Bacillus cereus is a bacterium found widely in the environment and raw food ingredients. This rod-shaped organism causes foodborne illness, resulting in both diarrheal and emetic types of poisoning. Understanding its physical characteristics, reaction to common dyes, and growth patterns provides distinctive markers necessary for laboratory identification. The organism’s ability to form a highly resistant internal structure allows it to survive harsh conditions, influencing its persistence in the food supply chain.
Cellular Morphology
Bacillus cereus is characterized by its relatively large size. The vegetative, or active, cells are straight or slightly curved rods, typically measuring 1.0–1.2 micrometers in width and 3.0–5.0 micrometers in length. The cells often possess blunt or squared-off ends, a feature common among the Bacillus genus. The bacterium is highly motile due to numerous peritrichous flagella, which enable the organism to move efficiently through liquid and across surfaces. While some related species produce a large capsule, B. cereus is generally described as non-capsulated or having only a minimal outer layer.
Staining Characteristics
B. cereus is classified as a Gram-positive bacterium. When subjected to the Gram staining procedure, the cell wall retains the primary crystal violet dye, causing the cells to appear purple under the microscope. This retention is due to the cell having a thick layer of peptidoglycan, which traps the crystal violet-iodine complex. This classification is most reliably observed in young, actively growing cultures. However, B. cereus tends to become Gram-variable, or even appear Gram-negative, as the culture ages. This change occurs because the cell wall degrades over time, diminishing its ability to retain the stain. Recognizing this variability is important for accurate identification.
Cellular Arrangements and Growth Patterns
When viewed in a prepared smear, individual B. cereus cells rarely appear alone. The rods frequently align to form short or long chains, an appearance sometimes described as “strepto-bacillus.” The junctions between cells in these chains are often clear and distinct, giving the arrangement a characteristic “bamboo pole” or boxcar-like structure. The macroscopic growth pattern on agar plates also aids identification. Colonies are typically large (2 to 5 millimeters in diameter), flat, dull, and spreading. The irregular perimeter is a visual representation of the organism’s swarming motility. On blood agar, the opaque, grayish-white colonies are surrounded by a zone of complete clearing, known as beta-hemolysis.
Endospore Formation: A Key Survival Feature
The ability of B. cereus and other members of its genus to form a specialized, dormant structure called an endospore is a key survival mechanism. This formation is triggered by environmental stress, such as nutrient depletion, and is not a reproductive process. The endospore encases the bacterium’s genetic material and a portion of its cytoplasm in a highly protective shell.
The structure includes a dehydrated core and an outer protective layer composed of multiple protein layers, known as the spore coat. The core contains dipicolinic acid, which accounts for up to ten percent of the spore’s dry weight and helps maintain dormancy and resistance. This tough casing provides exceptional protection against common threats like high heat, desiccation, ultraviolet radiation, and chemical disinfectants.
Under the microscope, endospores appear as oval or ellipsoidal structures that do not swell the mother cell during formation. They are typically located centrally or subterminally within the vegetative cell. The extraordinary resistance of these spores allows them to survive the initial cooking of foods, making the spore form directly responsible for the organism’s persistence and significance in food safety. The presence of these heat-resistant endospores is a primary diagnostic feature for the Bacillus group.