The term Bacillus refers to a large group of bacteria defined by their rod-like shape. These microbes are found in nearly every environment on Earth, from soil and water to the digestive tracts of animals. While some species cause disease, many others are harmless or provide benefits to industry and health. Understanding the structure of a Bacillus cell helps explain its diverse roles.
Understanding Basic Bacterial Cell Design
All bacteria, including Bacillus, are prokaryotic cells, which distinguishes them from the eukaryotic cells of plants and animals. The primary difference is that prokaryotes lack a membrane-bound nucleus, meaning their genetic material is not enclosed within a membrane. They also lack other complex, membrane-bound compartments called organelles.
Despite their relative simplicity, bacterial cells contain all the necessary components for life. The interior of the cell is filled with a gel-like substance called cytoplasm, which houses the cell’s internal structures. Enclosing this is a flexible plasma membrane that controls the movement of substances into and out of the cell. Scattered throughout the cytoplasm are thousands of ribosomes, small molecular factories responsible for building proteins.
A bacterium’s genetic material is a single, circular chromosome of DNA concentrated in a region of the cytoplasm called the nucleoid. This design allows bacteria to replicate and adapt quickly. While bacteria have several common shapes, including spheres (cocci) and spirals (spirilla), the Bacillus genus is defined by its rod-shaped morphology.
Detailed Structure of a Bacillus Cell
A Bacillus cell possesses fundamental prokaryotic components, but structures outside its plasma membrane give it distinct characteristics. The most notable of these is its cell wall, which provides rigid structural support and protection. Bacillus species are categorized as Gram-positive, a classification based on their reaction to the Gram stain procedure, which is a direct result of their cell wall architecture.
The Gram-positive cell wall has a thick layer of peptidoglycan, a mesh-like polymer of sugars and amino acids that can comprise up to 90% of the wall’s mass. During the Gram stain procedure, this thick peptidoglycan layer traps the applied crystal violet dye. This causes the cell to appear purple under a microscope, even after a decolorizing wash.
Teichoic acids are embedded within the peptidoglycan, helping stabilize the cell wall and participating in cell signaling. Some species have additional external layers. For example, Bacillus anthracis can produce a protective capsule made of protein that helps it evade a host’s immune system. Many Bacillus species are also equipped with long, flagella, whip-like appendages that rotate to propel the cell.
The Protective Endospore of Bacillus
The Bacillus genus can form endospores, which are dormant, durable structures developed in response to environmental stress, such as nutrient starvation or extreme temperatures. An endospore is a survival mechanism, not a reproductive cell, allowing the bacterium to endure conditions that would kill an active (vegetative) cell. This formation process is called sporulation.
Sporulation begins with an asymmetric cell division, where the bacterial cell divides into a larger mother cell and a smaller forespore. The mother cell then engulfs the forespore, wrapping it in multiple layers of membrane and protective materials. A thick layer of specialized peptidoglycan, called the cortex, is built between these membranes, and a tough protein coat assembles on the exterior.
The endospore’s core is partially dehydrated and contains high concentrations of dipicolinic acid, which complexes with calcium to stabilize its DNA. These features make endospores highly resistant to:
- Heat
- Desiccation
- Ultraviolet radiation
- Chemical disinfectants
When environmental conditions become favorable again, germination is triggered. This process allows the spore to rehydrate and transform back into a metabolically active cell.
Key Bacillus Species and Their Significance
Many Bacillus species are beneficial. Bacillus subtilis, for instance, is used as a probiotic to support digestive health. Its spores survive the stomach’s acidic conditions, allowing them to germinate in the intestines where they secrete enzymes to help break down food. These same enzyme-producing capabilities are harnessed industrially for detergents, food production, and scientific research.
Other Bacillus species are pathogens. Bacillus anthracis causes the disease anthrax, and its infectious particles are dormant endospores that can remain viable in soil for decades. Once in a host’s body, the spores germinate and release bacteria that produce lethal toxins.
Another species, Bacillus cereus, is a common cause of food poisoning. This bacterium produces different toxins that lead to two distinct types of illness: one causing vomiting and another diarrhea. Its spores can survive cooking, and if food is stored improperly, they can germinate and multiply. The vomiting (emetic) form is often associated with starchy foods like rice, as the toxin is heat-stable and can be pre-formed in the food, meaning reheating may not prevent illness.