Proteus mirabilis

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Proteus mirabilis


Introduction to bacteria

Bacterial structure and functions


Proteus mirabilis


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USMLE® Step 1 questions

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High Yield Notes

17 pages


Proteus mirabilis

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USMLE® Step 1 style questions USMLE

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A 44-year-old man presents to the emergency department due to severe abdominal pain for the past 12 hours. The patient also has nausea and dark red urine. Past medical history is significant for Crohn disease. Urinalysis shows numerous bacteria and pH >7. A CT scan of the abdomen is shown below. Which of the following characteristics is associated with the organism causing the kidney stone found in this patient?  

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Proteus mirabilis

cephalosporins p. 186

kidney stones p. 628

penicillins for p. 185

urinary tract infections p. 625

UTIs p. 179


Proteus mirabilis is a Gram-negative bacillus which belongs to a family of bacteria called the Enterobacteriaceae.

It is widely distributed in soil and water and can also be found in the normal human intestinal flora.

In humans, it causes urinary tract infections, or UTIs for short.

Now, Proteus mirabilis has a thin peptidoglycan layer, so it doesn’t retain the crystal violet dye during Gram staining. Instead, like any other Gram-negative bacteria, it stains pink with safranin dye.

And since it’s a Gram-negative bacillus, it looks like a little pink rod under the microscope.

Alright, now Proteus mirabilis is non-spore forming and highly motile.

It’s also facultative anaerobic which means it can survive in both aerobic and anaerobic environments, non-lactose fermenter, oxidase negative which means it doesn’t produce this enzyme, and urease positive which means it can produce an enzyme called urease.

Now, it grows well on blood agar and MacConkey agar.

On blood agar, it has a swarming growth, so it moves and forms a thin filmy layer of concentric circles, which look like the ripples after you throw a rock into a lake.

On MacConkey agar, however, it doesn’t swarm so it forms smooth, pale or colourless colonies.

Finally, the triple sugar iron test, or TSI for short can be done to assess hydrogen sulfide production.

This medium contains three sugars - lactose, glucose and sucrose, as well as iron and a pH sensitive dye.

Proteus mirabilis produces hydrogen sulfide, that reacts with the iron, and a black precipitate forms in the test tube.

Ok, now Proteus mirabilis has a number of virulence factors, that are like assault weaponry that help it attack and destroy the host cells, and evade the immune system.

First, Proteus mirabilis has flagella, which are lash-like appendages that comes out from the cell body, and confer the bacteria its motility.

Now, when Proteus mirabilis comes in contact with solid surfaces, especially urinary catheters, it has the ability to differentiate from short swimmer cells into elongated swarm cells that express hundreds to thousands of flagella, and interact with each other to form multicellular rafts.


Proteus mirabilis is a gram-negative, facultatively anaerobic, rod-shaped bacterium. It is commonly found in the human intestinal tract and urinary tract and is known for its ability to produce urease, an enzyme that breaks down urea, which can lead to the formation of struvite crystals in the urinary tract and cause kidney stones. This bacteria can be identified in a urine culture, and the treatment of its infections involves antibiotics like trimethoprim-sulfamethoxazole, fluoroquinolones, and cephalosporins.


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