Pseudomonas Fluorescens: Morphology, Gram Stain & Identification

Instructor: Angela Edwards

Angie has taught college science courses and has a master's degree in biology.

This lesson will discuss characteristics of the bacterium species, Pseudomonas fluorescens, including morphology and Gram stain, which aid in the identification of specific bacterial species.

A Good Bacteria?

For many people the word 'bacteria' is associated with infection, sickness, and disease. In fact, you can probably quickly think of a time when you had a bacterial infection and remember just how miserable it made you feel.

However, did you know that not all bacteria are pathogens, meaning they don't all cause harm? Some bacterial species can even have beneficial impacts to human health or the environment.

One of these non-pathogen bacterial species is Psuedomonas fluorescens (P. fluorescens). Under normal circumstances this bacteria does not cause harm to humans. In fact, it is often found in animal tissue, including the mouth, lungs, and stomach of humans, but guess what, we don't mind!

We need good bacteria like P. fluorescens to grow and persist in our bodies so they will out-compete with bad bacteria for space and nutrients.


So how do you know if you're dealing with good bacteria or bad bacteria, and with P. fluorescens specifically? Identification! Identification is critical in preventing the negative impacts of some species yet exploiting the benefits of others.

There are many laboratory tests to distinguish one bacterial species from another. Let's look at what makes P. fluorescens unique and thus identifiable.


One laboratory analysis looks at the bacterial characteristic of morphology. Morphology is defined as the size, shape, and arrangement of bacterial cells. There are three basic bacterial cell shapes:

  1. cocci - circle-shaped
  2. bacilli - rod-shaped
  3. spirilii - shaped like stringy spirals

Shape is determined microscopically by analyzing cultures of isolated samples. This is the first step in pinpointing the species present in the sample.

P. fluorescens is a rod-shaped bacteria and cells are single in arrangement, not linked together in chains. It is also a motile bacteria, which means it has flagella present. Flagella are whip-like structures on the bacterial cells that can allow the cells to move, and serve as a helpful characteristic when it comes to identification.

The different bacterial shapes. P. fluorescens is rod-shaped
Bacterial Cell Shapes

Gram Stain

The Gram stain is a popular microbiology technique used to differentiate between two groups of bacteria based on cell wall composition. Bacterial species are either Gram-negative or Gram-positive. The final result is determined by cell color via a microscope, after a staining procedure.

  • Gram-positive bacteria have thick cell walls and less fat or lipids. They hold the primary stain, crystal violet and, therefore, stain purple.
  • Gram-negative bacteria, like P. fluorescens, do not retain the primary stain due to their thin cell walls and higher lipid content. Therefore, they are able to hold the counter-stain, safranin, and stain red/pink.

Steps of the Gram stain:

  1. Apply a smear of bacteria to a slide and heat-fix it.
  2. Add crystal violet dye and let sit for one minute, then rinse with water. (Gram-positive bacteria will stain purple)
  3. Add iodine solution, let sit for 30 seconds, then rinse with water.
  4. Add alcohol solution, rinse with water.
  5. Add safranin, a counterdye that stains nuclei red. Let sit for 1 minute, rinse with water.
  6. Examine under a microscope.

P. fluorescens' are Gram-negative, they will not be purple in the end, but rather red.

Other Characteristics

Characteristics analyzed biochemically or genetically can also be used to identify bacterial species.

Often the result of these tests are only to help narrow the possible bacterial species down.

For example, P. fluorescens can't reduce nitrate because it lacks an enzyme some other bacteria have. This simply means it doesn't produce nitrogen gas, and therefore, it tests negative in a rapid nitrate test.

Another interesting characteristic is that P. fluorescens is most prevalent in the soil where it has advantageous properties for plant protection and growth. For example, it can protect grape plants from frost by out-competing a damaging bacterial species in low temperatures.

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