|Description:||Using a “dark field” microscopy technique, this photomicrograph revealed the presence of Treponema pallidum spirochetes, which are the bacterial agents responsible for causing syphilis.|
“A diagnosis of syphilis is confirmed by using dark field microscopy to demonstrate T. pallidum in material from suspected lesions, or regional lymph nodes (Creighton, 1990). A positive dark field result is an almost certain diagnosis of primary, secondary, or early congenital syphilis. In primary syphilis, the dark field examination may provide a means by which to identify the etiologic agent of syphilis and diagnose the disease before antibodies to T. pallidum can be detected.”
Principles of Dark field Microscopy
The standard bright field microscope can be equipped for dark field examination by replacing the bright field condenser with a dark field condenser. Illumination for dark field microscopy is obtained when light rays strike the object in the field at such an oblique angle that no direct rays enter the microscope objective, only the rays reflected from the object. Therefore, the object appears self-luminous against a dark background, hence the term dark field. When a fluid containing particles, including bacteria or treponemes, is placed on a slide, the oblique rays are reflected from the surfaces upward into the barrel of the microscope; these particles appear brightly illuminated against a black background.