Bacillus cereus is a ubiquitous, Gram-positive, rod-shaped bacterium belonging to the Bacillus genus. Commonly found in soil, it is notable for its ability to produce highly resistant spores. This spore-forming capacity allows the bacterium to survive harsh conditions, making it a significant concern in food safety.
External Appearance and Arrangement
Bacillus cereus presents as a straight or slightly curved rod shape, classifying it as a bacillus. Individual cells are relatively large for bacteria, typically measuring about 1.0 to 1.2 micrometers in width and 3.0 to 5.0 micrometers in length. The ends of the rods are often described as having a square or blunt appearance.
Under a microscope, the bacteria are frequently observed in pairs or short chains, sometimes forming longer, serpentine chains with clear junctions between cells. An ability to move is a distinguishing feature, as B. cereus is motile. Motility is achieved by peritrichous flagella, which are numerous, hair-like appendages distributed all around the cell surface.
The bacterium is a classic Gram-positive organism, meaning it retains the crystal violet stain due to its cell wall composition. However, as the culture ages or under certain stresses, some cells may appear Gram-variable or even Gram-negative because of changes in the cell wall structure. This variation highlights the importance of analyzing fresh cultures for accurate identification.
Cellular Components and Endospores
The vegetative cell possesses the typical Gram-positive cellular architecture, including a thick peptidoglycan layer surrounding the cell membrane. This robust cell wall provides structural integrity and is responsible for the positive reaction to the Gram stain. It also secretes a wide range of virulence factors, such as phospholipase C and metalloproteases, as exoenzymes.
The most defining structural feature is the endospore, which is a highly specialized, metabolically dormant cell type. The endospore structure is complex, consisting of a dehydrated core that contains the cell’s DNA and ribosomes. This core is surrounded by an inner membrane, a germ cell wall, and a thick peptidoglycan layer known as the cortex.
Outside the cortex lies a multi-layered protein coat, which provides significant protection against chemical damage. In B. cereus, this coat is further enclosed by a loose, balloon-like outer layer called the exosporium, which is composed of protein, lipid, and carbohydrate. A component contributing to the spore’s extreme heat resistance is dipicolinic acid, which is found in the core and helps stabilize the structure.
Defining Physiological Traits
Bacillus cereus is classified as a facultative anaerobe, meaning it can grow efficiently with oxygen but can also switch to fermentation in oxygen-limited environments. This metabolic flexibility is a significant factor in its survival in various food products. It is also a mesophilic organism, with an optimal growth temperature typically between 28°C and 37°C.
When the vegetative cell encounters unfavorable conditions, such as nutrient depletion, it initiates sporulation to form a protective endospore. This process involves asymmetric cell division, leading to the formation of a forespore within the mother cell. The mature spore is then released, surviving until conditions allow it to germinate back into a vegetative cell.
The bacterium produces several potent toxins that are responsible for its pathogenic potential. One is the emetic toxin, cereulide, a small, heat-stable cyclic peptide that is often pre-formed in food. The other group consists of diarrheal toxins, which are large, heat-labile proteins, including Hemolysin BL (Hbl) and Nonhemolytic Enterotoxin (Nhe). These diarrheal toxins are generally secreted by the vegetative cells after they have been ingested and begin to grow in the small intestine.
Context in Food Safety
The combination of spore resistance and toxin production makes B. cereus a common source of foodborne illness. Spores are highly resistant to heat, allowing them to survive normal cooking temperatures that destroy most vegetative bacteria. If cooked food is held at improper temperatures, the surviving spores can germinate, and the resulting vegetative cells multiply rapidly.
The illnesses caused by the bacterium manifest in two distinct forms, depending on which toxin is involved.
Emetic Syndrome
The emetic syndrome is caused by ingesting pre-formed cereulide toxin in the food. This leads to symptoms like nausea and vomiting, often occurring within a few hours.
Diarrheal Syndrome
The diarrheal syndrome occurs when a large number of vegetative cells or spores are consumed. These cells then produce enterotoxins in the small intestine, resulting in diarrhea and abdominal pain after a longer incubation period.
Contamination most frequently occurs in foods that have been cooked and then improperly cooled or stored, allowing time for spore germination and bacterial growth. Common food vehicles include starchy foods like rice and pasta, as well as sauces, soups, and meat products. Proper food handling, especially rapid cooling of leftovers, is a primary measure for preventing illness caused by this resilient microorganism.