Most
kidney stones are composed primarily of
calcium oxalate. Oxalobacter formigenes is a Gram-negative, anaerobic bacterium that metabolizes
oxalate in the intestinal tract and is present in a large proportion of the normal adult population. It was hypothesized that the absence of O. formigenes could lead to increased colonic absorption of
oxalate, and the subsequent increase in urinary
oxalate could favor the development of stones. To test this hypothesis, a case-control study involving 247 adult patients with recurrent
calcium oxalate stones and 259 age-, gender-, and region-matched control subjects was performed. The prevalence of O. formigenes, determined by stool culture, was 17% among case patients and 38% among control subjects; on the basis of multivariate analysis controlling demographic factors, dietary
oxalate, and
antibiotic use, the odds ratio for colonization was 0.3 (95% confidence interval 0.2 to 0.5). The inverse association was consistently present within strata of age, gender, race/ethnicity, region, and
antibiotic use. Among the subset of participants who completed a 24-h urine collection, the risk for
kidney stones was directly proportional to urinary
oxalate, but when urinary factors were included in the multivariable model, the odds ratio for O. formigenes remained 0.3 (95% confidence interval 0.1 to 0.7). Surprisingly, median urinary
oxalate excretion did not differ with the presence or absence of O. formigenes colonization. In conclusion, these results suggest that colonization with O. formigenes is associated with a 70% reduction in the risk for being a recurrent
calcium oxalate stone former.