Acid-fast bacteria appear what color microscopically?

Acid-fast bacteria, such as those from the genus Mycobacterium, exhibit a distinct red color when subjected to special staining techniques, like the Ziehl-Neelsen stain or the Kinyoun stain. This characteristic staining process involves using a strong dye called carbol fuchsin, which binds to the mycolic acids present in the cell walls of acid-fast bacteria. After applying heat and rinsing the slide with acid-alcohol, only the acid-fast organisms retain the red dye, resulting in a bright red appearance against the blue counterstain background.

The other colors mentioned do not pertain to acid-fast bacteria. For instance, in common staining methods, blue is typically associated with non-acid-fast organisms when using counterstains like methylene blue, while green is often indicative of other specific bacterial types, and purple can be linked to Gram-positive bacteria in Gram staining protocols. Hence, acid-fast bacteria's unique red coloration is a key feature that differentiates them from other bacteria during microscopic examination.