An 5-minute read.
During your skin care career and education, you probably heard the terms Gram-positive or Gram-negative bacteria and wondered what these terms mean. Bacteria are classified in various ways based on different criteria, such as mobility, temperature preferences, or oxygen requirements. In our courses, the category of bacteria is introduced according to their shape. However, Gram staining appears in literature enough that providing some high-level insight into this method deserves a little bit of the spotlight.
Gram staining, introduced by Danish bacteriologist Hans Christian Gram in 1884, was used to identify pneumonia-causing organisms. The Gram staining process differentiates bacteria into two groups based on their cell wall structure.
The Gram stain process starts with a slide sample of bacteria stained with a crystal violet dye. Next, iodine is added to ensure the dye sticks to the bacteria’s cell walls. A decolorizer is then applied to wash the sample. This step highlights the difference between Gram-positive and Gram-negative bacteria. Gram-positive bacteria have thick cell walls that hold the dye and are colored purple. In contrast, Gram-negative bacteria have thinner cell walls, lose the dye when the decolorizer is applied, and turn pink.[1]
Gram-positive bacteria possess a thick cell wall called peptidoglycan (pep-TIE-doh-GLY-kan), a polymer of sugars and amino acids.[2] This thick layer is responsible for the characteristic staining pattern of Gram-positive bacteria, which appear purple or blue under a microscope after the Gram stain test.
Key points:
- Cell Wall Composition: Thick peptidoglycan layer
- Staining Color (Gram Stain Test): Purple or blue
- Examples: Staphylococcus, Streptococcus
In contrast, Gram-negative bacteria have a thin peptidoglycan layer. However, a significant feature of their cell wall is the outer membrane, which consists of lipids and proteins called lipopolysaccharides (li-poh-POH-lee-SAK-uh-rides).[2] This thin structure influences their reaction to Gram staining, resulting in a different coloration than Gram-positive bacteria. This difference affects their staining pattern, making Gram-negative bacteria appear pink or red after the Gram stain test.
Key points:
- Cell Wall Composition: Thin peptidoglycan layer and an outer membrane containing lipopolysaccharides
- Staining Color (Gram Stain Test): Pink or red
- Examples: E. coli, Salmonella
Being knowledgeable of the Gram staining technique is another tool you can use in microbiology discussions to help understand topics concerning bacteria. So next time you come across the terms Gram stain, Gram positive, or Gram negative, you'll have a little insight into how this categorization technique works.
Learn More: Gram Staining
References
1. Tripathi N, Sapra A. Gram Staining. [Updated 2023 Aug 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562156/
2. Salton MRJ, Kim KS. Structure. In: Baron S, editor. Medical Microbiology. 4th ed. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 2. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8477/
