Answer
The reason why Treponema pallidum is the only bacterium that is displayed in the FTA-ABS test is because the fluorochrome used has been combined with a treponemal antibody. The treponemal antibody is specific to Treponema bacteria, and will only attach to that bacteria. Thus, when the sample is illuminated with ultraviolet light, only the Treponema bacteria will be displayed.
Work Step by Step
1. Immunofluorescence works by chemically combining a fluorochrome with a specific antibody.
2. An antibody will only attach to the antigen (e.g. bacteria) that it is specific to.
3. In the example provided, the antibody that was combined with the fluorochrome was a treponemal antibody. This is specific to Treponema bacteria.
4. Thus, when immunofluorescence was performed, only the Treponema bacteria was illuminated with ultraviolet light.
5. Any other bacteria or antigen that was present in the sample would not have had the treponemal antibodies attached, and thus would not have been displayed.
