Gram Stain: Purpose, principle, procedure – complete guide

A Gram stain is a laboratory test commonly used to quickly diagnose bacterial infections. Healthcare providers often order this test with a bacteria culture to determine the type of bacterium causing the infection. 

The test involves checking for bacteria at the site of a suspected infection or in certain bodily fluids. 

A medical laboratory scientist processes the Gram stain, which provides relatively quick results. This allows healthcare providers to determine if bacteria are present and, if so, their general type(s). This information can then guide further identification tests and treatment options.

Background

Gram staining is an essential technique in microbiology that has been used for hundreds of years. The origin of this technique can be traced back to Danish bacteriologist Hans Christian Gram, who first introduced it in 1882. Gram initially developed the technique to identify organisms causing pneumonia. Since then, it has become widely used for diagnosing bacterial infections by differentiating bacteria based on their cell walls’ chemical and physical properties.

Bacteria are a large group of single-celled organisms that can live in various places in your body and on your skin. While some bacteria are harmless or beneficial to your health, others can cause infections and diseases.

A Gram stain is a laboratory test that helps diagnose harmful bacteria by differentiating them based on their cell walls’ chemical and physical properties. 

Under a Gram stain, different 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. This information can help healthcare providers determine the best course of treatment for bacterial infections.

Gram stain test principle

The Gram stain test principle involves the cell wall’s ability to retain the primary stain (crystal violet) after being fixed by a mordant (iodine) and decolorized with alcohol. The cell walls of Gram-positive bacteria have a thick layer of protein-sugar complexes called peptidoglycan and low lipid content. 

Decolorizing the cell causes this thick cell wall to dehydrate and shrink, closing the pores in the cell wall and preventing the stain from exiting the cell. As a result, the ethanol cannot remove the crystal violet-iodine complex bound to the thick layer of peptidoglycan of Gram-positive bacteria, and they appear blue or purple.

In contrast, Gram-negative bacteria have a thin peptidoglycan layer and a thick outer layer of lipids. When exposed to alcohol, the decolorizer dissolves the lipids in the cell walls, allowing the crystal violet-iodine complex to leach out of the cells. Then, when stained again with the counterstain (safranin), they take up the stain and appear red.

Sample collection

A healthcare provider can collect a sample for a Gram stain test in several ways. 

Sample collection methods include:

• Brushing or scraping tissue from the surface of a part of your body.  Sites collected by a swab with brushing and scraping may include your throat, nose, vagina, or rectum.
• Using fine-needle aspiration to draw a fluid sample from an area in your body. Areas that require the use of fine-needle aspiration include an abscess or other fluid-filled area. 
• Collecting samples directly into a sterile container. You can collect samples directly into a sterile container include urine, sputum (spit), or stool.

Once collected, the provider puts the sample in a sterile container and sends it to a laboratory for testing.

Gram staining procedure 

1. Prepare a bacterial smear: Transfer a small bacterial culture to a clean microscope slide using a sterile loop or needle. Spread the bacteria into a thin, even layer, allowing the smear to air dry.
2. Heat fix the smear: Once the smear is completely dry, pass the slide through the flame of a Bunsen burner several times to heat-fix the bacteria to the slide. This kills the bacteria and helps them adhere to the slide during staining.
3. Stain with crystal violet: Cover the smear with crystal violet stain and let it sit for 1 minute. Rinse the slide gently with water to remove excess stain.
4. Add iodine: Cover the smear with Gram’s iodine solution and let it sit for 1 minute. This acts as a mordant, forming a complex with the crystal violet that is more tightly bound to the bacterial cell wall. Rinse the slide gently with water.
5. Decolorize: Rinse the slide briefly with 95% ethanol or acetone, then immediately rinse it with water. This step differentiates between Gram-positive and Gram-negative bacteria by removing the crystal violet-iodine complex from Gram-negative bacteria while leaving it intact in Gram-positive bacteria.
6. Counterstain with safranin: Cover the smear with safranin and let it sit for 1 minute. Rinse the slide gently with water to remove excess stain.
7. Blot dry: Gently blot the slide dry with bibulous paper, careful not to rub or disturb the bacterial smear.

After completing these steps, you can observe the slide under a microscope using oil immersion. Gram-positive bacteria will appear purple, while Gram-negative bacteria will appear pink.

Gram stain examination

After the Gram staining process, a medical laboratory scientist examines the slide under a microscope. During the microscopic evaluation, they categorize any bacteria by color and shape.

Typically, gram-positive bacteria appear purple to blue, and gram-negative bacteria appear pink to red. The most common shapes include round (cocci) or rod-shaped (bacilli). The medical laboratory scientist also looks for additional sample characteristics by observing the groupings of the bacteria on the slide. Examples include cocci that are present singly, in pairs, in groups of four, in clusters, or chains, and bacilli that are thick, thin, short, long, or have enlarged spores on one end. The medical laboratory scientist then puts together a report and sends it to your healthcare provider.

What does white cells in gram stain mean?

The presence of white blood cells (WBCs) in a Gram stain of a clinical specimen, such as sputum or pus, can indicate an infection. White blood cells are part of the immune system and help fight infections by attacking and destroying invading pathogens.

When an infection is present, the body produces more white blood cells to fight the infection, which can be seen in a Gram stain of the infected tissue. In a Gram stain, white blood cells will appear as colorless or lightly stained cells, often with irregular shapes and multi-lobed nuclei. They can be differentiated from bacterial cells, which will appear either purple (Gram-positive) or pink (Gram-negative) depending on the characteristics of their cell walls.

White blood cells in a Gram stain can provide valuable information to healthcare providers when diagnosing and treating infections. However, it is important to note that white blood cells alone are not enough to diagnose an infection, and additional tests may be necessary to confirm a bacterial infection.

E coli gram stain

Escherichia coli, commonly known as E. coli, is a rod-shaped, Gram-negative bacterium commonly found in warm-blooded organisms’ lower intestines. While most strains of E. coli are harmless and even beneficial to their hosts, some strains can cause food poisoning and other illnesses.

When subjected to a Gram stain, E. coli cells appear pink under the microscope. This is because E. coli is a Gram-negative bacterium, meaning that it has a thin peptidoglycan layer in its cell wall and an outer membrane that allows the crystal violet-iodine complex to be washed away during the decolorization step of the Gram stain procedure. The cells are then counterstained with safranin, which gives them their characteristic pink color.

The ability to quickly and accurately identify bacterial species using techniques such as the Gram stain is crucial in both clinical and research settings. With E. coli, knowing whether a particular strain is present in a food or water sample can help prevent outbreaks of food poisoning and other illnesses.

Gram-positive organisms

Gram-positive bacteria are a type of bacteria that has a thick cell wall. When viewed under a microscope using the Gram staining technique, they appear blue or purple. This is because their cell wall retains the stain color used in the technique developed by Hans Christian Gram in 1884.

Gram-positive bacteria and their associated conditions

• Streptococcus pneumoniae – Bacterial pneumonia.
• Clostridium- Food poisoning
• Staphylococcus aureus– toxic shock syndrome, Bacterial pneumonia, Methicillin-resistant Staphylococcus aureus (MRSA), and bacterial pneumonia.
• Group A Streptococcus species- Strep throat infections.
• Listeria- Food poisoning.
• Clostridium butyricum and Clostridium botulinum– Botulism.

Gram-negative organisms

Gram-negative bacteria are a type of bacteria that do not retain the crystal violet stain used in the Gram staining method. They have a cell envelope composed of a thin peptidoglycan cell wall between an inner cytoplasmic cell membrane and a bacterial outer membrane.

Gram-negative bacteria and their associated conditions

• Escherichia coli – food poisoning, urinary tract infections, and food poisoning.
• Legionella pneumoniae–  Legionnaires’ disease.
• Salmonella – typhoid fever
• Klebsiella wound infections, bacterial meningitis, bloodstream infections, pneumonia.
• Neisseria gonorrheae– Gonorrhea.
• Proteus- Urinary tract infections (UTI).
• Morganella morganii– pericarditis, UTIs, pneumonia, wound infections, sepsis, central nervous system infections.
• Aeromonas- joint infections, osteomyelitis, bacterial meningitis.

Limitations

While Gram staining is a useful technique for diagnosing bacterial infections, it has limitations. Not all forms of bacteria can be tested using the Gram stain method. 

Gram stains rarely provide a diagnosis alone. Instead, they broadly help to determine the type of bacteria present. Further identification tests and treatment options may be necessary to diagnose and treat bacterial infections. Healthcare providers must consider these limitations when using Gram stains to diagnose bacterial infections.

Gram stain vs. bacteria culture

Gram stains and bacteria cultures are two laboratory tests that healthcare providers used to diagnose bacterial infections. While both tests provide valuable information, they have some key differences. A Gram stain involves applying a stain to a sample and looking at it under a microscope to determine if bacteria are present. If bacteria are present, the medical laboratory scientist can see if they’re gram-negative or gram-positive based on their color under the stain. In contrast, a bacteria culture test involves taking cells from the same sample used for the Gram stain and putting them in a special environment to encourage cell growth. Once the cells have multiplied and grown enough, a medical laboratory scientist examines the cultured bacteria to determine their identity.

One advantage of the Gram method is that they provide quicker results than bacteria cultures. However, in most cases, it can’t diagnose the specific type of bacterium causing the infection. Bacteria cultures can provide more detailed information about the type of bacterium present but may take longer to produce results. Healthcare providers often use both tests together to diagnose bacterial infections.4

Conclusion

Gram staining is an important and useful technique for diagnosing bacterial infections. By differentiating bacteria based on their cell walls’ chemical and physical properties, healthcare providers can determine if bacteria are present and their general type (s). This information can then guide further identification tests and treatment options. While Gram staining has limitations, it remains a valuable tool for diagnosing and treating bacterial infections.

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