What is a Gram stain?

A Gram stain is a laboratory test that checks for bacteria at the site of a suspected infection or in certain bodily fluids. A medical laboratory scientist processes the Gram stain, which gives relatively quick results, so healthcare providers can know if bacteria are present, and, if so, the general type(s). This can help guide further identification tests and treatment options.

Bacteria are a large group of one-celled organisms. They can live in different places in your body and on your skin. While some types of bacteria are harmless or even beneficial, others can cause infections and disease. A Gram stain helps diagnose harmful bacteria.

Under a Gram stain, different kinds of bacteria change one of two sets of colors (pink to red or purple to blue) under a special series of stains and are categorized as “gram-negative” or “gram-positive,” accordingly. Gram staining works by differentiating bacteria by the chemical and physical properties of their cell walls.

However, not all forms of bacteria can be tested using the Gram stain method, and Gram stains don’t usually provide a diagnosis alone. Instead, they help to broadly determine the type of bacteria.

Gram staining is an essential staining technique in microbiology that scientists have used for hundreds of years. It’s named after Danish bacteriologist Hans Christian Gram, who first introduced it in 1882, mainly to identify organisms causing pneumonia.

What are gram-positive and gram-negative bacteria?

If bacteria are present on a Gram stain slide, a medical laboratory scientist classifies them as gram-negative or gram-positive based on which color they turn under a series of stains. There are also gram-variable bacteria, which means they stain irregularly, and there’s a mix of pink- and purple-colored bacteria.

It’s important to note that gram-negative doesn’t mean there are no bacteria, and “positive” and “negative” don’t mean “good” and “bad” in this case.

Gram-positive organisms

Gram-positive bacteria have cell walls that contain thick layers of peptidoglycan, a substance that forms the cell walls of many bacteria. The peptidoglycan forms about 90% of the cell wall in gram-positive bacteria. This causes them to appear blue to purple under a Gram stain.

Gram-positive organisms include:

• Staphylococcus species.
• Streptococcus species.
• Corynebacterium species.
• Clostridium species.
• Listeria species.

Gram-negative organisms

Gram-negative bacteria have cell walls with thin layers of peptidoglycan (10% of the cell wall) and high lipid (fatty acid) content. This causes them to appear red to pink under a Gram stain.

Gram-negative organisms include:

• Neisseria gonorrheae and Neisseria meningitides.
• Moraxella species.
• Escherichia coli (E. coli).
• Pseudomonas species.
• Proteus species.
• Klebsiella species.

What’s the difference between a Gram stain and a bacteria culture?

Healthcare providers use both Gram stains and bacteria cultures if they suspect you have a bacterial infection. However, these tests provide different information.

Providers usually request bacteria culture tests alongside Gram stain tests. In a bacteria culture test, a medical laboratory scientist takes cells from the sample that’s also for the Gram stain and puts it in a special environment to encourage cell growth. Results are often available within a few days, but some types of bacteria grow slowly, which may take several days or longer. Once the cells have multiplied and grown enough, a medical laboratory scientist examines the cultured bacteria to determine their exact identity.