The difference in staining between gram-positive and gram-negative bacteria is due to a difference in cell walls.
- Gram-positive bacteria have a thick peptidoglycan layer which stains a vivid violet colour
- Gram-negative bacteria have only a thin peptidoglycan layer, which allows the violet stain to wash out with ethanol, and thus it washed away during the staining process, and are thus instead stained pink or red by the counterstain used in the staining process.
Gram staining – 140 years after its discovery – is still often the first and one of the useful methods of identifying bacteria.
The differentiation is particularly important as it helps to identify which types of antibiotics will be most effective against an organism – often much sooner (within a a couple of hours) than can be determined by culture and sensitivities – which can take up to 48 hours.
Gram-positive bacteria all have a thick peptidoglycan outer layer. This can be targeted by antibiotics – in particular penicillins and cephalosporins.
Many common infections are caused by Gram-positive bacteria, bacterially by staphylococcus and streptococcus:
- Staphylococci – cellulitis and wound infections
- Streptococci – impetigo, tonsillitis, Scarlett fever, pneumonia