Proteus mirabilis

by Pam Murphy

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Life History

Proteus mirabilis is part of the normal flora of the human gastrointestinal tract. It can also be found free living in water and soil. When this organism, however, enters the urinary tract, wounds, or the lungs it can become pathogenic. Proteus mirabilis commonly causes urinary tract infections and the formation of stones. 

Microbiological Characteristics

Proteus mirabilis is part of the Enterobacteriaceae family. It is a small gram-negative bacillus and a facultative anaerobe. Proteus mirabilis is characterized by its swarming motility, its ability to ferment maltose, and its inability to ferment lactose. P. mirabilis has the ability to elongate itself and secrete a polysaccharide when in contact with solid surfaces, making it extremely motile on items such as medical equipment.


The most common infection involving Proteus mirabilis occurs when the bacteria moves to the urethra and urinary bladder. Although Proteus mirabilis mostly known to cause urinary tract infections, the majority of urinary tract infections are due to E. coli. One-hundred thousand cfus per milliliter in the urine are usually indicative of a urinary tract infection. Urinary tract infections caused by P. mirabilis occur usually in patients under long-term catherization. The bacteria have been found to move and create encrustations on the urinary catheters. The encrustations cause the catheter to block. 

Symptoms for urethritis are mild including frequency of urination and pyuria (presence of white blob cells in the urine). Cystitis (bladder infection) symptoms are easier to distinguish and include back pain, concentrated appearance, urgency, hematuria (presence of red blood cells in the urine), and suprapubic pain as well as increased frequency of urination and pyuria. 

Pyelonephritis (kidney infection) can occur when the bacteria migrates from the lower urinary tract. Although it is seen as a furtherance of infections, not all patients have the symptoms associated with urethritis and cystitis. Pyelonephritis is marked by nausea and vomiting.

Proteus mirabilis
 can enter the bloodstream through wounds. This happens with contact between the wound and an infected surface. The bacteria induce inflammatory response that can cause sepsis and systemic inflammatory response syndrome (SIRS). SIRS has a mortality rate between 20 and 50 percent. 

P. mirabilis can also, though less common, colonize the lungs. This is the result of infected hospital breathing equipment and causes pneumonia. Symptoms for pneumonia include fever, chills, chest pain, rales, and cough.

Prostatitis can occur as a result of P. mirabilis infection, causing fever, chills, and tender prostate in men.

Virulence Factors

swarming on  agarThe flagellum of P. mirabilis is crucial to its motility, a characteristic that helps the organism colonize. The flagellum has also been linked to the ability of P. mirabilis to form biofilms, aiding in the bacteria’s resistance to defenses of the host and select antibiotics. P. mirabilis also relies on its pili for adhesion to avoid being flushed out of the urinary tract system.
Important to Proteus mirabilis is urease, responsible for raising the pH and consequently making it easier to thrive. Increased pH allows stone formation to take place. On occasion the stones fill the entire renal pelvis. 
Also present are endotoxins, responsible for induction of the inflammatory response system and pore-forming hemolysins. (Picture from


Infections caused by P. mirabilis are seen most often in nursing home patients. These infections are commonly caused by infected medical equipment including catheters, nebulizers (responsible for inhalation), and examination gloves (responsible for wound infections). The length of catherization is directly related to incidence of infection. Each day of catherization gives an infection rate of 3-5%.
Urinary tract infections caused by P. mirabilis also occur commonly in sexually active women and men, especially those engaging in unprotected intercourse. Younger women are at greater risk than younger men; however, older men are at greater risk than older women due to the occurrence of prostate disease.

Treatment and Prevention

Proteus mirabilis infections can be treated with broad-spectrum penicillins or cephalosporins except in severe cases. It is not susceptible to nitrofurantoin or tetracycline and has experienced increasing drug resistance of ampicillin, trimethoprim, and ciprofloxin. In cases with severe stone formation, surgery is necessary to remove the blockage.

Proteus mirabilis is part of the normal flora of the gastrointestinal tract, and as a result the bacteria enters the urinary tract or infects medical equipment by the fecal route. Consequently, prevention includes good sanitation and hygiene, including proper sterilization of medical equipment. It is also suggested that patients not requiring catherization should not receive catherization, despite its convenience for the caretaker.