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EDITORIAL
Classic Spotlight: How the Gram Stain Works
George A. O’Toole
Department of Microbiology and Immunology, Geisel School of Medicine at Dartmouth, Hanover, New Hampshire, USA
F rom the teaching laboratory to the clinical microbiology lab-
oratory, one of the best-known tools to study microbes is the
Gram stain. This stain, originally developed to help distinguish
tute of iodine to directly visualize the crystal violet precipitate via
electron microscopy. This work confirmed the differential reten-
tion of the crystal violet precipitate and thus helped establish the
bacteria from host cells in tissue, has evolved as a key assay to help mechanism by which this ubiquitous assay functions. Further-
clinicians decide which antibiotics should be used to treat infec- more, the authors’ diagrams that model how Bacillus subtilis ver-
tions, given that Gram-positive and Gram-negative organisms sus Escherichia coli responds to the ethanol-destaining step are a
Downloaded from http://jb.asm.org/ on October 30, 2019 by guest
display differential responses to some classes of antimicrobial must-see for any student of microbiology.
agents. While not providing any phylogenetic information, the
purple versus pink staining mirrors the different biologies of these REFERENCE
two groups of organisms and reflects relationships between many 1. Beveridge TJ, Davies JA. 1983. Cellular responses of Bacillus subtilis and
Escherichia coli to the Gram stain. J Bacteriol 156:846 – 858.
clinically relevant microbes as defined by 16S rRNA gene-based
analyses.
Since its development in the late 1800s, evidence has accumu- Citation O’Toole GA. 2016. Classic spotlight: how the Gram stain works. J Bacteriol
lated that the Gram stain differentiates between microbes based 198:3128. doi:10.1128/JB.00726-16.
on their ultrastructure; typically the thick cell wall of Gram-posi- Address correspondence to georgeo@dartmouth.edu.
tive organisms is credited with helping to retain the purple crystal Copyright © 2016, American Society for Microbiology. All Rights Reserved.
violet-iodine complex upon treatment with ethanol. In a 1983 The views expressed in this Editorial do not necessarily reflect the views of the journal or
publication in the Journal of Bacteriology, Beveridge and Davies of ASM.
(1) used a variation of Gram stain with an electron-dense substi-
3128 jb.asm.org Journal of Bacteriology December 2016 Volume 198 Number 23
