Table of Contents
- Introduction
- Morphological Characteristics
- Basic Characteristics of Bacillus cereus
- Fermentation Profile of Bacillus cereus
- Enzymatic Reactions of Bacillus cereus
- Key Biochemical Tests of Bacillus cereus
- Conclusion
- References
Introduction
- Bacillus cereus is a Gram-positive, rod-shaped bacterium that often appears in pairs or chains under the microscope.
- It is spore-forming, producing highly resistant endospores that can survive harsh environmental conditions, including heat and disinfectants.
- This bacterium is facultatively aerobic, meaning it can grow in both the presence and absence of oxygen.
- It is widely distributed in soil, dust, vegetation, and various environmental surfaces, especially in agricultural and food production settings.
- B. cereus is known to contaminate raw and cooked food, particularly rice, pasta, dairy products, and meats, leading to foodborne outbreaks.
- It produces toxins responsible for two types of gastrointestinal illnesses: the emetic (vomiting) type, caused by cereulide toxin, and the diarrheal type, due to enterotoxins.
- Beyond food poisoning, Bacillus cereus can also cause serious opportunistic infections, including endophthalmitis, wound infections, bacteremia, and meningitis, especially in immunocompromised individuals.
- Its ability to form biofilms on medical equipment and surfaces increases its clinical significance in hospital settings.
- Biochemical testing is a crucial step in identifying Bacillus cereus and distinguishing it from closely related species such as B. subtilis, B. thuringiensis, and B. anthracis.
- These tests help microbiologists confirm its identity based on specific enzymatic activities, sugar fermentation profiles, and substrate utilization patterns, contributing to accurate diagnosis and contamination control.
Morphological Characteristics
Gram Stain:
- Bacillus cereus appears as Gram-positive rods under the microscope.
- In older cultures or under stress, some cells may appear Gram-variable due to cell wall changes.
Cell Shape and Arrangement:
- Cells are straight rods with squared-off ends, often arranged in pairs or short chains.
- Average cell size ranges from 1.0–1.2 µm in width and 3.0–5.0 µm in length.
Spore Formation:
- Forms oval, centrally to subterminal located spores that do not swell the cell.
- Spores are highly resistant to environmental stress, contributing to survival in adverse conditions and food products.
Motility:
- Bacillus cereus is motile due to the presence of peritrichous flagella (flagella distributed all around the cell surface).
- This trait aids in colonization and movement across surfaces, including food and tissue environments.
Hemolysis:
- Shows strong beta-hemolysis on 5% sheep blood agar, forming clear zones around colonies due to complete lysis of red blood cells.
- This is a distinctive feature that helps differentiate it from non-hemolytic Bacillus species like Bacillus anthracis.
Colony Morphology:
- Colonies are usually large, flat, rough, opaque, and grayish-white in appearance.
- Often display irregular edges and may exhibit swarming on nutrient agar due to motility.
Basic Characteristics of Bacillus cereus
Bacillus cereus is a Gram-positive, rod-shaped bacterium that demonstrates strong catalase activity, motility, and spore formation. It shows beta-hemolysis on blood agar and can grow in potassium cyanide (KCN) medium. These core traits are essential for distinguishing it from related species in clinical and food microbiology.
| Basic Characteristic | Result |
|---|---|
| Catalase | Positive (+ve) |
| Citrate | Positive (+ve) |
| Gelatin Hydrolysis | Negative (−ve) |
| Gram Staining | Positive (+ve) |
| Growth in KCN | Positive (+ve) |
| Hemolysis | Positive (+ve) |
| Indole | Negative (−ve) |
| Motility | Positive (+ve) |
| MR (Methyl Red) | Negative (−ve) |
| Nitrate Reduction | Variable |
| Oxidase | Negative (−ve) |
| Pigment | Negative (−ve) |
| Shape | Rods |
| Spore | Positive (+ve) |
| VP (Voges Proskauer) | Positive (+ve) |
Fermentation Profile of Bacillus cereus
Fermentation tests are useful in understanding the metabolic flexibility of Bacillus cereus. It can ferment a variety of sugars such as glucose, glycerol, maltose, and trehalose, producing acid as a byproduct. However, it does not ferment lactose, mannitol, or xylose—traits that assist in its differentiation from other enteric bacteria.
| Carbohydrate | Result |
|---|---|
| Adonitol | Negative (−ve) |
| Arabinose | Negative (−ve) |
| Arabitol | Negative (−ve) |
| Cellobiose | Variable |
| Fructose | Positive (+ve) |
| Galactose | Negative (−ve) |
| Glucose | Positive (+ve) |
| Glycerol | Positive (+ve) |
| Glycogen | Positive (+ve) |
| Inositol | Negative (−ve) |
| Inulin | Negative (−ve) |
| Lactose | Negative (−ve) |
| Maltose | Positive (+ve) |
| Mannitol | Negative (−ve) |
| Mannose | Negative (−ve) |
| Melibiose | Negative (−ve) |
| Raffinose | Negative (−ve) |
| Rhamnose | Negative (−ve) |
| Ribose | Positive (+ve) |
| Salicin | Variable |
| Sorbitol | Negative (−ve) |
| Starch | Positive (+ve) |
| Sucrose | Variable |
| Trehalose | Positive (+ve) |
| Xylose | Negative (−ve) |
Enzymatic Reactions of Bacillus cereus
The enzymatic profile of Bacillus cereus includes several reactions useful in identification. The bacterium hydrolyzes casein, esculin, and tyrosine, and shows positive lecithinase activity. These enzymes contribute to nutrient breakdown and virulence. Reactions such as arginine and acetate utilization may vary depending on the strain and conditions.
| Enzymatic Test | Result |
|---|---|
| Acetate Utilization | Variable |
| Arginine Dehydrolase | Variable |
| Casein Hydrolysis | Positive (+ve) |
| Esculin Hydrolysis | Positive (+ve) |
| Lecithinase | Positive (+ve) |
| Lysine | Negative (−ve) |
| Ornithine Decarboxylase | Negative (−ve) |
| Phenylalanine Deaminase | Negative (−ve) |
| Tyrosine Hydrolysis | Positive (+ve) |
Key Biochemical Tests of Bacillus cereus
| Biochemical Test | Result |
|---|---|
| Catalase Test | Positive ✅ |
| Oxidase Test | Negative ❌ |
| Indole Test | Negative ❌ |
| Methyl Red (MR) Test | Positive ✅ |
| Voges-Proskauer (VP) Test | Positive ✅ |
| Citrate Utilization | Positive ✅ |
| Urease Test | Variable (+/-) |
| Nitrate Reduction | Positive ✅ |
| Gelatin Hydrolysis | Positive ✅ |
| Starch Hydrolysis | Positive ✅ |
| Lecithinase Production | Positive ✅ (used in identification) |
| Casein Hydrolysis | Positive ✅ |
| Glucose Fermentation | Positive (acid, no gas) ✅ |
| Mannitol Fermentation | Negative ❌ |
| Sucrose Fermentation | Variable |
| Lactose Fermentation | Negative ❌ |
| Xylose Fermentation | Positive ✅ |
Conclusion
- Biochemical tests play a crucial role in the accurate identification of Bacillus cereus from clinical, environmental, and food samples.
- The organism is typically identified by a positive catalase test, positive lecithinase activity, and a positive Voges-Proskauer (VP) test.
- It also shows strong hydrolytic activity on substrates like starch, gelatin, and casein, which aids in its differentiation from other Bacillus species.
- The distinctive beta-hemolysis on blood agar further supports its identification in lab diagnostics.
- Understanding the biochemical profile of Bacillus cereus is essential not only for diagnosing infections but also for monitoring food safety and preventing outbreaks.
- Correct interpretation of test results allows timely action in both public health and clinical microbiology settings.
References
- Bergey’s Manual of Determinative Bacteriology, 9th Edition.
- Madigan MT, et al. Brock Biology of Microorganisms, 15th Edition.
- Forbes BA, Sahm DF, Weissfeld AS. Bailey and Scott's Diagnostic Microbiology, 13th Edition.
- Public Health Agency of Canada. Pathogen Safety Data Sheet – Bacillus cereus.
- CDC (Centers for Disease Control and Prevention) – Bacillus cereus Foodborne Illness.
