Bacillus cereus is a gram-positive rod-shaped bacterium of the genus Bacillus and the order Bacillales, belonging to the class Bacilli and family Bacillaceae of the division Firmicutes. The bacterium is ubiquitous in the environment and is ingested with raw materials or food in the form of germs. In spoiled foods with high protein content, more than 1000 individuals of the species occur per gram, so the consumer can expect food poisoning.
What is Bacillus cereus?
Bacillus is a genus of rod-shaped bacteria that includes over 200 species with Gram-positive staining behavior. Within the Bacteria domain, the genus belongs to the division Firmicutes, the class Bacilli, and the order Bacillales, of which it is a member of the family Bacillaceae. Many species from the bacterial genus are capable of active locomotion and carry so-called pili for this purpose. One of these actively motile bacteria is the species Bacillus cereus. Together with other species, this bacterial species forms the so-called Bacillus cereus group. Genetically close relationship distinguishes the individual representatives. Their common core genome consists of over 3000 genes. Bacillus cereus is considered an opportunistic pathogen and is therefore human pathogenic under certain conditions. The human pathogenicity of the bacterial species is primarily related to the toxins they produce. As soon as an above-average number of toxins of Bacillus cereus are produced in the digestive tract of a human being, typical symptoms of poisoning occur. In immunologically weakened people, even small amounts of toxins are sufficient for the development of poisoning symptoms.
Occurrence, distribution, and properties
Bacillus cereus is ubiquitously present in the environment and is found in raw materials of numerous foods in addition to soil. The bacterial species is also found in relatively high concentrations in natural soil and occurs in an individual number of about one million per one gram of soil sample. Thus, the species Bacillus cereus is one of the most abundant species of soil bacteria at all. The individuals form spores that are extremely resistant to temperature and other influences. For this reason, the processing of individual raw materials can harm them just as little as the boiling of food. The species enjoys optimal culture conditions in a temperature range of 28 to 35 degrees Celsius. Individual strains of Bacillus cereus are sensitive to acids. The metabolic pathway of the species is facultative anaerobic. This means that the bacteria can metabolize without oxygen and survive in an oxygen-poor environment. In this case, they resort to substances other than O2 and metabolize them for growth and energy production purposes. However, if oxygen is available, they can also run their metabolism on oxygen. The bacteria possess the enzyme catalase, which can convert H2O2 to O2 and H2O. Thus, they are able to produce oxygen and water from hydrogen peroxide. Since the bacteria are ubiquitous in the environment, a small amount of them can also be found in the human body. This is especially true in the gastrointestinal tract of humans, as the bacteria are ingested with all raw materials. In small quantity, the bacterial species is not human pathogenic for people of normal constitution. The bacteria produce a so-called enterotoxin through their metabolic pathway. In higher quantities, these toxins show an effect on the gastrointestinal tract. In spoiled raw materials, the bacterial count of Bacillus cereus and thus the amount of enterotoxins produced is usually far above the human tolerance threshold.
Diseases and complaints
Enterotoxins are cytotoxic proteins that can cause pore formation in the cell membrane, altering the permeability of epithelial cells of the intestinal mucosa or even inducing cell death in the intestinal epithelium. The failure of epithelial cells manifests itself in a loss of function of the physiological mucosal barrier of the intestine. Fluid and electrolytes are lost. Damage by enterotoxins manifests clinically in the form of diarrhea. Bacillus cereus can be present in spoiled food in such large bacterial numbers that typical symptoms of food poisoning occur. Germ counts of more than 1,000 individuals per gram of food cause symptoms in people of normal constitution.In the case of such a high infestation, the food usually tastes unpleasant because large amounts of protease can be detected in it. Symptoms of food poisoning by enterotoxins include diarrhea, nausea, vomiting, and stomach cramps as well as abdominal pain. The highest risk for high exposure to Bacillus cereus is for protein-rich foods such as fish, meat, dairy and egg products. These products provide an ideal nutritional base for bacteria of the species and thus can lead to their proliferation. People with constitutionally stressful pre-existing conditions and correspondingly weakened immune systems are much less able to withstand enterotoxins before developing symptoms of poisoning. Thus, they are generally more susceptible to food poisoning than average. In addition to diseases and immunosuppressants, psychological stress states can also weaken the immune system. Older people also generally have weaker immune systems than younger people. Young children also often have limited immunological resistance. Thus, seniors and young children are much more likely to be affected by food poisoning caused by toxins from the bacterial species Bacillus cereus than adolescents and adults. Various measures are available to prevent food poisoning caused by the pathogen. On the one hand, good kitchen hygiene is absolutely essential. On the other hand, even slightly spoiled food should not be eaten. Since the germs of Bacillus cereus have been shown to be resistant to heat, boiling food does not help against food poisoning.
