The Gram stain is a common technique that categorizes bacteria into Gram-positive and Gram-negative groups based on their cell wall structure. It can quickly and efficiently confirm the presence and initiate the identification of bacteria.
The staining process takes only a few minutes and involves adding a primary stain, Crystal Violet, that is retained by Gram-positive bacteria but not by Gram-negative bacteria. Bacteria with a thick peptidoglycan layer hold the stain, turning purple. These are termed gram-positive and generally cause strep and staph infections. Bacteria with a thin peptidoglycan layer and outer membrane do not hold the stain and appear pink. These are gram-negative and generally more resistant to antibiotics.
Knowing whether a bacterium is Gram-positive or Gram-negative can aid in selecting the initial antibiotic for treatment.
While the Gram stain cannot identify individual bacterial species within a group, it remains a valuable tool for quickly detecting bacterial presence.
Figure 1: Up - Purple, Gram-positive bacteria, after a gram stain, as viewed with a light microscope. Down - Pink, Gram-negative bacteria, after a gram stain, as viewed with a light microscope.