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  • Writer's pictureSandra Witt

Gram Stains

I hear so many crazy things coming from Parrot FaceBook groups, one of which is the interpretation of Gram stain results. If you really want to understand what these test results are telling you, please read the following information from an article by Dr. Scott McDonald, DVM.

Gram staining is simple test used to check for the presence and quantities of various type of bacteria and yeast from the oral cavity, cloaca, or some other site. To perform the test, a sterile cotton-tipped applicator stick is used to swab the collection site. The sample is smeared on to a microscope slide and transported back to the laboratory. The slide is then stained with several dyes in order to "color" the bacteria and yeast for identification.

When the process is complete, bacteria with larger pores that stain pink are called Gram negative bacteria, and bacteria with small pores are Gram positive bacteria.

Bacteria also present in different shapes. The two primary shapes are cocci (round or oval) and bacilli (rod-shaped). Other less common shapes are coccobacillic (rounded, slightly rod-shaped) and spirochetes (curved spirals).

All cocci are Gram positive bacteria and they make up the majority of the normal bacteria flora seen in healthy psittacine birds.

Gram positive Cocci include Streptococcus sp. and Staphylococcus sp. and they make up the majority of the normal bacteria flora seen in healthy psittacine birds. Lactobacillus is an example of this type of bacteria which we are all familiar with in yogurt or in a number of commercial products used to inoculate a bird's GI tract with "normal" bacteria.

Gram negative rods consist of most of the bacteria considered potentially pathogenic or disease-producing in birds. Examples include E. coli, Pseudomonas, Proteus, Citerbacter and Enterbacter sp.

Coccobacillic-shaped bacteria (CB) are Gram negative and the most common that is disease producing is Klebsiella sp.; Spirochetes are also Gram negative. These are rare but they have been associated with disease in the respiratory tract of cockatiels.

It is useful to estimate the number of bacteria and general proportions of Gram positives to Gram negatives present on each slide.

Estimates are classified as follows:

  • Rare - only a few bacteria on slide

  • +1 occasional bacteria on slide

  • +2 5-15 bacteria per oil immersion field

  • +3 15-75 bacteria per oil immersion field

  • +4 more than 75 per oil immersion field

By determining an estimate of the number and types of bacteria present, the veterinarian can better determine if specific cultures or other tests, treatments, or management changes should be carried out.

In general, most healthy psittacines have primarily Gram positive bacteria in their GI tract. Gram negative bacteria may be found in smaller numbers or percentages than Gram positive bacteria. In a potentially sick bird, we are looking for increased numbers and proportions of Gram negative bacteria compared to Gram positive bacteria.


Yeasts are types of fungi; the most common is Candida sp. Spores of this fungi are ubiquitous; however, they may become pathogenic in debilitated or immuno-suppressed individuals. Their presence, numbers, and forms help determine if treatment is necessary.

Yeast appear in three forms - single, budding, or pseudohyphae (chains). These all stain Gram positive and are up to four times larger than bacteria. Small numbers of single yeast cells are common in normal healthy psittacines; a lot may be present if the bird has been eating bread. Larger numbers of budding yeast or the presence of pseudohyphae indicates the yeast are multiplying in the GI tact and that a disease state may exist. Examples include young birds with sour crop or infection in the oral cavity or birds on long term antibiotics which kill normal bacteria causing a yeast overgrowth.

Gram Stain
  • Samples are obtained either choana (roof of the mouth), cloaca (just inside the vent), feces, or some other site.

  • Gram Negative Rods are the most common disease-producing bacteria in psittacines. Examples include E. coli, Pseudomonas, Enterbacter sp. May be found in small numbers in healthy birds but should not be the predominate bacteria seen.

  • Gram Negative CB: Coccobacillic-shaped bacteria. Most common example is Klebsiella sp. may produce disease if found in large numbers.

  • Gram Positive Rods: Normal flora in psittacines. Most common example is Lactobacillus sp. May be the predominate organism found.

  • Gram Positive Cocci: Normal flora in psittacines. Examples include Staphylococcus and Streptoccus bacteria. May be the predominate organism found. In some instances, certain species of Staph or Strep may cause disease.

  • Yeast: Fungi; Candida sp. is the most common example seen. May be normal in small numbers but large numbers of growing organisms may indicate a problem.

  • Other: occasionally parasites or other interesting artifacts are seen.

Original publication date December 20, 2017.

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