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-negative bacteria have a complex wall structure than gram-positive bacteria, with multiple layers – in particular an additional outer lipopolysaccharide (LPS) layer. This means that they are not (as) susceptible to some of the antibiotics used to treat gram-positive infections.
- The ureido penicillins are perhaps the most commonly used antibiotics to treat gram-negative infections and include amoxicillin, ampicillin and piperacillin
- These drugs are often combined with a beta-lactamase inhibitor (in blue below) – to prevent the beta-lactam being broken down by the bacteria, such as:
- Tazocin – piperacillin + tazobactam
- Augmentin – amoxicillin + clavulinic acid
The LPS is also toxic to humans, and thus gram-negative infections can be more likely to result in sepsis and signs of septic shock.