Bacillus subtilis is a Gram-positive bacterium known for its remarkable adaptability and its role as a model organism in microbiology. This organism is ubiquitous, commonly found in soil, and is sometimes incorporated into commercial probiotic products. Its success in diverse environments is directly attributable to its morphology, which is the study of the organism’s form and structure. The unique architecture of B. subtilis allows it to transition between a rapidly growing cell, a highly motile explorer, a durable resting form, and a complex community structure.
The Characteristic Rod Shape
The fundamental form of the active, dividing B. subtilis cell is the rod shape, which is why it is classified in the genus Bacillus. This shape is physically defined by its size, with individual cells typically measuring about 0.7 to 0.8 micrometers in diameter and 2.0 to 3.0 micrometers in length. The rod-like structure is maintained by a thick, multi-layered peptidoglycan cell wall, which is characteristic of all Gram-positive bacteria. This robust cell wall provides the necessary structural integrity to withstand internal turgor pressure and preserve the cell’s shape.
The peptidoglycan layer is a mesh-like polymer of sugars and amino acids that encases the cell, giving it rigidity and protection. During rapid growth, the rod-shaped cells often remain loosely connected end-to-end, forming short chains. This tendency is due to incomplete separation after cell division, though the cells can also exist singly or in pairs.
How Bacillus Subtilis Moves
The active vegetative cell of B. subtilis is capable of remarkable movement, a behavior mediated by specialized structures called flagella. B. subtilis is characterized as peritrichous, meaning it possesses numerous flagella distributed randomly all over the entire cell surface. Each flagellum is a complex, rotating machine built with a membrane-embedded basal body, a hook, and a long, helical filament that acts as a propeller.
This flagellar arrangement facilitates two primary modes of movement: swimming and swarming. Swimming motility occurs in liquid environments, where the individual flagella bundle together behind the cell and rotate to propel the bacterium forward in a run. Swarming is a coordinated, multicellular movement across moist, solid surfaces, which requires the cells to differentiate slightly and secrete a surfactant to reduce surface tension. This collective movement allows the population to rapidly colonize a new territory, showcasing a dynamic morphological adaptation to the physical environment.
The Endospore Transformation
The most dramatic morphological adaptation of B. subtilis is the formation of an endospore, a highly resistant, dormant cell type. This complex differentiation process, known as sporulation, is triggered by severe environmental stress, such as nutrient depletion, and involves an asymmetric cell division. The resulting structure, the spore, is a complete morphological departure from the active vegetative cell, designed for long-term survival.
The endospore contains the bacterium’s genetic material in a highly dehydrated core, which accounts for its resistance to heat and radiation. Surrounding the core is the cortex, a thick layer of specialized peptidoglycan that helps to maintain the dehydrated state of the core. The outermost protective shell is the spore coat, a complex, multilayered protein structure. This coat is organized into distinct layers, including the basement layer, inner coat, outer coat, and an outermost layer called the crust.
These protective layers shield the spore from desiccation, harsh chemicals, and extreme temperatures, allowing the organism to survive for years in a metabolically inactive state. The mature endospore is highly refractile, meaning it appears bright under a light microscope, a physical trait that distinguishes it from the dark, metabolically active vegetative cell. This transformation ensures the species’ longevity in the environment until conditions become favorable enough for the spore to germinate and resume vegetative growth.
Biofilm and Colony Structures
Beyond the individual cell, B. subtilis exhibits complex community morphologies when growing on surfaces. Under laboratory conditions, single cells aggregate to form colonies that are often visibly wrinkled or rough on the surface. This macro-morphology is a structured community known as a biofilm.
The physical structure of the biofilm is maintained by the Extracellular Polymeric Substance (EPS) matrix, a self-secreted, sticky substance that embeds the cells. The EPS is a viscoelastic gel composed of several components, including an exopolysaccharide, protein fibers like TasA that promote cell-to-cell attachment, and a hydrophobin protein called BslA. This matrix alters the physical structure of the entire community, providing a protective barrier that helps the cells resist external stressors such as desiccation and antimicrobial agents.