Gram Stain
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A gram stain test involves placing a sample (such as fluid, tissue, or a swab) onto a microscope slide and applying special stains so microorganisms can be seen under a microscope. It is used to rapidly detect and help identify bacteria (and sometimes fungi) in suspected infections, particularly in normally sterile body sites.
Why get tested?
To identify the cause of a bacterial infection so appropriate treatment can be given.
When to get tested?
Seek advice from your doctor if you have an area of inflamed, red or a painful skin, a wound which does not seem to be healing or any other concerns which might lead you to believe you may have a bacterial infection.
Sample required?
A skin swab or fluid/pus (if present) from the site of infection.
Test preparation needed?
None
What is being tested?
A Gram stain is used to determine if bacteria are present in an area of the body that is normally sterile, such as spinal fluid. A sample from the infected area is smeared on a glass slide and allowed to dry. A series of stains are applied and then the stained slide is examined under a microscope where bacteria appear either purple (gram positive) or pink (gram negative). The test is named after Dr. Christian Gram, who invented the process.
A Gram stain can predict the type of bacteria causing an infection, such as pneumococcal pneumonia or a staphylococcal abscess. Viruses cannot be seen with a Gram stain since they lack the cell wall, which takes up the stain.
How is the sample collected for testing?
Usually, samples are collected using sterile swabs to obtain cells or exudate (fluid or pus containing cellular matter) at the site of suspected infection. Body fluids may be collected in sterile containers or by needle and syringe.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
Common questions
This is one of the most commonly performed microbiology tests used to identify the cause of an infection. Often, 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 to be completed. Absence or presence of white blood cells in the Gram stain can determine the adequacy of the specimen.
A Gram stain is performed when a bacterial infection is suspected.
Determining the Gram status of an organism allows the doctor to select an appropriate antibiotic before culture results are available. Gram stains cannot predict the organism’s identity or susceptibility to antibiotics. Only the culture of the material can supply this information.
Bacterial infections should not be ignored, even if the symptoms are mild. You should consult your doctor so treatment, if necessary, can begin promptly, and the spread and severity of the disease can be limited. If left untreated, bacterial infections can migrate throughout the body and cause tissue and organ damage.
Antibiotics are the main treatment of bacterial infections.
In addition to colour, which indicates whether the organism is Gram positive or Gram negative, the shape of the organism (such as rod-shaped) and the formation of groups of organisms is informative. For example, staphylococcus (staph) bacteria are found in clusters while streptococcus (strep) bacteria are found in chains.
The Gram stain is usually done in conjunction with a culture. This allows for more exact identification of the bacteria causing the infection and determination of the most appropriate antibiotic.
No. This test must be performed in an accredited laboratory by trained laboratory professionals.