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What is the Gram Stain?
What is the Gram Stain?
The Gram stain is a foundational stain in microbiology. Originally created in 1882 by Hans Christian Gram to better visualize the causative agent of pneumonia, the Gram stain is able to differentiate between gram positive and gram negative bacteria. The Gram stain is so influential because determining what type of bacteria (gram positive/negative) is in a sample, scientists and doctors can quickly proceed to gather more information and/or pursue treatment.
The Gram stain is a foundational stain in microbiology. Originally created in 1882 by Hans Christian Gram to better visualize the causative agent of pneumonia, the Gram stain is able to differentiate between gram positive and gram negative bacteria. The Gram stain is so influential because determining what type of bacteria (gram positive/negative) is in a sample, scientists and doctors can quickly proceed to gather more information and/or pursue treatment.
Tripathi N, Sapra A. Gram Staining. [Updated 2023 Aug 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562156/
Smith, A.C. and Hussey, M.A. (2005). Gram Stain Protocols. American Society for Microbiology. https://asm.org/getattachment/5c95a063-326b-4b2f-98ce-001de9a5ece3/gram-stain-protocol-2886.pdf
What's the difference between gram positive and gram negative bacteria?
What's the difference between gram positive and gram negative bacteria?
The main differences between gram positive and gram negative bacteria lay within their cell wall structure. Gram positive bacteria (Streptococcus, Staphylococcus, etc.) have one inner membrane and a thick cell wall outside the membrane. Gram negative bacteria (Cholera, Gonorrhea, E. coli, etc.) have outer and inner membranes, with a thin cell wall in between. Differences in cell surfaces mean that gram positive and gram negative bacteria interact differently with their environments.
Aryal, S. (2022). Gram Staining- Principle, Reagents, Procedure, Steps, Results. MicrobeNotes. https://microbenotes.com/gram-stain-principle-reagents-procedure-and-result-interpretation/
Safety:
Safety:
The reagents used in a Gram stain are flammable, toxic to aquatic life, a serious eye irritant, and organ toxin (central nervous system). When doing a gram stain, wear lab goggles, gloves, and lab-appropriate clothing. Working under a hood is strongly advised. Avoid inhaling the stain. If the stain comes into contact with your eyes, rinse continuously. Keep stain and stain components away from heat sources and open flames.
Aryal, S. (2022). Gram Staining- Principle, Reagents, Procedure, Steps, Results. MicrobeNotes. https://microbenotes.com/gram-stain-principle-reagents-procedure-and-result-interpretation/
Staining Analysis:
Staining Analysis:
Pink:
Gram-positive bacteria
Purple:
Gram-negative bacteria
Kumar, V. (2023). Bacterial Cell Walls. Microbiology Notes. https://microbiologynotes.org/bacterial-cell-walls/
Chambers J, Yarrarapu SNS, Mathai JK. Anthrax Infection. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. [Figure, Anthrax Bacillus anthracis, Gram-Positive rods...]Available from: https://www.ncbi.nlm.nih.gov/books/NBK535379/figure/article-19041.image.f3/
Sizar O, Leslie SW, Unakal CG. Gram-Positive Bacteria. [Updated 2023 May 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. [Figure, Gram Stain of Staphylococcus aureus Contributed by Scott Jones, MD] Available from: https://www.ncbi.nlm.nih.gov/books/NBK470553/figure/article-22388.image.f1/
Morphological changes induced by β-lactam antibiotics observed on Gram staining. Bitterman, R. et al.
Clinical Microbiology and Infection, Volume 23, Issue 1, 26
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