A urinary tract infection (UTI) is a common condition that occurs when bacteria enter the urinary system. While many people associate these infections with the bacterium Escherichia coli, other microorganisms can also be responsible. One such bacterium is Proteus mirabilis, a gram-negative organism known for causing complicated UTIs. Although it accounts for a smaller percentage of total UTIs compared to E. coli, P. mirabilis possesses distinct biological features that make it a significant cause of infection, particularly in specific patient populations and healthcare settings. This bacterium’s unique characteristics influence how it establishes an infection, the types of complications that can arise, and the approaches required for effective treatment.
The Mechanism of a Proteus Mirabilis Infection
Proteus mirabilis employs a sophisticated set of tools to infect the urinary tract. Central to its strategy is the production of an enzyme called urease. This enzyme’s primary function is to break down urea, a waste product abundant in human urine, into ammonia and carbon dioxide.
The generation of ammonia is a defining feature of a P. mirabilis infection because it alters the pH of urine. Ammonia is an alkaline substance, and its accumulation raises the urine’s pH, sometimes to 9.0 or higher. This alkaline environment is favorable for the bacterium’s survival and is also damaging to the urothelial cells that line the urinary tract, facilitating further bacterial colonization.
Another characteristic is its ability to move, a behavior known as swarming motility. When P. mirabilis comes into contact with a solid surface, such as the lining of the urinary tract or a urinary catheter, it undergoes a physical transformation. The bacteria elongate and produce a greater number of flagella, which are tail-like appendages that enable movement. This coordinated swarming helps the bacteria ascend the urinary tract from the bladder to the kidneys, increasing the likelihood of a more widespread infection.
Complications and High-Risk Groups
The biochemical changes initiated by Proteus mirabilis can lead to serious complications, most notably the formation of kidney stones. The alkaline urine it creates facilitates the development of stones composed of struvite, a mixture of magnesium, ammonium, and phosphate. In a normal, acidic urinary environment, these minerals remain dissolved, but in the high-pH environment engineered by the bacteria, they precipitate out of the urine and crystallize.
These “infection stones” can grow very large, sometimes forming a cast of the kidney’s collecting system known as a staghorn calculus. The stones can harbor bacteria within their structure, protecting them from the host’s immune system and antibiotics. This often leads to persistent or recurrent infections that are difficult to eradicate. Beyond stone formation, a P. mirabilis infection can lead to pyelonephritis (a severe kidney infection) or bacteremia, where bacteria enter the bloodstream.
Certain groups are more vulnerable to P. mirabilis UTIs. The most prominent high-risk group includes individuals with long-term indwelling urinary catheters. Catheters provide a surface for bacteria to form crystalline biofilms, which are communities of bacteria in a protective matrix that can block the catheter. The elderly, particularly in long-term care facilities, and individuals with structural abnormalities of the urinary tract are also at an increased risk.
Symptoms and Diagnosis
The symptoms of a UTI caused by Proteus mirabilis often overlap with those of other UTIs. These can include a persistent urge to urinate, a burning sensation during urination, and passing frequent but small volumes of urine. The urine itself may appear cloudy or have a noticeably strong, sometimes described as “fishy,” odor.
When the infection ascends to the kidneys, more severe symptoms of pyelonephritis can develop. These include high fever, chills, and persistent pain in the flank, back, or side. Nausea and vomiting may also occur, indicating a serious illness that requires prompt medical attention. A history of recurrent kidney stones can also point toward a chronic P. mirabilis infection.
Diagnosing a P. mirabilis UTI begins with a urinalysis, which examines a urine sample for signs of infection like white blood cells and bacteria. A finding that suggests P. mirabilis is a urine pH that is alkaline. To confirm the cause, a urine culture is performed to grow the bacteria from the sample, identify it, and determine which antibiotics will be effective. If kidney stones are suspected, imaging tests like a kidney ultrasound or a CT scan are ordered to visualize the urinary tract and detect any obstructions.
Treatment Approaches
The primary treatment for a Proteus mirabilis UTI is a course of antibiotics. Because this bacterium can be resistant to several common antibiotics, the choice of medication is guided by a urine culture and its susceptibility testing. This testing identifies which antibiotics are capable of killing the particular strain causing the infection. While uncomplicated infections may be treated with oral antibiotics, more severe infections like pyelonephritis may require intravenous antibiotics in a hospital setting.
A significant challenge in treatment arises when struvite stones have formed. Since antibiotics alone are often insufficient, the complete removal of these infection stones is part of the treatment plan. Depending on the size and location of the stones, this may be achieved through non-invasive procedures like shock wave lithotripsy, which uses sound waves to break stones apart, or through surgical interventions to remove them directly.
For patients with urinary catheters, managing the device is an important part of treatment. The catheter can become coated in a bacterial biofilm, making the bacteria difficult to eliminate. As a result, the existing catheter is removed and replaced as part of the treatment protocol. Proper catheter care and limiting the duration of catheterization are important strategies to prevent recurrence.