When a healthcare practitioner suspects that you have a bacterial (or sometimes fungal) infection; often whenever a culture is requested
A Gram stain is a laboratory procedure used to detect the presence of bacteria and sometimes fungi in a sample taken from the site of a suspected infection. It gives relatively quick results as to whether bacteria or fungi are present and, if so, the general type(s).
The Gram stain involves applying a sample from the infected area onto a glass slide and allowing it to dry. The slide is then treated with a special stain and examined under a microscope by a trained laboratorian. Any bacteria that may be present are categorized by color and shape during the microscopic evaluation:
- Color — typically bacteria may be either "Gram positive" (purple) or "Gram negative" (pink)
- Shape — the most common shapes include round (cocci) or rod-shaped (bacilli)
Additional information may be obtained by observing the groupings of the bacteria on the slide, such as cocci that are present singly, in pairs, in groups of four, in clusters or in chains, or bacilli that are thick, thin, short, long, or have enlarged spores on one end. Any bacteria that are present within white blood cells (intracellular) are also noted.
The Gram stain color and the bacterial shape give clues as to what bacteria might be causing the infection. One example of gram-positive cocci is Staphylococcus aureus, the bacteria associated with staph infections. An example of gram-negative bacteria is Escherichia coli, the cause of many urinary tract infections.
Though Gram stains are useful as initial tests for detecting and identifying general types of bacteria or fungi, results are usually considered preliminary. Results of a culture and/or other tests such as antigen, antibody, or molecular testing for particular types of bacteria are necessary to confirm a diagnosis. Sometimes, susceptibility testing is necessary to determine which antibiotic will be most effective in treating the infection.
How is the sample collected for testing?
Several different types of samples may be collected for Gram stains. Some samples are collected using sterile swabs to obtain cells or exudate at the site of a suspected infection. Other samples, such as urine or sputum, may be collected in a sterile container. Some body fluids may be collected by needle and syringe. A swab may be used to collect a sample of bacteria grown and isolated in a culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
A Gram stain is used, along with a culture of the material from an infected site, to identify the cause of a bacterial infection. The Gram stain provides preliminary results on whether bacteria are present and the general type, such as the shape and whether they are Gram-positive or Gram-negative.
Often, detecting the presence of bacteria and determining whether an infection is caused by a microbe that is Gram-positive or Gram-negative will be sufficient to allow a healthcare practitioner to prescribe treatment with an appropriate antibiotic while waiting for more specific tests, such as a culture, to be completed. Absence or presence of white blood cells in the Gram stain can help establish that an adequate sample was obtained as white blood cells are frequently present with an infection.
A Gram stain may also be performed as part of the evaluation of a culture. When bacteria grow on/in a nutrient media in the laboratory, a Gram stain is performed to help determine the type of bacteria present and to help determine what other tests may need to be performed to definitively identify the cause of infection.
Fungi, including yeast, may also be detected with a Gram stain.
When is it ordered?
What does the test result mean?
A negative Gram stain is often reported as "no organism seen." This may mean that there is no bacterial infection present or that there were not enough bacteria present in the sample to be seen with the stain under a microscope.
Positive Gram stain results usually include a description of what was seen on the slide. This typically includes:
- Whether the bacteria are Gram-positive (purple) or Gram-negative (pink)
- Shape — round (cocci) or rods (bacilli)
- Size, relative quantity, and/or arrangement of the bacteria, if relevant
- Whether there are bacteria present within other cells (intracellular)
- Presence of red blood cells or white blood cells
- Fungi (in the form of yeasts or molds) may be seen on a Gram stain and are reported. Yeast may appear as single cells that may have buds, while molds may appear as a wide variety of plant-like branches called hyphae. Further testing may be performed to identify the specific type. (See Fungal Tests for more on this.)
This information, along with signs and symptoms and other clinical findings, will help the healthcare practitioner decide which treatment may be most effective, sometimes before culture results are available.
However, Gram stain results are usually considered preliminary, and results of a culture and/or other tests such as antigen and antibody testing for particular types of bacteria are necessary to confirm a diagnosis. Sometimes, susceptibility testing is necessary to determine which antibiotic will be most effective in treating the infection.
A few examples of some medically significant bacteria and what they look like on a Gram stain include:
- Gram-positive cocci—Staphylcoccus aureus (Staph aureus) can cause skin infections and toxic shock syndrome; Streptococcus pneumoniae can cause pneumonia.
- Gram-negative cocci—Neisseria meningitidis causes meningitis while Neisseria gonorrhoeae causes the sexually transmitted disease gonorrhea.
- Gram-positive bacilli—Bacillus anthracis (anthrax) can cause skin infections or pneumonia (also a bioterrorism agent); Listeria monocytogenes can cause foodborne illnesses.
- Gram-negative bacilli—Escherichia coli is a common cause of urinary tract infections.
Samples such as blood and cerebrospinal fluid are generally sterile. Bacteria are not usually present in these samples when there is no bacterial infection. Bacteria may initially be present in low numbers with an infection, and this type of sample may require extra processing in order to concentrate the bacteria so that they can be detected by a Gram stain.
Other body fluids and sites, such as sputum or skin, typically have cells and normal flora present in addition to any bacteria that are causing an infection. Gram stains on these types of samples require careful examination by a trained laboratorian to determine which cells may be normal and which may be due to an infection.
Is there anything else I should know?
Bacterial infections should not be ignored, even if the symptoms are mild. Prompt treatment can limit their spread and severity. If left untreated, bacterial infections can spread and may eventually cause tissue and organ damage.
Viruses cannot be detected with a Gram stain.
If a healthcare practitioner suspects that a person has a mycobacterial infection, such as tuberculosis, an AFB (acid-fast bacillus) smear and culture may be ordered on a sputum sample instead of a Gram stain and culture.
What are the treatment options for bacterial infections?
Why isn't a Gram stain performed on all samples sent for culture?
Gram stains are not helpful when the disease-causing bacteria (pathogen) cannot be differentiated from normal bacterial flora. For example, a Gram stain of a sample from the throat cannot tell the healthcare practitioner what bacteria are causing the infection because both the "good bacteria" and the "bad bacteria" look the same under the microscope.
What happens if my healthcare practitioner needs more information than the Gram stain provides?
The Gram stain is usually done in conjunction with a culture and may be followed by susceptibility testing. This allows for more exact identification of the bacteria causing the infection and determination of the most appropriate antibiotic. Several other tests are available to help make a diagnosis, including antigen and antibody testing, and molecular tests that detect the genetic material of the bacteria.
On This Site
Tests: Susceptibility Testing, Bacterial Wound Culture, Blood Culture, Body Fluid Analysis, CSF Analysis, Urine Culture, AFB Testing, Gonorrhea Testing, Stool Culture, Fungal Tests, Sputum Culture, Synovial Fluid Analysis, Pleural Fluid Analysis, Pericardial Fluid Analysis, Peritoneal Fluid Analysis
Conditions: Wound and Skin Infections, Staph Wound Infections, Meningitis and Encephalitis, Fungal Infections, PID, Septic Arthritis, Urinary Tract Infection, Pneumonia, Sepsis, Vaginitis and Vaginosis
Elsewhere On The Web
Microbial Life Educational Resources: Gram Staining
MedlinePlus Medical Encyclopedia: Bacterial Infections
Mayo Clinic: What is the difference between a bacterial infection and a viral infection?