Laboratory Diagnosis of Bacillus cereus
SPECIMENS: Faeces, vomitus, remaning food (if any), eye specimen (corneal swab)
DIRECT DETECTION METHODS
- Microscopically the organisms appear as large gram positive rods in singles, pairs, or serpentine with square ends after Gram staining.
- Endospores formation are seen as unstained oval or round region within centre of cell. Spores are oval (ellipsoidal) and not swelling the mother cell.
Figure: Gram stain of Bacillus cereus
- Growth on 5% sheep blood agar, chocolate agar, routine blood culture media, and nutrient broths.
- Detectable growth within 24 hours following incubation on media incubated at 35° C, in ambient air, or in 5% carbon dioxide (CO2).
- Colony character on blood agar: Large, feathery, spreading, dull, gray, granular, spreading colonies and opaque with a rough matted surface and irregular perimeters, beta-hemolytic.
- Bacillus cereus can be isolated from faeces by using selective media such as: MYPA (mannilol, egg yolk, polymyxin, phenol red and agar), PEMBA (polymyxin , egg yolk, mannilol, bromodiymol, blue agar).
- These media take advantage of the phospholipase C positive reaction on egg yolk agar, no production of acid from mannitol, and incorporation of pyruvate or polymyxin as the 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
- Serologic methods are available for the detection of B. cereus toxin in food and feces.
- Microslide gel diffusion test is generally used as toxin detection system.
- The toxigenic potential of B. cereus isolates, genes encoding emetic-toxin cereulide (ces) and enterotoxins (nhe, hbl and cytK) can be analysed by multiplex PCR.
Treatment of Bacillus cereus
- Persons with B. cereus food poisoning require only supportive treatment.
- Oral rehydration or, occasionally, intravenous fluid and electrolyte replacement for patients with severe dehydration is indicated. Antibiotics are not indicated.
- Patients with invasive disease require antibiotic therapy. Bacillus cereus is susceptible to clindamycin, erythromycin, vancomycin, aminoglycosides and tetracycline. It is resistant to penicillin and trimethopri
Prevention and Control of Bacillus cereus
- Diarrheal and vomiting intoxications by this organism are readily preventable by appropriate food-handling procedures.
- Meat and vegetables should not be held at temperatures between 10 and 45 °C for long periods, and rice held overnight after cooking should be refrigerated and not held at room temperature.
- Prevention of infection in patients following surgery or in those who are immunocompromised or who are otherwise predisposed to infection, depends on good practice.