Gram Stain Interpretation
The Gram stain is a quick test that is used by the microbiology laboratory to differentiate different types of bacteria based on the biochemical properties of their cell walls
Gram staining requires the bacterial culture to be smeared to a glass slide, a violet stain is then added to the slide which stains Gram-positive bacteria violet
The slide is then rinsed with a decolorizing agent which washes away the violet stain from Gram-negative bacteria
Following this step the slide is counterstained with a red dye which colors the Gram-negative bacteria pink
Have a thick cell wall which is made up of peptidoglycan (50-90% of cell wall)
This thick peptidoglycan layer allows the organisms to retain the violet stain and prevents it from being washed away during the decolorization step
- Have a thinner layer of peptidoglycan (about 10% of the cell wall), and they lose the violet stain during the decolorization step but retain the counter stain (pink)
- It helps us differentiate Gram-positive bacteria from Gram-negative bacteria which can help guide empiric treatment
The microbiology lab also uses the morphology and arrangement of bacteria to further differentiate bacteria:
Spherical morphology = cocci
- Arrangement: chains or clusters
- Gram-positive cocci in clusters usually indicate Staphylococcus species
- Gram-positive cocci in chains usually indicate Streptococcus or Enterococcus species
Rod-shaped morphology = bacilli
- Ex. Escherichia, Klebsiella, Enterobacter, Pseudomonas, etc.
- Some organisms are Gram-variable – they may stain either negative or positive, and some organisms are not susceptible to Gram staining, for example when there is no cell wall structure (Ex. Mycoplasma).
Any bacteria detected on Gram stain of a sterile site specimen should be considered significant, although a negative Gram stain does not exclude infection
Nonsterile specimens also generally contain human cells such as epithelial cells and white blood cells
- For example: a high-quality sputum specimen has high numbers of white cells and few epithelial cells; a low-quality specimen has high numbers of epithelial cells and low numbers of white cells. Low-quality sputum specimens may be rejected by the laboratory as unfit for culture
How the patient is doing. Clinical status of patient in addition to Gram stain results should guide therapy
Presence of a multidrug-resistant organism. Assess patient risk factors, recent antimicrobial therapy and prior microbiology when considering the need to cover for multidrug-resistant organisms
- For example, MRSA risk factors include: history of MRSA infection or colonization, household contact with a MRSA colonized individual, IV drug use, homelessness, incarcerated persons, recent travel to or residing in an MRSA endemic region or community
Content derived from: NB Provincial Health Authorities Anti-Infective Stewardship Committee. Gram Stain Interpretation Guideline. 2020-10.