“Urease positive” refers to microorganisms that produce an enzyme called urease. This characteristic is observed in various bacteria and fungi, signifying their ability to break down a specific compound in their environment. Understanding which organisms are urease positive is important in microbiology and medicine, as this enzymatic activity often plays a role in how these microbes interact with their hosts and contribute to certain health conditions.
The Urease Enzyme and Its Function
Urease is a metalloenzyme containing two nickel ions, which are necessary for its function. This enzyme catalyzes the hydrolysis of urea, a nitrogen-containing waste product naturally present in the body. The reaction converts urea into ammonia and carbon dioxide.
Ammonia production is a significant outcome of urease activity because it is a basic compound. This leads to an increase in the pH of the surrounding environment, a process known as alkalinization. This localized pH change is a defining feature of urease activity, impacting microbial survival and disease development.
Identifying Urease Activity
Detecting urease activity in a laboratory relies on observing this pH change. The principle of the urease test involves placing a microbial sample into a medium that contains urea and a pH indicator, such as phenol red. When urease is produced by the microorganisms, it breaks down the urea, releasing ammonia.
The ammonia then raises the medium’s pH, causing the pH indicator to change color and providing a visible sign of urease activity. For instance, phenol red changes from a yellowish or light orange color at an acidic pH (around 6.8) to a bright pink or magenta color at an alkaline pH (around 8.1). Rapid urease-positive organisms can show this color change within minutes to 24 hours, while weakly positive ones might take several days. Common tests include the Rapid Urease Test (RUT) for Helicobacter pylori and urea broth or agar tests for other bacteria.
Common Urease-Positive Organisms and Related Conditions
Several bacteria are recognized for their urease activity, which contributes to their ability to cause disease. Helicobacter pylori is a spiral-shaped bacterium that strongly produces urease. This urease activity is essential for its survival in the highly acidic environment of the stomach, as the ammonia it produces creates a more neutral “cloud” around the bacterium, allowing it to colonize the gastric lining. H. pylori is a major cause of stomach ulcers, chronic gastritis, and is also linked to an increased risk of stomach cancer.
Proteus species, including Proteus mirabilis and Proteus vulgaris, are also strong urease producers. These bacteria are frequently associated with urinary tract infections (UTIs). Their urease activity breaks down urea in urine, leading to an increase in urine pH. This alkaline environment promotes the formation of struvite stones, a type of kidney stone composed of magnesium ammonium phosphate and calcium phosphate. Proteus mirabilis is particularly noted for forming crystalline biofilms on urinary catheters, contributing to complicated UTIs.
Klebsiella pneumoniae also exhibits urease activity. While it is a common cause of various infections, including pneumonia and bloodstream infections, its urease production contributes to UTIs. Like Proteus, its urease can lead to urine alkalinization and contribute to urinary stone development, though it is less frequently associated with struvite stones.
Other urease-positive organisms include Ureaplasma urealyticum and Morganella morganii. Ureaplasma urealyticum is a bacterium found in the urogenital tract that can cause genitourinary infections, such as urethritis and bacterial vaginosis. Its urease activity can also contribute to the formation of struvite stones in the urinary tract. Morganella morganii is a Gram-negative bacterium often implicated in hospital-acquired infections, including UTIs, and its urease production has been linked to the formation of urinary stones.
Clinical Significance and Management
The identification of urease-positive organisms is important in diagnosis and guiding treatment strategies. For instance, the Rapid Urease Test (RUT) is a widely used diagnostic tool for Helicobacter pylori infection. A small tissue sample from the stomach lining is placed in a urea-containing medium, and a rapid color change indicates the presence of H. pylori urease, allowing for prompt diagnosis. Another non-invasive test for H. pylori is the urea breath test, where a patient ingests labeled urea, and the presence of labeled carbon dioxide in their breath signifies urease activity.
Understanding urease activity influences treatment. This often involves antibiotics to eradicate the infection. In cases of stone formation, strategies focus on stone removal and preventing recurrence.