Biofilms and Encrustation
Infection of the catheterised urinary tract by bacteria that produce the enzyme urease can be particularly troublesome. These organisms colonize the catheter surfaces forming extensive biofilm communities. They are capable of generating ammonia from urea, and elevating the pH of the urine and biofilm. Under these conditions, large crystals of struvite and small amorphous crystalline forms of carbonate apatite precipitate in the urine and in the developing biofilm. Aggregates of crystals and bacteria form in the urine and these also become deposited onto the catheter and incorporated into the developing bacterial community to form a crystalline biofilm.
Electron micrograph of the urine drainage channel of an encrusted catheter in false colour. The catheter material is shown in blue, with the crystalline biofilm in pink.
Struvite crystals are composed of magnesium ammonium phosphate. The carbonate apatite is a poorly defined crystalline form of calcium phosphate in which varying proportions of the phosphate groups have been replaced by carbonate ions.
Crystal formation
In the presence of bacterial urease, the urea which is present in urine in large amounts is hydrolysed to produce ammonia and carbonic acid.
In aqueous solution, the carbonic acid and two ammonia molecules produced from these reactions exist in equilibrium with the ions hydrogencarbonate and ammonium.
The ammonium production results in an increase in the proportion of hydroxide ions so the urine becomes more alkaline. Normal urine has a pH of about 6 to 6.5, but in the presence of an infection with a urease positive organism, this can increase to 8 or 9.
Every sample of urine has a characteristic pH, termed the nucleation pH, above which the calcium and magnesium salts will crystallize. When the urease activity elevates the pH of the urine above the pHn, the apatite and struvite will form.
The urease producing organism most commonly found associated with the resulting crystalline biofilms is Proteus mirabilis.
^ back to the top