The gram staining procedure is not appropriate for staining mycobacteria, but the acid-fast stain is appropriate for these organisms. Why the gram staining procedure is not appropriate for staining mycobacteria is because mycobacteria as acid-fast bacteria have a high content of mycolic acids in their cell wall. Hence their cell wall tends to have characteristics of both gram-positive and gram-negative bacteria. The thickness of their cell wall together with the mycolic acids makes these organisms not respond well to staining agents. For these bacteria, acid-fast stains are used to differentiate them with a staining technique called Ziehl–Neelsen staining. This bacteriological stain was first introduced by Paul Ehrlich. In this staining technique, acid-fast bacteria like the mycobacteria will stain red whereas the nonacid fast bacteria will stain blue or green.