Lyme disease is a condition transmitted through tick bites, affecting thousands annually. This illness stems from specific bacterial cells, primarily Borrelia burgdorferi in North America and Eurasia. Other Borrelia species, such as B. mayonii, B. afzelii, and B. garinii, are also involved depending on the geographic location.
The Bacterium Behind Lyme Disease
Borrelia burgdorferi is a gram-negative bacterium with a distinctive spiral, or spirochete, morphology. Its cell envelope includes an inner protoplasmic layer, a cell membrane, a thin peptidoglycan layer, and an outer membrane composed largely of lipids and proteins.
The bacterium measures approximately 0.3 micrometers in width and 5 to 20 micrometers in length. It possesses internal flagella, located between its inner and outer membranes. These flagella allow the bacterium to move with a corkscrew-like motion, helping it navigate through host tissues.
The outer surface of B. burgdorferi is covered with outer surface proteins (Osp), including OspA, OspB, and OspC. These proteins change expression based on the bacterium’s location and infection stage. For example, OspA and OspB are abundant in unfed ticks, but their expression decreases, and OspC expression increases once the tick begins feeding.
How the Bacterium Infects the Body
Infection with Borrelia burgdorferi begins when an infected Ixodes tick bites a human. The bacterium resides in the tick’s midgut and migrates to the salivary glands when the tick begins feeding. During this process, B. burgdorferi adapts to the mammalian environment by altering its surface proteins.
Once transmitted through the tick’s saliva, the bacteria adhere to host tissues and multiply at the bite site. They then disseminate from the initial skin lesion, often causing a characteristic expanding rash known as erythema migrans.
Following local replication, Borrelia burgdorferi can spread throughout the body via the bloodstream and lymphatic system, affecting various organs and tissues. This dissemination can lead to symptoms as the bacterium colonizes different sites, including the skin, joints, nervous system, and heart. The resulting inflammation and cellular damage, rather than toxins produced by the bacteria, are believed to be the primary causes of Lyme disease symptoms.
The Bacterium’s Evasion Tactics
Borrelia burgdorferi employs strategies to evade the host’s immune system, contributing to its persistence. One tactic is antigenic variation, where the bacterium changes its surface proteins to avoid antibody detection. The vlsE locus on a linear plasmid is a site for this recombination, allowing the bacterium to continually alter its outer surface lipoprotein.
The bacterium interferes with the host’s complement system, a part of the innate immune response. B. burgdorferi acquires human complement regulators, such as Factor H and Factor H-like protein 1, onto its surface. This acquisition inhibits the activation of the complement pathway, preventing components that would lead to bacterial destruction.
B. burgdorferi can modulate or suppress host immune responses. It can inhibit germinal centers, which are important for generating long-lived antibody responses. This interference can lead to a lack of sustained antibody affinity maturation, allowing the bacterium to survive despite the host’s immune activation.
Targeting the Bacterium with Treatment
Treating Lyme disease primarily involves antibiotics, which interfere with bacterial processes essential for Borrelia burgdorferi’s survival. Common antibiotic classes include tetracyclines, penicillin-based drugs, and cephalosporins. These medications target different aspects of bacterial cell function to eliminate the infection.
Tetracyclines, such as doxycycline, inhibit bacterial protein synthesis by binding to the 30S ribosomal subunit, preventing amino-acyl tRNA from attaching to the ribosome. This disrupts the bacterium’s ability to produce necessary proteins for growth and replication. Penicillin-based drugs and cephalosporins, like ceftriaxone, primarily inhibit bacterial cell wall synthesis. They interfere with the peptidoglycan layer, a structural component of the bacterial cell wall, leading to cell death.