Maria Aguero-Rosenfeld, M.D., New York Medical College and Bellevue Hospital Center
Microbiologists share some of the concerns that patients have about the current diagnostic tests for Lyme borreliosis and other tick-borne diseases. The laboratory diagnostic challenges stem from the organism’s complex antigenic composition and its variation in expression depending on the environment where the organism is located. Borrelia burgdorferi has both linear and circular plasmids along with chromosomal DNA, and, in contrast to many other bacteria, a large portion of its genes are in plasmid DNA. The plasmid genes encode outer membrane components allowing the pathogen the flexibility of switching on and off antigens depending on the environment. This mechanism allows the pathogen to survive during the inactive tick stage (wintering) and to replicate during blood feeding on a suitable host.
Unlike other spirochetes, B. burgdorferi can be cultured in vitro. However, researchers are just beginning to understand the difference between immune responses to antigens expressed in vivo and antibodies detected using antigens from B. burgdorferi cultured in vitro. The Borrelia pathogen expresses some antigens as it first comes in contact with the host mammal leading to the early antibody response. Then, as more antigens are presented, the mammalian host develops the corresponding immune response in a sequential fashion. The intensity and type of antibodies developed depend on the duration of disease prior to antimicrobial treatment, the host immune system, and, perhaps, pathogenetic properties of the microorganism. Researchers have found that there are antigens expressed in vivo and others expressed in vitro. Therefore, the assays that are used to identify antibodies need to include those antigens expressed in vivo.
Two methods are available for directly detecting the presence of the pathogen in humans: culturing and polymerase chain reaction (PCR). Both of these have had mixed results in detecting B. burgdorferi (Table 7-1). Culturing spirochetes from a patient’s blood or synovial fluid has been difficult because the concentration of spirochetes is low. The key to this approach would be to optimize the culture methods that could allow scientists to detect these spirochetes efficiently. Microbiologists attain the best results from culture when using skin from a patient’s erythema migrans rash.
Modifications to the medium used to culture a patient’s blood have had mixed results. The sensitivity of the test depends on the volume of cultured blood and evidence of early disease dissemination, when the organism is most likely to be present in the bloodstream. However, only 40 percent