Bacillus anthracis is the causative agent of anthrax. This Gram-positive, rod-shaped bacterium possesses unique biological features that allow it to persist in the environment and cause infection. The organism’s ability to switch between an active, multiplying state and a highly resistant dormant state is rooted in specialized structures.
The Basic Cellular Structure
The active form of the organism is called the vegetative cell. Individual cells measure between 1.0 and 1.2 micrometers in width and 3 to 5 micrometers in length. These cells are distinctive for having somewhat flattened or square ends, giving them a boxcar-like appearance under a microscope.
As a Gram-positive bacterium, B. anthracis possesses a thick layer of peptidoglycan in its cell wall, which retains the crystal violet stain. In laboratory cultures or within an infected host, these vegetative cells often remain attached to one another after division. This arrangement leads to the formation of long, serpentine chains that can resemble a string of boxcars or a bamboo rod.
The Protective Poly-D-Glutamic Acid Capsule
When B. anthracis is active inside a host, it produces an external, slimy layer known as a capsule. This capsule is chemically unique among bacteria because it is a polypeptide composed of poly-D-glutamic acid, rather than polysaccharide sugars. The formation of this protein-based capsule is regulated by genetic material found on the bacterium’s pXO2 plasmid.
The primary function of this negatively charged capsule is to shield the bacterium from the host’s immune system. The layer protects the vegetative cell from phagocytosis, the process where immune cells engulf and destroy invading microbes. This protective layer is only produced by the vegetative cells during infection. Without the capsule, the bacteria are highly susceptible to immune clearance.
Formation and Resilience of Endospores
The endospore is the most remarkable feature of B. anthracis and the form responsible for nearly all anthrax infections. Spores are a dormant survival strategy triggered when vegetative cells encounter harsh environmental conditions, such as nutrient depletion or exposure to free oxygen. The formation of a single, highly protected spore occurs internally within the parent cell, a process known as sporulation.
Sporulation involves packaging the bacterium’s DNA and cytoplasm into a specialized structure surrounded by multiple protective layers. The spore core becomes highly dehydrated, and dipicolinic acid accumulates to stabilize the core’s proteins and DNA against heat damage. Surrounding the core are the inner membrane, the peptidoglycan cortex, and a thick, proteinaceous spore coat.
The resulting endospore is an extremely resilient structure capable of withstanding conditions that would rapidly kill the vegetative cell, including high temperatures, desiccation, and chemical disinfectants. This robustness allows the spores to remain viable in soil for decades, acting as the reservoir for the disease. The spore is oval-shaped and typically located centrally or subterminally within the vegetative cell.
When these dormant spores find a favorable environment, such as the warm, nutrient-rich conditions inside a host, they undergo germination. This transition involves the rapid uptake of water, loss of resistance, and degradation of the spore coat, leading to the emergence of a metabolically active vegetative cell. Germination marks the beginning of an active infection, as the newly formed vegetative cells begin to multiply and produce the capsule and toxins.
Non-Motility and Basic Growth Characteristics
B. anthracis is characterized by its complete lack of motility, unlike many of its close relatives within the Bacillus genus. The vegetative cells do not possess flagella, the whip-like appendages used by many bacteria for movement. This non-motile characteristic is a consistent trait used by microbiologists to distinguish B. anthracis from other similar-looking, but motile, Bacillus species.
B. anthracis is classified as a facultative anaerobe, meaning it can grow with or without the presence of oxygen. It generally achieves optimal growth when incubated at body temperature, around 37°C, and often benefits from a slightly elevated carbon dioxide atmosphere. On laboratory agar plates, the colonies are typically non-hemolytic, meaning they do not break down red blood cells, and often exhibit a distinct rough, flat appearance with irregular projections.