Treponema pallidum can be best identified using

The dark-field microscope is a light microscope in which the lighting system has been modified to reach the specimen from the sides only. This creates a "dark field" that contrasts against the highlighted edge of the specimens. Resolution by dark-field microscopy is quite high. Thus, this techniquehas been paicularly useful for observing organisms such as Treponema pallidum, a spirochete that is smaller than 0.2 mm in diameter and therefore cannot be observed with a brightfield or phase contrast microscope.Ref: Jawetz, Melnick, & Adelberg's Medical Microbiology; Twenty-Seventh Edition; Chapter 2; Cell Structure