What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Proteus mirabilis: Veterinary Infections and Lab ID

Name: Proteus mirabilis: Veterinary Infections and Lab ID
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Proteus mirabilis is a Gram-negative, motile, urease-positive rod that swarms across non-selective agar and is a leading cause of complicated urinary tract infection in dogs. In the diagnostic laboratory it is identified by its lactose-negative growth on MacConkey agar, a strongly positive urease reaction, and its characteristic concentric swarming rings on blood agar.

This organism matters because of what it does to urine chemistry. Its urease enzyme splits urea into ammonia and carbon dioxide, raising urine pH and pushing magnesium and phosphate out of solution as struvite crystals. Those crystals aggregate into stones, the stones traumatize the urothelium, and the damaged mucosa becomes a foothold for recurrent infection. The result is a self-perpetuating cycle that is one of the most frustrating problems in small animal urology.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What Proteus mirabilis Is

P. mirabilis is a member of the family Enterobacterales (formerly Enterobacteriaceae), a facultatively anaerobic Gram-negative bacillus. It is widely distributed in the environment and is a normal inhabitant of the gastrointestinal tract of mammals, including dogs and cats [1]. Outside the gut it behaves as an opportunistic pathogen, and the urinary tract is its preferred site of infection in companion animals [2].

The genus Proteus contains several species, and P. mirabilis is the one most frequently isolated from clinical veterinary specimens. The name comes from the Greek sea god Proteus, who could change his shape, a reference to the organism's ability to form pleomorphic, elongated swarm cells on solid surfaces.

Core structural and biochemical features

Gram stain: Gram-negative rod, roughly 0.4 to 0.8 micrometers wide and 1 to 3 micrometers long in the vegetative form.
Motility: Highly motile by means of peritrichous flagella (flagella distributed over the entire cell surface). This is the engine behind swarming.
Urease: Strongly positive. Urease hydrolyzes urea to ammonia, which is the biochemical root of struvite stone formation.
Lactose: Negative. Colonies on MacConkey agar are colorless because the organism cannot ferment lactose.
Oxidase: Negative. This separates it from Pseudomonas and other oxidase-positive Gram-negative rods.
Hydrogen sulfide: Positive on triple sugar iron (TSI) agar, producing a black butt.
Indole: Negative, which distinguishes P. mirabilis from P. vulgaris.

Fimbriae and adhesion

Adhesion is the first step in establishing a urinary tract infection, and P. mirabilis carries several fimbrial types that bind uroepithelial cells. Two major fimbrial subunit genes, pmpA and ucaA, were cloned from canine uropathogenic P. mirabilis strains, and the ucaA gene product shares about 56% amino acid identity with the F17A and F111A major fimbrial subunits of bovine enterotoxigenic Escherichia coli [3]. Hybridization experiments showed that both fimbrial types are present in many urinary tract infection-related canine isolates, but could not be demonstrated in P. vulgaris or other Proteus-related species [3]. This species-level specificity is one reason P. mirabilis is treated as a distinct uropathogen rather than a generic Proteus problem.

The Swarming Phenomenon

Swarming is the single most recognizable laboratory feature of P. mirabilis, and it is also a virulence factor. When the organism is inoculated onto a moist, non-selective agar surface, individual cells differentiate into elongated, hyperflagellated swarm cells. These cells migrate outward in coordinated groups, and the population advances across the plate in waves. Because the waves pause and resume, the colony develops concentric rings of growth that can cover the entire plate within hours.

A 2019 study in the Journal of Bacteriology clarified how this works. The authors measured the relationship between agar concentration (a proxy for surface rigidity), single-cell phenotypes, and swarm colony phenotypes. They found that cell elongation and single-cell motility are coupled with population migration on low-percentage hard agar (1% to 2.5%) and become decoupled on high-percentage hard agar (greater than 2.5%) [4]. The same work showed that lipopolysaccharide, specifically the O-antigen component, is not essential for elongation and motility of individual cells. Instead, LPS functions to broaden the range of agar concentrations on which cell elongation and motility remain coupled with population migration [4].

Swarming is not a laboratory curiosity. It is a coordinated multicellular behavior that lets the organism spread across a surface, and it correlates with virulence gene content. In a 2025 study of 91 P. mirabilis isolates, swarming motility was graded as weak in 11%, intermediate in 38.5%, and strong in 50.5% of isolates. The hlyA gene (hemolysin) frequency was directly associated with swarming motility and inversely associated with biofilm formation, while the rsmA gene showed a reverse association with swarming [5]. Biofilm formation and swarming are therefore two distinct surface behaviors with different genetic drivers, and the organism can shift between them.

Temperature also controls swarming. A 2021 study found that the median minimum paralyzing temperature for P. mirabilis swarming was 44 degrees Celsius, and that incubation at elevated temperature altered antimicrobial susceptibility profiles in disk diffusion testing [6]. This is a practical reminder that incubation conditions affect both the appearance of the organism and the interpretation of susceptibility results.

Why swarming matters in the diagnostic lab

Swarming is a clinical problem before it is a scientific one. A swarming Proteus isolate can overgrow an entire blood agar plate and obscure other organisms in a mixed culture. In a polymicrobial infection, that can mean a co-pathogen is missed entirely. This is why laboratories use selective and differential media, and why technologists often subculture from a single isolated colony rather than from the swarm edge.

Urease, Urine pH, and Struvite Stones

Urease is the biochemical signature of P. mirabilis and the reason it causes stone disease. The enzyme catalyzes the hydrolysis of urea:

Urea + water → ammonia + carbamic acid → ammonia + carbon dioxide

Ammonia combines with water to form ammonium hydroxide, which is strongly alkaline. As urine pH rises above roughly 7.0, the solubility of magnesium ammonium phosphate (struvite) and calcium carbonate apatite falls sharply, and crystals begin to precipitate. Struvite is the mineral magnesium ammonium phosphate hexahydrate, and it is the stone type most closely linked to urease-producing bacteria.

The cycle runs like this:

P. mirabilis colonizes the bladder, often after catheterization or in a patient with a pre-existing urinary abnormality.
Urease raises urine pH and ammonia concentration.
Struvite and apatite crystals form and aggregate.
Stones form, often rapidly, and can reach a clinically significant size in weeks.
The stones traumatize the urothelium and provide a biofilm-covered surface that antibiotics cannot penetrate.
Bacteria persist inside the stone, and infection recurs whenever treatment stops.

A 2025 case report documented this sequence in a Golden Retriever with urinary incontinence, hematuria, stranguria, and foul-smelling urine. Radiography and ultrasonography confirmed multiple nephroliths in the right kidney and a urethral calculus measuring 5.28 mm. The dog had elevated BUN, creatinine, and phosphorus, anemia, hematuria, proteinuria, and struvite crystalluria. P. mirabilis was isolated and confirmed by conventional and molecular methods, including 16S rRNA gene sequencing. The urethral calculus was relieved by retrograde urohydropropulsion, and the dog was treated with amoxicillin-clavulanate plus dietary and supportive therapy. By the fourth week after treatment, urinalysis showed no abnormalities [7].

That case illustrates the two-part treatment principle for struvite urolithiasis: dissolve or remove the stone, and eliminate the urease-producing infection. Neither alone is sufficient.

Clinical Relevance in Dogs and Cats

Urinary tract infection

P. mirabilis is one of the most common causes of bacterial urinary tract infection in dogs. A six-year retrospective study of canine and feline urine samples in Phoenix, Arizona, found E. coli as the most isolated organism at 43.4%, followed by Enterococcus faecalis at 11.5%, Proteus mirabilis at 10.7%, Staphylococcus pseudintermedius at 9.9%, Enterococcus faecium at 6.9%, and Klebsiella pneumoniae at 6.3% [8]. In that dataset, culture-positive canine urine samples were more frequent than positive feline samples, and summer months produced the majority of suspected UTI samples for both species [8].

Two clinical patterns should raise suspicion for Proteus specifically. The first is infection after urinary catheterization or other instrumentation, because the organism is a classic device-associated uropathogen. The second is recurrent infection in a dog with a history of struvite stones or alkaline urine. The 1975 experimental work by Wooley and colleagues induced cystitis in dogs and cats using P. mirabilis, confirming the organism's ability to establish bladder infection under controlled conditions [9].

Otitis externa

P. mirabilis is also recovered from the ear canals of dogs with otitis externa. A study of 592 dogs with clinical otitis externa in Romania isolated E. coli, P. mirabilis, and Enterobacter cloacae complex in 9.12%, 6.2%, and a smaller proportion of samples respectively, using blood agar and MacConkey agar followed by biochemical testing and MALDI-TOF mass spectrometry [10]. The authors noted that some isolates produced extended-spectrum beta-lactamase or AmpC beta-lactamase enzymes, which complicates treatment [10].

Skin and soft tissue infection

P. mirabilis can participate in polymicrobial wound infections. A case report described a male mixed-breed dog with two large wounds extending through the epidermis, dermis, and fascia, with necrotic and crepitating foci. Histopathology and bacterial culture led to a diagnosis of polymicrobial necrotizing fasciitis, and MALDI Biotyper identification recovered Macrococcus caseolyticus, Proteus mirabilis, and Escherichia coli from the lesions [11]. A separate case report described a 5-year-old dog with generalized pyoderma in which P. mirabilis was initially isolated as the sole agent and treated according to the antibiogram with no effect. Repeat microbiological testing identified methicillin-resistant, multidrug-resistant Staphylococcus pseudintermedius as a co-agent. The dog died a little over two months after the first symptoms with clinical signs of septic shock [12]. Both cases make the same point: when a patient worsens despite appropriate therapy for the first isolate, repeat sampling is warranted.

Diarrhea

P. mirabilis is an opportunistic pathogen that causes diarrhea as well as urinary tract infection in companion animals [13]. A study of feces from 35 dogs with diarrhea hospitalized in Beijing found a P. mirabilis prevalence of 28%. Thirteen of 35 isolates (37%) produced extended-spectrum beta-lactamases, with blaTEM detected in all ESBL-producing isolates. All isolates were susceptible to imipenem, cefoxitin, and cefotaxime/clavulanic acid, and the only aminoglycoside resistance gene identified was rmtB, at a prevalence of 51% [13].

Antimicrobial Resistance

Resistance in P. mirabilis is a growing concern in companion animal medicine. The Beijing diarrhea study found that 37% of isolates produced ESBLs [13]. A 2024 report from Korea described the first P. mirabilis isolate harboring blaNDM-1 (New Delhi metallo-beta-lactamase 1) recovered from a companion dog living with a human owner. Whole-genome sequencing revealed 20 different antimicrobial resistance genes against various classes of antimicrobial agents, and the genomic analysis placed the isolate in a phylogroup alongside other P. mirabilis strains carrying diverse resistance genes [1].

Cephamycin susceptibility in P. mirabilis is generally favorable compared with other Enterobacterales. In a study of 218 ESBL-producing Enterobacteriaceae isolates from dogs and cats, P. mirabilis isolates that produced only ESBL showed susceptibility rates of 82.7% to cefmetazole, 96.6% to flomoxef, and 100% to latamoxef [14]. The same study found that Klebsiella pneumoniae and Enterobacter cloacae co-producing ESBL and AmpC beta-lactamase had significantly lower susceptibility rates than isolates producing ESBL alone, which shows that beta-lactamase profile and species both influence drug choice [14].

Clonal relatedness between animal and human isolates is well documented. A 2019 study compared P. mirabilis from companion animals (107 isolates) and humans (76 isolates) with urinary tract infection using pulsed-field gel electrophoresis. Nine clusters included isolates from community and hospital patients, including strains with 100% similarity. A high number of clusters (43.6%, 17 of 39) included strains from both companion animals and humans, and one strain from a dog was 100% similar to a human community-acquired isolate [15]. This genetic overlap is the basis for the One Health approach to surveillance that the Korean authors called for [1].

Laboratory Identification

Specimen collection

For urinary tract infection, urine collected by cystocentesis is preferred because it avoids contamination from the distal urethra and genital tract. The Northwest Italy antimicrobial stewardship study used cystocentesis to collect urine from 16 dogs and 12 cats presenting with acute UTI signs, then selected therapy according to minimal inhibitory concentration results [16]. Free-catch and catheterized samples are acceptable in some situations but carry a higher risk of contamination and are harder to interpret.

Culture media and colony appearance

Blood agar (non-selective). P. mirabilis grows readily and produces the classic swarming pattern. The colony spreads in concentric waves that can cover the plate. On a mixed culture, the swarm may obscure other organisms, which is the single most important practical caveat in the laboratory identification of this organism.

MacConkey agar (selective and differential). P. mirabilis grows as colorless, non-lactose-fermenting colonies. MacConkey agar inhibits Gram-positive organisms and distinguishes lactose fermenters (pink) from non-fermenters (colorless). The Romanian otitis externa study used blood agar and MacConkey agar as its primary culture media, followed by biochemical testing and MALDI-TOF confirmation [10].

CLED agar. Cystine-lactose-electrolyte-deficient agar is sometimes used for urine culture because it prevents swarming and allows discrete colony counting. This is a practical alternative when a quantitative urine culture is needed.

Biochemical tests

Urease test. Inoculate the organism into Christensen urea agar or a urea broth. A positive result turns the medium bright pink as ammonia raises the pH. P. mirabilis is strongly urease-positive, usually within a few hours.

Triple sugar iron agar. P. mirabilis produces an alkaline slant, an acid butt, gas, and hydrogen sulfide, which appears as a black precipitate in the butt.

Indole test. P. mirabilis is indole-negative, which separates it from P. vulgaris.

Motility test. Motility can be demonstrated in motility test medium or by the swarming pattern on agar.

Molecular and mass spectrometry methods

MALDI-TOF mass spectrometry is now a standard identification method in veterinary diagnostic laboratories. The necrotizing fasciitis case used the Bruker MALDI Biotyper to identify M. caseolyticus, P. mirabilis, and E. coli from wound cultures [11], and the otitis externa study used MALDI-TOF as its confirmatory method [10]. For cases where species-level confirmation is needed or where conventional biochemistry is ambiguous, 16S rRNA gene sequencing is available, as used in the canine urolithiasis case [7].

Comparison with common look-alikes

Feature	Proteus mirabilis	Escherichia coli	Klebsiella pneumoniae
Gram stain	Gram-negative rod	Gram-negative rod	Gram-negative rod
Motility	Highly motile (peritrichous flagella)	Motile (peritrichous flagella)	Non-motile
Urease	Strongly positive	Negative	Variable, usually negative
Lactose fermentation	Negative	Positive (with metallic green sheen on EMB)	Positive (mucoid colonies)
Swarming on blood agar	Yes, concentric rings	No	No
Capsule	No	No	Yes (mucoid, prominent capsule)
Typical veterinary source	Canine UTI, struvite stones, otitis externa, wounds	Canine and feline UTI, diarrhea, wounds	Canine and feline UTI, respiratory infection
Key biochemical clue	Urease positive, indole negative, H2S positive	Indole positive, lactose positive	Non-motile, mucoid, lactose positive

The three organisms overlap in the urinary tract but diverge sharply in the laboratory. Motility and urease are the fastest ways to separate P. mirabilis from E. coli and K. pneumoniae on a bench plate.

Clinical Relevance, Limitations and Common Mistakes

The most common mistake in clinical practice is treating a Proteus urinary tract infection with an antibiotic without addressing the underlying stone or urine pH problem. If struvite uroliths are present, the infection will recur as soon as therapy stops, because bacteria persist within the stone matrix. The canine urolithiasis case showed that relieving the urethral obstruction and treating with amoxicillin-clavulanate plus dietary therapy resolved the clinical and urinalysis abnormalities by week four [7], but that outcome depended on removing the calculus.

The second common mistake is missing a co-pathogen because Proteus swarmed over the plate. In the pyoderma case, the initial culture identified only P. mirabilis, and the dog did not improve until repeat sampling revealed multidrug-resistant S. pseudintermedius [12]. When a patient fails to respond to therapy that matches the antibiogram, repeat culture is the correct next step.

The third mistake is assuming that all Proteus isolates behave the same. Swarming ability varies widely between isolates, with some strains showing weak or absent swarming [5][17]. A non-swarming isolate is still P. mirabilis and still a pathogen. Identification should not depend on the swarm pattern alone.

The fourth mistake is overlooking the zoonotic and reverse-zoonotic dimension. The genetic overlap between canine and human isolates [15] and the emergence of NDM-1-producing strains in companion animals [1] mean that resistance surveillance in pets has public health implications.

Individual cases require veterinary assessment. This article provides background knowledge, not a treatment protocol.

Quick Review

P. mirabilis is a Gram-negative, motile, urease-positive, lactose-negative rod.
Swarming on non-selective agar produces concentric rings and can obscure other organisms in mixed culture.
Urease raises urine pH and drives struvite (magnesium ammonium phosphate) stone formation.
It is a leading cause of canine UTI, especially after catheterization and in recurrent cases.
Laboratory identification uses MacConkey agar (colorless colonies), urease testing (positive), and swarming on blood agar.
ESBL and carbapenemase resistance is emerging in companion animal isolates.
Treating the infection without removing the stone leads to recurrence.

Frequently Asked Questions

What does Proteus mirabilis look like on a culture plate?

On blood agar it forms concentric rings of swarming growth that can cover the plate. On MacConkey agar it forms colorless colonies because it does not ferment lactose.

Why does Proteus mirabilis cause bladder stones?

Its urease enzyme splits urea into ammonia, which raises urine pH. At alkaline pH, magnesium and phosphate precipitate as struvite crystals that aggregate into stones.

Is Proteus mirabilis contagious to other pets or to people?

It is a normal gut inhabitant of mammals and can be shared between species. Genetic studies have found closely related strains in dogs, cats, and humans, which is why a One Health approach to surveillance is recommended [15][1].

How is Proteus mirabilis identified in the laboratory?

By growth on MacConkey agar, a strongly positive urease test, a negative indole test, hydrogen sulfide production on TSI agar, and the characteristic swarming pattern on blood agar. MALDI-TOF mass spectrometry and 16S rRNA sequencing are used for confirmation.

Can a Proteus urinary tract infection be treated without removing the stone?

No. Antibiotics alone do not clear bacteria embedded in a struvite stone. The stone must be dissolved or surgically removed for the infection to resolve.

Why is swarming a problem in the diagnostic laboratory?

Swarming growth can overgrow an entire agar plate and hide other bacteria in a mixed infection, leading to an incomplete diagnosis.