Overview of Bacillus cereus
Bacillus cereus, part of the Bacillus genus, is an opportunistic pathogen with relatively low virulence compared to Bacillus anthracis. However, it is one of the most significant species after B. anthracis due to its ability to cause diseases like gastroenteritis, eye infections, and, in rare cases, severe pneumonia. Bacillus cereus was first isolated when a new type of bacteria was discovered in barn air on agar plates. In the 2010s, the FDA identified B. cereus as a major cause of microbiological contamination in U.S. pharmaceutical facilities.
Microbiological Characteristics
Bacillus cereus shares morphological similarities with B. anthracis. When cultured on sheep blood agar at 35°C in regular air, it forms large, beta-hemolytic colonies, often between 4-7 mm in diameter. It is motile and breaks down salicin. If lecithinase-positive, it is likely B. cereus. During foodborne illness investigations, the bacterial count in suspected food can be quantified, with counts exceeding 105 CFU/g indicating the likely cause of food poisoning. Though part of the normal flora in small amounts, quantitative cultures are necessary for diagnosis.
Toxin Production and Pathogenesis
Bacillus cereus-induced gastroenteritis is linked to two types of enterotoxins. The heat-stable toxin causes emetic (vomiting) symptoms, while the heat-labile toxin leads to diarrheal symptoms. The heat-labile toxin works similarly to those of E. coli and Vibrio cholerae, stimulating adenylate cyclase and causing severe diarrhea. However, the exact mechanism of the heat-stable toxin is less understood.
Bacillus cereus can also cause eye infections, though the mechanism remains unclear. Three toxins—necrotizing toxin, cereolysin (a potent hemolysin named after the species), and phospholipase C (a strong lecithinase)—are involved. These toxins, along with other unknown factors, contribute to the rapid destruction of the eye tissue in these infections.
Epidemiology
Bacillus cereus and other Bacillus species are ubiquitous in the environment, especially in soil. Most infections originate from environmental contamination, and the presence of these bacteria in clinical samples is often considered insignificant unless tied to a specific infection.
Clinical Manifestations
Food Poisoning
Bacillus cereus causes two distinct types of food poisoning: vomiting and diarrheal types. The vomiting form is often linked to contaminated rice, where heat-resistant spores survive cooking. If rice is improperly stored, spores germinate, and bacteria proliferate, producing a heat-stable toxin that remains even after reheating. This toxin is ingested, leading to illness within 1-6 hours, causing vomiting, nausea, and abdominal pain. In severe cases, liver failure can occur due to mitochondrial fatty acid metabolism disruption, though this is rare.
The diarrheal form typically occurs after consuming contaminated meat, vegetables, or sauces. With a longer incubation period, the bacteria multiply in the intestines, producing a heat-labile toxin that causes diarrhea, nausea, and abdominal cramps. Symptoms usually last more than a day.
Eye Infections and Other Diseases
Eye infections caused by Bacillus cereus, particularly panophthalmitis, often follow trauma from contaminated objects. The infection can progress rapidly, resulting in complete blindness within 48 hours. Additionally, intravenous drug users are at risk of developing eye infections from systemic Bacillus cereus infections.
Other infections include bloodstream infections related to intravenous catheters, central nervous system shunt infections, and endocarditis (especially in drug users). Immunocompromised patients may develop pneumonia, bacteremia, or meningitis. In rare cases, Bacillus cereus can cause anthrax-like severe pneumonia in immunosuppressed patients, as documented in a few cases involving metalworkers in Texas and Louisiana. These strains carried the pXO1 plasmid, associated with B. anthracis virulence, highlighting the potential for B. anthracis genes to spread among Bacillus cereus strains.
Laboratory Diagnosis
Bacillus cereus can be easily cultured from clinical samples, especially in cases of food poisoning. Since it can temporarily colonize humans, suspected food sources (such as rice, meat, or vegetables) must be tested for confirmation. Routine testing for toxins is not commonly performed, and diagnosis is generally based on epidemiological evidence. Bacillus species grow rapidly and can be identified by Gram staining and culturing samples from affected areas like the eye, blood, or other tissues.
Treatment and Prevention
For Bacillus cereus-induced gastroenteritis, symptomatic treatment is usually sufficient as the disease resolves quickly and without complications. However, infections caused by other Bacillus species may progress rapidly and are often resistant to multiple antibiotics. Bacillus cereus is typically resistant to penicillin and cephalosporins, necessitating the use of antibiotics like vancomycin, clindamycin, ciprofloxacin, or gentamicin. Prompt treatment is critical for eye infections.
Food poisoning can be prevented by consuming cooked foods quickly or storing them properly under refrigeration. Reheating contaminated foods will not destroy heat-stable toxins.
Conclusion
Bacillus cereus is a significant cause of foodborne illness, eye infections, and opportunistic infections in immunocompromised individuals. Proper food handling and storage, along with timely medical treatment for infections, are essential in controlling its impact.
Related Terms
Bacillus anthracis, beta-hemolysis, adenylate cyclase-cyclic adenosine monophosphate, panophthalmitis, penicillin, cephalosporin, vancomycin, clindamycin, ciprofloxacin, gentamicin, colony, motility, toxin, food poisoning, bacteremia
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