The Bacillus genus encompasses a diverse group of bacteria, often recognized for Bacillus anthracis, the bacterium responsible for anthrax. However, B. anthracis represents only a small fraction of this vast genus. Many Bacillus species are harmless and offer valuable benefits to humans and the environment. This article explores the broad characteristics and diverse roles of Bacillus species beyond B. anthracis.
General Characteristics of Bacillus Species
Bacillus species are typically rod-shaped, Gram-positive bacteria, meaning they retain a purple stain in a common laboratory test due to their cell wall structure. They range in size, with diameters generally between 0.5 to 2.5 micrometers and lengths from 1.2 to 10 micrometers, often appearing singly or in chains. They are predominantly aerobic, though some species are facultative anaerobes.
A defining feature of Bacillus species is their ability to form dormant structures called endospores. These endospores are highly resistant to harsh environmental conditions, including extreme temperatures, desiccation, radiation, and many disinfectants. This allows Bacillus to survive for extended periods in unfavorable environments, such as soil, until conditions become suitable for growth, contributing to their widespread presence across various natural habitats.
Beneficial Applications of Bacillus Species
In agriculture, Bacillus thuringiensis (Bt) is a widely used biopesticide, offering an environmentally friendly alternative to chemical pesticides. Bt produces crystalline proteins, known as delta-endotoxins, which are toxic to specific insect larvae when ingested, causing damage to their gut lining and leading to cessation of feeding and eventual death. Different strains of Bt are effective against specific insect orders, including moths, butterflies, mosquitoes, and beetles.
Bacillus species also serve as probiotics, particularly Bacillus subtilis and Bacillus coagulans, which support human and animal health. These spore-forming bacteria can survive the acidic conditions of the stomach and reach the intestines, where they contribute to a balanced gut microbiota. They may help in breaking down food, absorbing nutrients, and supporting gut barrier function. Some Bacillus probiotics are found in fermented foods like natto and are available as dietary supplements.
Bacillus is also a significant source of enzymes for industrial applications. Various species produce large quantities of extracellular enzymes like proteases and amylases, which are used in detergents, food processing, and textile industries. For example, Bacillus amyloliquefaciens is a source of alpha-amylase used in starch hydrolysis. Additionally, some Bacillus strains are engineered to produce vitamins, nucleotides, and other valuable biochemicals.
Pathogenic Bacillus Species Beyond Anthrax
Bacillus cereus is a common cause of human illness, primarily through foodborne infections. B. cereus is ubiquitous in soil and can contaminate a wide range of foods. Food poisoning caused by B. cereus typically presents in two distinct forms: emetic (vomiting) syndrome and diarrheal syndrome.
The emetic syndrome is characterized by nausea and vomiting, usually appearing within 30 minutes to 6 hours after consuming contaminated food. This rapid onset is due to a heat-stable toxin, cereulide, pre-formed in the food. Cooked rice that has been left unrefrigerated is a common source for this type of illness.
The diarrheal syndrome, on the other hand, typically manifests 6 to 15 hours after ingestion, causing watery diarrhea and abdominal pain. This syndrome results from enterotoxins produced by the bacteria once they multiply within the small intestine. Foods like meats, sauces, soups, and vegetables are often implicated in diarrheal outbreaks. Most B. cereus food poisoning cases are mild and resolve within 24 hours.
Beyond foodborne illness, other Bacillus species, including B. cereus, B. subtilis, and B. licheniformis, can cause opportunistic infections, particularly in individuals with compromised immune systems. These infections are less common but can include bacteremia, endocarditis, meningitis, and infections of wounds or the eyes. Such infections are often associated with trauma or the presence of foreign bodies.
Identifying Bacillus anthracis
Distinguishing Bacillus anthracis from other Bacillus species is important due to its pathogenic nature. B. anthracis possesses specific characteristics that differentiate it in laboratory settings. A key distinguishing feature is its unique capsule, which is composed of poly-D-glutamic acid (PGA). Unlike most bacteria that have polysaccharide capsules, this polypeptide capsule helps B. anthracis evade the host’s immune system by inhibiting phagocytosis.
Bacillus anthracis also produces a tripartite protein toxin, known as anthrax toxin, which consists of protective antigen (PA), edema factor (EF), and lethal factor (LF). The combination of PA and LF forms lethal toxin, while PA and EF combine to form edema toxin. These toxins are encoded on a plasmid and contribute significantly to the disease’s severity by causing tissue swelling and lethality. Furthermore, B. anthracis is typically non-motile, a characteristic that sets it apart from many other Bacillus species, which are generally motile due to peritrichous flagella. In culture, B. anthracis colonies are often described as having a “ground-glass” appearance and a tenacious, sticky consistency, and they do not cause hemolysis on sheep blood agar.