Borrelia burgdorferi is the bacterial species responsible for Lyme disease, which is the most common vector-borne illness in the Northern Hemisphere. This organism belongs to the Spirochaetes phylum, a group of bacteria defined by their unique physical characteristics and cellular organization. The spirochete’s distinct structure allows it to survive in two vastly different environments: the tick vector and the mammalian host. This unique cellular arrangement, from its specialized cell envelope to its complex genetic material, dictates how the bacterium interacts with its surroundings and causes infection. This article examines the structural and cellular characteristics that make B. burgdorferi a persistent pathogen.
Overall Morphology and Classification
Borrelia burgdorferi exhibits a distinctive morphology, classifying it as a spirochete, which means “coiled hair” in Greek. Its physical shape is helical and highly flexible, often described as a flat-wave or corkscrew. This serpentine form allows the bacterium to contort its body, a necessary trait for navigating dense connective tissues in the host. The organism is relatively long and thin, typically measuring about 0.3 micrometers (µm) in diameter and ranging from 10 to 30 µm in length. This long, slender body is the protoplasmic cylinder, which contains the cell’s internal components. The classification within the phylum Spirochaetota is defined by this characteristic coiling and the location of its flagella. The mechanical interaction between the cell cylinder and the internal flagella is what generates its unique flat-wave appearance and movement.
The Specialized Cell Envelope
The cell envelope of B. burgdorferi shares similarities with Gram-negative bacteria but is atypical, consisting of an inner membrane, a thin peptidoglycan layer, a periplasmic space, and an outer membrane. The inner, or cytoplasmic, membrane encloses the protoplasmic cylinder and serves as the primary barrier regulating the transport of nutrients and waste. This membrane is the anchor point for the motors that power the bacterium’s unique motility mechanism. The peptidoglycan layer is unusually thin, which contributes to the flexibility of the cell body, allowing the spirochete to bend and twist. The peptidoglycan is located between the inner and outer membranes within the periplasmic space. A key feature is the distinct composition of the outer membrane, which lacks the classical Lipopolysaccharide (LPS) found in most Gram-negative bacteria. Instead, the outer membrane is heavily decorated with lipoproteins, including Outer Surface Proteins (Osps). These Osps are exchanged or modified depending on whether the bacterium is in the tick gut or the mammalian bloodstream, facilitating immune evasion and host adaptation. The periplasmic space also houses the entire flagellar apparatus, known as endoflagella or axial filaments, completely enclosed by the outer membrane.
Mechanism of Motility
The movement of B. burgdorferi is powered by its endoflagella, a unique arrangement that distinguishes it from bacteria with external flagella. These flagella are entirely confined within the periplasmic space, running along the length of the cell body. Typically, a bundle of seven to eleven flagellar filaments is anchored at each end of the bacterium, with the bundles overlapping in the middle of the cell. The flagella are attached to rotary motors embedded in the inner membrane, similar to the motors in other bacteria. The rotation of the endoflagella generates torque that causes the entire flexible protoplasmic cylinder to twist and coil. This twisting force results in a propagating wave that moves along the cell body, driving the spirochete forward with a characteristic corkscrew motion. This type of motility is highly effective for movement through viscous, gel-like environments, such as the dense extracellular matrix and connective tissues of the mammalian host.
Genetic Organization
The genome of B. burgdorferi is highly segmented and complex among bacterial species. Unlike most bacteria, which possess a single, circular chromosome, B. burgdorferi has a single linear chromosome, typically around 910 to 1000 kilobase pairs (kb) in size. The linear ends of this chromosome are capped with covalently closed telomeres, a structural feature commonly associated with eukaryotic chromosomes. Accompanying this linear chromosome is a large collection of accessory genetic elements. The genome includes over twenty linear and circular plasmids, which vary in size from approximately 5 to 62 kb. These plasmids account for a significant portion of the total genetic material and are not merely dispensable elements. The genes necessary for survival and adaptation within the host and vector environments are often encoded on these multiple plasmids. For example, the genes responsible for the Outer Surface Proteins, like OspC, involved in early mammalian infection, are frequently found on these accessory replicons. This segmented genome structure allows the bacterium flexibility, enabling it to rapidly adapt its surface protein expression in response to environmental changes encountered during its life cycle.