Pre-formed urease activity of Helicobacter pylori as determined by a viable cell count technique--clinical implications.

The pre-formed urease activity of three NCTC reference strains and five clinical isolates of Helicobacter pylori was determined at room temperature (21 degrees C) and 37 degrees C by a viable cell count technique with a conventional urea slope test (Christensen's agar) as well as the commercial CLO-test. The urease activity of two gastroduodenal commensals, Proteus mirabilis and Klebsiella pneumoniae, was also tested. H. pylori strains produced positive reactions with viable cell counts of 10(6)-10(8) cfu within 30 min and with counts of 10(3)-10(6) cfu within 2 h. For some strains, smaller numbers of organisms were needed with the CLO-test than with the conventional test, and incubation of the CLO-test strips at 37 degrees C slightly decreased the number of organisms required for positive results. P. mirabilis produced a positive result on urea slopes with an initial inoculum of 10(7)-10(8) cfu at 2 h, but no positive reaction occurred for K. pneumoniae at 12 h, even with an initial inoculum of 10(11) cfu. However, both P. mirabilis and K. pneumoniae gave a positive result after incubation for 24 h with initial inocula of < 10(1) cfu and 10(3)-10(4) cfu respectively. Incubation at 37 degrees C significantly reduced the inoculum size of these organisms required for a positive result after incubation for 4 h when tested with the slopes, but not with the CLO-test. These findings indicate that H. pylori possesses much greater pre-formed urease activity than P. mirabilis and K. pneumoniae. False negative results for clinical detection of H. pylori in gastroduodenal biopsies may be due to small numbers of organisms, especially after treatment with antimicrobial agents, and false positive results may arise from gastroduodenal commensals or contaminants.