers, touching animals, or handling garbage.
- Washing fruits and vegetables with running cold water and a brush, especially if they have been in contact with soil, dust, or animal feces. Peeling or cutting off bruised or damaged parts of fruits and vegetables.
- Using separate utensils, cutting boards, and plates for raw and cooked foods to avoid cross-contamination. Cleaning and sanitizing all kitchen surfaces and equipment after each use with hot water and detergent or bleach solution.
- Avoiding unpasteurized milk and dairy products, as well as homemade cheese, yogurt, ice cream, or custard that may contain B. cereus spores. Also avoiding raw or undercooked eggs, meat, poultry, fish, or shellfish that may harbor B. cereus bacteria.
- Seeking medical attention promptly if symptoms of food poisoning or other infections occur, such as vomiting, diarrhea, abdominal cramps, fever, eye pain, blurred vision, respiratory distress, wound infection, or sepsis. Informing the doctor of any recent food consumption or exposure to soil or animals that may be related to the illness.
- Reporting suspected cases of food poisoning or outbreaks to the local health authorities to help identify the source of contamination and prevent further spread of B. cereus. Providing samples of stool, vomit, food, or eye swabs for laboratory testing if requested.
By following these prevention and control measures, the risk of Bacillus cereus infection can be reduced significantly. However, it is important to remember that B. cereus is a ubiquitous organism that can survive in harsh conditions and produce toxins that are resistant to heat and acid. Therefore, it is essential to maintain good food safety practices at all times to protect oneself and others from this tenacious bacterium.
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