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Gram's Stain Does Not Cross the Bacterial Cytoplasmic Membrane
Michael J Wilhelm1, Joel B Sheffield2, Mohammad Sharifian Gh1
1†Department of Chemistry, Temple University, 1901 N. 13th Street, Philadelphia, Pennsylvania 19122, United States.
ACS Chemical Biology
|April 17, 2015
Summary
Gram staining
Area of Science:
- Microbiology
- Cell Biology
- Biophysics
Background:
- The Gram stain is a cornerstone of bacterial identification.
- Its molecular mechanism has been widely accepted for over a century.
- Current understanding of dye uptake in Gram staining is largely unverified.
Purpose of the Study:
- To re-evaluate the molecular mechanism of Gram staining.
- To quantitatively characterize crystal-violet uptake in bacteria in real-time.
- To challenge the established model of dye traversal.
Main Methods:
- Utilized complementary time-resolved methods: second-harmonic light-scattering and bright-field transmission microscopy.
- Focused on membrane-specific, quantitative characterization of crystal-violet (CV) uptake.
- Observed bacterial interactions with crystal-violet during the staining procedure.
Main Results:
- Crystal-violet (CV) is unable to traverse the bacterial cytoplasmic membrane (CM).
- CV is kinetically trapped within the peptidoglycan mesh (PM) on the timescale of the Gram stain procedure.
- Observations contradict the prevailing model of CV diffusion into the bacterial cytosol.
Conclusions:
- The established mechanism for Gram staining is incorrect.
- Crystal-violet's unique interaction with the peptidoglycan mesh explains its efficacy as a Gram stain.
- This finding necessitates a re-evaluation of bacterial cell envelope permeability studies.
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