Bacteria are single-celled microorganisms found in nearly every environment on Earth. One of the first characteristics scientists observe is their physical form, a consistent and genetically determined trait used for classification. This visual distinction is not just superficial; it is connected to how a bacterium survives and interacts with its surroundings.
Common Bacterial Shapes and Arrangements
Three primary shapes are most frequently observed: spheres, rods, and spirals. Spherical bacteria, known as cocci, can have different appearances based on their grouping after cell division. They can remain in pairs (diplococci), form long chains (streptococci, like Streptococcus pyogenes), or create grape-like clusters (staphylococci, exemplified by Staphylococcus aureus).
Rod-shaped bacteria, or bacilli, also exist in specific arrangements. Diplobacilli are two rods paired together, while streptobacilli form chains. A common example is Escherichia coli, found in the human gut. Some bacteria, called coccobacilli, are intermediate between a sphere and a rod, appearing as short ovals.
Spiral-shaped bacteria include several distinct forms. Vibrios are curved like a comma, such as Vibrio cholerae, the cause of cholera. Spirilla are rigid, corkscrew-shaped bacteria, while spirochetes, like Treponema pallidum which causes syphilis, are more flexible and helical.
How Bacterial Shape is Determined
A bacterium’s shape is an actively maintained structure defined by its cell wall. This wall is constructed from a molecule called peptidoglycan, which forms a rigid, bag-like layer (sacculus) around the cell. This layer provides structural support and protects the cell from bursting due to internal osmotic pressure. The organization of the peptidoglycan determines if the cell is a sphere, rod, or another shape.
The cell wall’s construction is guided by the bacterial cytoskeleton. In rod-shaped bacteria, proteins like MreB form spiral filaments beneath the cell membrane. These filaments act as tracks, directing the machinery that synthesizes peptidoglycan and ensuring the cell elongates into a rod. Bacteria that naturally lack the MreB gene are spherical.
Another cytoskeletal protein, FtsZ, directs cell division in most bacteria. It assembles into a ring at the center of the cell, marking the division site. This FtsZ ring constricts, guiding the synthesis of a new cell wall that separates the parent cell into two daughters. This process ensures the shape is maintained through generations.
Functional Significance of Shape
A bacterium’s shape is closely tied to its survival, influencing functions like nutrient acquisition, movement, and attachment to surfaces. These traits are shaped by evolutionary pressures that optimize a bacterium for its specific environment.
Elongated shapes like rods and spirals have a higher surface-area-to-volume ratio than spherical cocci. This is an advantage in nutrient-scarce environments, allowing for more efficient uptake. Conversely, a sphere’s lower ratio makes it more resistant to drying out and other physical stresses. Some bacteria even develop thin extensions called prosthecae to further increase their surface area.
Motility is another function influenced by shape. Spiral-shaped bacteria, particularly spirochetes, are efficient at moving through viscous liquids like mucus. Their corkscrew motion allows them to bore through difficult environments. The curved shape of Vibrio species can cause them to swim in circles near surfaces, helping them remain in nutrient-rich areas.
Shape also influences a bacterium’s ability to attach to surfaces, including host tissues, which is a preliminary step for forming biofilms or causing infections. A bacterium’s form can also serve as a defense mechanism. Elongated or filamentous shapes are more difficult for predatory immune cells (phagocytes) to engulf.