Some bacteria, especially Proteus, Pseudomonas, Klebsiella, and Aerobactersp, possess urease, which is an enzyme that catalyzes the hydrolysis of urea to CO₂ and ammonia. By accepting protons to become NH₄⁺, the pH of the urine is raised. Therefore, the concentration of PO₄^3– increases, and the ion associates with NH₄⁺ and magnesium ion to form the crystalline struvite stone.1,52 Because the amounts of urea, phosphate, and magnesium generally are large, struvite stones grow rapidly to form staghorns. The other product of urea hydrolysis, CO₂, is readily converted to HCO₃^– in the tubules. The HCO₃^–, in turn, becomes carbonate (CO₃^2–) as it loses a proton to NH₃. The CO₃^2–ion crystallizes with calcium to form calcium carbonate, which helps to increase the mass of stone material, and accounts for the almost universal finding of calcium salts in struvite stones.53