AAT Bioquest

Are acid-fast bacteria gram positive or gram negative?

Posted June 1, 2020


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.

Additional resources

MycoLight™ Live Bacteria Fluorescence Imaging Kit

MycoLight™ Flow Cytometric Live Bacteria Assay Kit

Madison, B. M. (2001). Application of stains in clinical microbiology. Biotechnic & Histochemistry, 76(3), 119-125.

Almand, E. A., Moore, M. D., & Jaykus, L. A. (2017). Virus-bacteria interactions: an emerging topic in human infection. Viruses, 9(3), 58.