Gram Stain

A Gram stain and culture of the material from an infected site are the most commonly performed microbiology tests used to identify the cause of an infection. Often, detecting the presence of microorganisms and determining whether an infection is caused by an organism that is Gram-positive or Gram-negative will be sufficient to allow a doctor 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.

A Gram stain may be performed as part of the bacterial culture when a bacterial infection is suspected. It is performed on the same sample as the culture, and the test results are reported out promptly to help guide treatment.

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 microorganisms 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) as well as their shape — round (cocci) or rods (bacilli). Sometimes size, relative quantity, and grouping of the bacteria, if relevant, are also reported. If there are bacteria present within other cells (intracellular), this will also be noted as well as; for example, the presence of red blood cells or white blood cells. This information, along with signs and symptoms and other clinical findings, will help the doctor decide which treatment may be most effective, often 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 antimicrobial will be most effective in treating the infection.

Some body sites are generally sterile, such as the blood and cerebrospinal fluid. Bacteria will not usually be present in these sites when there is not a bacterial infection. They may initially be present in low numbers with an infection, and a 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.

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.

If fungi (in the form of yeasts or molds) are present on the Gram stain, they are reported and further testing is pursued as necessary to identify the specific type. Yeast may appear as single cells that may have buds, while molds may appear as a wide variety of plant-like branches called hyphae.

If a doctor 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.