Acid-fast bacteria appear what color microscopically?

Acid-fast bacteria, such as those belonging to the Mycobacterium genus (most notably Mycobacterium tuberculosis), are characterized by their unique cell wall structure, which contains mycolic acid. This property influences the staining procedure used to visualize these bacteria.

In a Ziehl-Neelsen stain or other acid-fast staining techniques, acid-fast organisms retain the primary stain (typically carbol fuchsin) even after decolorization with acid-alcohol. This retention results in these bacteria appearing red under the microscope. The bright red color is easily distinguishable against a contrasting blue or green background, which represents non-acid-fast bacteria that have taken up a secondary counterstain.

Understanding this staining phenomenon is critical for microbiologists and medical technologists, as it aids in the identification and diagnosis of infections caused by acid-fast bacteria.