Bacillus cereus is a ubiquitous, rod-shaped bacterium widely distributed in the environment, particularly in soil and dust, which contributes to its presence in various food products. This microorganism is a member of the B. cereus group, a collection of closely related species. Understanding the specific morphological, hemolytic, and biochemical characteristics of B. cereus is fundamental to its identification in clinical and food safety laboratories. The bacterium is recognized globally as a common cause of foodborne illness.
Defining the Physical Structure
Microscopic examination reveals B. cereus as a relatively large, Gram-positive bacillus, characterized by straight rods that often possess square or slightly rounded ends. These vegetative cells typically arrange themselves in single units or short chains. In older cultures or under specific environmental stress, the cell wall structure can weaken, sometimes causing the organism to appear Gram-variable or even Gram-negative.
A defining feature is its capacity to form a protective endospore, a dormant structure that allows survival under adverse conditions like high heat or desiccation. The endospore is oval or ellipsoidal in shape and is typically situated centrally within the vegetative cell. Furthermore, B. cereus is motile, a characteristic imparted by peritrichous flagella, which are numerous, hair-like appendages distributed over the entire cell surface.
Hemolytic Activity and Culture Appearance
When grown on specialized media, B. cereus exhibits distinct macroscopic and microscopic traits, one of the most prominent being its hemolytic activity. Hemolysis refers to the ability of a bacterium to lyse, or break down, red blood cells in a culture medium such as a Blood Agar Plate (BAP). B. cereus is characterized as a beta-hemolytic organism, meaning it produces a complete clearing of the medium around the colony. This clearing results in a transparent, yellowish zone on the BAP.
On the surface of the agar, colonies are typically large, measuring between 2 to 5 millimeters in diameter after 24 hours of incubation. They are often described as having a flat, dull, gray-white or gray-yellow appearance with irregular, spreading margins. The texture is commonly opaque and granular, sometimes likened to a “ground glass” texture, which assists in preliminary visual identification.
Metabolic Differentiation Traits
The definitive identification of B. cereus relies heavily on a battery of biochemical tests that analyze its enzymatic activity. One widely used trait is its strong positive reaction to the catalase test, indicating the presence of the enzyme catalase, which breaks down hydrogen peroxide. The organism is also characteristically positive for lecithinase, an enzyme that hydrolyzes lecithin, a lipid found in egg yolk.
This lecithinase activity is routinely visualized on specialized media, such as Mannitol Egg Yolk Polymyxin (MYP) agar, where a precipitate forms around the colony. Another distinguishing metabolic feature is the inability of B. cereus to ferment the carbohydrate mannitol. Contrastingly, it demonstrates the ability to utilize and break down starch, a process known as starch hydrolysis. These combined traits—catalase-positive, lecithinase-positive, and mannitol-negative—are useful for distinguishing B. cereus from other Bacillus genus members, such as the non-hemolytic B. anthracis. Most strains are also positive for the Voges-Proskauer (VP) test.
Relevance to Food Safety
The study of B. cereus’s characteristics is directly linked to its public health significance as a foodborne pathogen. The bacterium causes two distinct types of gastrointestinal illness, which are differentiated by the toxins produced and the resulting symptoms.
Emetic Syndrome
The emetic (vomiting) syndrome is a food intoxication caused by the ingestion of cereulide, a heat-stable cyclic peptide toxin that is pre-formed when the bacteria grow in contaminated food. This form of illness is frequently associated with starchy foods like cooked rice and has a rapid onset, typically between 1 to 5 hours after consumption.
Diarrheal Syndrome
The diarrheal syndrome is an infection that occurs when vegetative cells or spores are ingested, which then produce proteinaceous enterotoxins, such as hemolysin BL and non-hemolytic enterotoxin, within the small intestine. This diarrheal form has a longer incubation period, usually 8 to 16 hours, and is linked to a wider variety of foods, including meats, vegetables, and milk products. The survival of its heat-resistant spores, especially in improperly cooled cooked foods, allows the organism to grow to levels capable of toxin production, underscoring the importance of proper food handling and storage temperatures.