What is Proteus mirabilis?
Proteus mirabilis is a type of bacteria that can infect the urinary tract, accounting for approximately 1–10% of all urinary tract infections [1]. It is a facultative anaerobe, meaning that it can survive in both the presence and absence of oxygen. It is commonly found in the human digestive tract as well as in the environment.
When it comes to UTIs, P. mirabilis is associated with complicated infections, such as catheter-associated urinary tract infections (CAUTIs) and other urological device-associated infections, being a resident of a long-term care facilities, old age, and bladder dysfunction. While not particularly challenging to detect, P. mirabilis can be challenging to treat.
What causes Proteus mirabilis UTI?
Proteus mirabilis is thought to reach the urinary tract the same way most bacteria do — migrating from the gastrointestinal tract. It's disproportionately involved in what doctors call complicated UTIs. There are a few groups of complicated UTI patients that experience P. mirabilis infection more often:
- People who use urinary catheters, both short- and long-term
- People with kidney stones
- People with abnormalities of the urinary tract
Risks and complications
UTIs caused by P. mirabilis have distinct risks as compared to UTIs caused by other bacteria and fungi, making accurate and timely identification important.
Kidney stones: P. mirabilis produces an enzyme called urease, which breaks down a chemical in urine called urea. This process raises the pH of the urine, making it alkaline. These changes, caused by P. mirabilis, promote the formation of struvite stones — also called infection stones — as they can re-seed the bladder after antibiotic treatment, causing recurrent or chronic UTI symptoms [1, 2].
Biofilms: Through the same process that P. mirabilis can cause struvite stones, it can also cause hard mineral encrustation on catheters. This makes it more difficult for antibiotics to reach the bacteria, as the bacteria can be encased in the mineral deposits on the surface of the catheter [1, 2].
Antibiotic resistance: P. mirabilis is intrinsically resistant to nitrofurantoin (the drug in Macrobid), tetracyclines, and colistin. This means that if you are prescribed one of these medications and your UTI is caused by P. mirabilis, the medication will not work. If you've been placed on one of these as a preventive antibiotic or first-line treatment and you are still experiencing symptoms, it is worth discussing the specific organism causing your UTI with your clinician. Learn more about antimicrobial resistance and UTI treatment.
How is Proteus mirabilis infection detected?
Unlike some UTI-causing organisms, Proteus mirabilis usually grows in standard urine culture. It is commonly recognized for its "swarming" behavior, where rather than growing individual colonies in the petri dish, it typically grows in a distinctive bulls'-eye pattern. The value of advanced testing for this organism is less so about finding it, but more about providing additional information to help inform your next steps.
Polymicrobial Infections: Proteus mirabilis often co-occurs with other organisms [2]. This is particularly true for patients who use catheters. Polymicrobial infections are often misreported as contamination when found by standard urine culture. Next-generation sequencing-based tests can help capture the full picture when multiple species may be at play.
Resistance Profiling: Knowing an infection is Proteus mirabilis immediately takes certain antibiotics off the table. Getting insight into resistance markers across the board can help optimize treatment selection.
Virulence Factors: Virulence factors are characteristics of a pathogen that enable it to more efficiently infect a host, evade the immune system, and cause disease. While the clinical-relevance is not yet known, clinical metagenomic-based tests enable identification of specific virulence factors a pathogen may have. More research is needed to know how the presence of specific virulence factors should change treatment. Virulence factors of P. mirabilis are listed in the table below.
| Virulence Factor | Role in Infection |
|---|---|
| Urease | Reacts with urea in urine, increasing the pH of urine, thus enabling the formation of mineral deposits and struvite stones |
| Fimbriae | Fimbriae are hair-like appendages found on the surface of bacterial cells that enable them to adhere to different surfaces, such as the lining of the urinary tract or a catheter |
| Flagella | Flagella are appendages that enable the cells to move across surfaces, playing a role in ascension in the urinary tract or across the surface of a catheter |
How is P. mirabilis treated?
As P. mirabilis is a type of bacteria, it is treated through the use of antibiotics. However, not just any antibiotic will work. P. mirabilis is intrinsically resistant to nitrofurantoin, tetracyclines, and colistin. This is why it is important to know exactly which pathogen(s) may be causing your UTI. If P. mirabilis is identified, your provider will likely prescribe an antibiotic that P. mirabilis is more likely to be susceptible to, such as ciprofloxacin, trimethoprim-sulfamethoxazole (TMP/SMZ), amoxicillin, or a cephalosporin. They will take into consideration your current symptoms, your medical history, as well as local resistance rates to determine the best treatment option.
Catheter-associated and other complicated UTIs may require additional intervention to ensure that P. mirabilis is eliminated from the urinary tract. Recurrent, chronic, or persistent urinary symptoms combined with consistent P. mirabilis detection may suggest the presence of biofilms or struvite stones. In these cases, treatment may include completely replacing the current urological device as well as procedures to eliminate the struvite stones from the bladder or kidneys.
The bottom line
UTIs caused by Proteus mirabilis are not uncommon, especially for people who have certain risk factors, such as those who use a urinary catheter or live in long-term care facilities. While standard diagnostics tend to accurately identify P. mirabilis, when it is found it may be worth pursuing advanced diagnostic testing to rule out polymicrobial infection, as P. mirabilis is often found alongside other UTI-causing bacteria. Because of its ability to alter the chemical characteristics of urine, P. mirabilis UTIs can lead to the formation of struvite stones or catheter encrustation — both of which limit the effectiveness of antibiotics and may serve as reservoirs, leading to recurrent or chronic UTI symptoms. In some cases, treatment may involve certain procedures in addition to antibiotics.
The BIOTIA-ID Urine Test uses next-generation sequencing to identify over 40 urogenital pathogens, including Proteus mirabilis, alongside antibiotic resistance markers to help guide treatment.
Frequently asked questions
Is a Proteus mirabilis UTI more serious than other UTIs?
UTIs caused by Proteus mirabilis can be more serious than UTIs caused by other pathogens. This is mainly due to P. mirabilis' ability to create struvite stones and mineral encrustation on urological devices — both of which can play a role in recurrent or chronic infection of the urinary tract and the development of antibiotic resistance.
Can Proteus mirabilis cause kidney or bladder stones?
Yes. P. mirabilis produces urease, which increases the pH of urine and promotes the formation of struvite stones. These stones often contain the bacteria within them, which in turn become a reservoir that can cause recurrent or chronic infection. In these cases, eliminating the struvite stones is important to ensure symptom resolution.
Can Proteus mirabilis cause biofilms?
Yes. P. mirabilis can cause biofilms. Through the same process it can cause kidney or bladder stones, if P. mirabilis is adhered to catheters or other urological devices it can cause the formation of mineral deposits, called encrustation. Once formed, it can be difficult to treat the infection with antibiotics alone — complete removal of the device is often necessary to eliminate the infection. This is because, like struvite stones, the mineral deposits can form around the bacterial cells, protecting them from the antibiotic.
How is Proteus mirabilis diagnosed?
Proteus mirabilis is readily grown in standard urine culture and is often identified by its characteristic growth pattern called swarming. However, advanced diagnostic testing such as next-generation sequencing can provide more comprehensive information about antibiotic resistance and potential polymicrobial infection as compared to standard urine culture alone. This information is important for people at risk of complications from UTI.
Who is most at risk for Proteus mirabilis UTIs?
A number of people are at higher risk for UTIs caused by Proteus mirabilis. These include patients with urinary catheters, diabetes, and/or urinary tract abnormalities, as well as patients who are hospitalized, in long-term care facilities, or in nursing homes.
References
- [1] Schaffer JN, Pearson MM. Proteus mirabilis and Urinary Tract Infections. Microbiol Spectr. 2015 Oct;3(5):10.1128/microbiolspec.UTI-0017-2013.
- [2] Armbruster CE, Mobley HLT, Pearson MM. Pathogenesis of Proteus mirabilis Infection. EcoSal Plus. 2018 Feb;8(1):10.1128/ecosalplus.ESP-0009-2017.
- [3] Wasfi R, Hamed SM, Amer MA, Fahmy LI. Proteus mirabilis Biofilm: Development and Therapeutic Strategies. Front Cell Infect Microbiol. 2020 Aug 14;10:414.