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Treponema pallidum

Treponema pallidum is a a gram-negative spirochete bacteria. Subspecies of this bacterium cause treponemal diseases, such as syphilis.
Rabbit polyclonal antibodies are available for IHC use and tissue sections generally do not require heat-induced epitope retrieval (HEIR) or enzyme retrieval. The author recommends an alkaline phosphatase (AP) detection system, as skin biopsies are often submitted for IHC testing and melanin pigment can be problematic. This particular bacterium is easier to see with a fast red (FR) chromogen, especially with low yield infections. The use of a metal enhancement solution, such as 1% copper sulfate, will enhance and darken the reaction of chromogenic substrate DAB when a horseradish peroxidase (HRP) labeled detection system is used for demonstration of T. pallidum.
In the top image, T. pallidum (spirochetes) are demonstrated in a cutaneous biopsy from an HIV patient with epidermal ulcerations and dermal lymphohistiocytic inflammation. DAB chromogen with metal enhancement is utilized. In the lower image, FR chromogen is used in conjunction with the AP detection system.