Borrelia, a genus of bacteria known for causing conditions such as Lyme disease (primarily Borrelia burgdorferi), is often classified using Gram staining. This laboratory technique helps reveal fundamental aspects of a bacterium’s biology and structure.
Borrelia’s Gram Classification
Borrelia is consistently classified as Gram-negative. This applies to all species within the genus, including Borrelia burgdorferi. Its Gram-negative status is determined by the distinct composition and arrangement of its cell wall components. As a member of the spirochete phylum, Borrelia possesses a unique spiral or helical shape, contributing to its characteristic movement.
The Science of Gram Staining
Gram staining differentiates bacteria into Gram-positive and Gram-negative groups. This technique relies on differences in the bacterial cell wall structure to either retain or lose a primary stain during a decolorization step. The process begins by applying crystal violet, a purple dye, which stains all bacterial cells. Next, an iodine solution is added, forming a large complex with the crystal violet inside the cells.
The crucial differentiating step involves washing the cells with an alcohol-based decolorizer. Gram-positive bacteria, with their thick, mesh-like peptidoglycan layer, retain the crystal violet-iodine complex and remain purple. The decolorizer dehydrates the peptidoglycan, shrinking its pores and trapping the dye complex within the cell. In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer and an outer membrane. The alcohol dissolves this outer membrane, allowing the crystal violet-iodine complex to wash out, leaving the Gram-negative cells colorless.
Finally, a counterstain, typically safranin, is applied. Gram-positive cells remain purple, while decolorized Gram-negative cells take up the safranin and appear pink or red under a microscope. This color difference allows for visual differentiation of bacterial types.
Borrelia’s Unique Cell Structure
Borrelia’s Gram-negative classification stems from its distinctive cell envelope, which includes an inner cytoplasmic membrane, a thin peptidoglycan layer, and an outer membrane. The thin peptidoglycan layer, which is between the inner and outer membranes, is not robust enough to retain the crystal violet stain during the decolorization step of Gram staining. The presence of the outer membrane also contributes to the inability to retain the stain.
A characteristic feature of Borrelia, like other spirochetes, is its spiral shape and the location of its flagella. Instead of extending freely from the cell surface, Borrelia’s flagella, known as endoflagella or axial filaments, are located within the periplasmic space, which is the region between the inner and outer membranes. These internal flagella enable a unique corkscrew-like motility that aids the bacterium in moving through viscous environments and host tissues. While Borrelia possesses an outer membrane, it notably lacks lipopolysaccharide (LPS), a molecule commonly found in the outer membrane of many other Gram-negative bacteria. Instead, its outer membrane contains numerous lipoproteins that interact with the host.
Significance of Gram-Negative Status
The Gram-negative classification of Borrelia provides important insights into its fundamental biology and how it interacts with its environment and host organisms. The outer membrane, a defining feature of Gram-negative bacteria, serves as a protective barrier against various external factors. This membrane can help shield the bacterium from certain environmental stresses or components of the host’s immune system.
Understanding these structural attributes helps researchers investigate how Borrelia establishes infection and evades host defenses. This knowledge is important for ongoing research into the bacterium’s behavior and its relationship with its hosts.