Physical Traits of Bacteria

Furthermore, many bacteria have flagella, which are long, whiplike appendages, or cilia, which are short hairlike protrusions. Other bacteria are atrichous, meaning they have neither cilia nor flagella. These characteristics also aid in classification. The flagellum consists of three main parts: a hook, a motor, and a filament. The filament, a long, thin structure, is the part that is most visible outside the bacterial cell. The hook is an L-shaped structure that connects the filament to the body of the bacterium. The motor, embedded in the cell wall, turns the hook, which in turn causes the filament to whip back and forth, generating motion for the cell. Bacteria are also identified by a Gram stain, a method of staining used to differentiate types of bacteria based on cell wall structure. The Gram stain was developed in 1884 by Danish scientist Hans Christian Gram (1853–1938). The stain differentiates bacteria based on the makeup of their cell walls. The bacteria are washed first in a purple stain called crystal violet, then in a pink stain called safranin. Gram-positive bacteria contain a thick mesh of polymer chains of sugar and protein molecules called peptidoglycan in their cell walls, which retains both stains, but the crystal violet masks the safranin and thus stains purple. The gram-negative bacteria contain considerably less peptidoglycan, so they retain only the safranin and stain pink. Gram staining is frequently used as a first step in identifying a bacterium. Some bacteria are difficult to definitively classify using the Gram stain. However, most bacteria are frequently referenced according to their Gram stain classification (gram-negative or gram-positive).