Acid-fast bacteria are gram-positive in terms of structure because they contain peptidoglycan in the cell wall. However, they stain poorly with Gram stain procedure, appearing weakly Gram-positive. The reason for weak Gram staining is that the outer membrane of the acid-fast cell wall contains large amounts of glycolipids, especially mycolic acids that in the genus Mycobacterium, make up approximately 60% of the acid-fast cell wall. Such a high lipid content makes Gram dyes hard to penetrating into the cell and further stain the peptidoglycan.