Potential contribution of optional urease-positive bacteria to idiopathic urinary calcium stone formation. I. Expression of urease activity in bacteria from the urinary tract that are commonly classified as urease-negative.

The aetiology of calcium oxalate stones, which are commonly believed to be sterile, has not yet been fully elucidated. Recent bacteriological studies and investigations using the scanning electron microscope have also shown microorganisms to be present in this type of stone. These microorganisms were assumed not to be able to split urea. To list the most common urease-negative bacteria established in the human urinary system, we isolated apparently urease-negative microorganisms from a consecutive series of 58 urinary stone-forming patients by using standard selecting agars. Pure strains were incubated in an inductive medium lacking all sources of nitrogen except urea. Induction of urease activity was monitored by a test based on the reaction of phenol/hypochlorite with ammonium ions. This test revealed whether the urease negativity of a strain indicated by the selective agar was optional or absolute. All strains we investigated by this method and which were classified by standard methods as urease-negative we found produced urease activity which was clearly measurable, though it was often comparatively small. In the light of these results, the matrix theory of calcium oxalate stone development will need some modifications.