Antibiotics can decrease colonization of a common intestinal bacterium that metabolizes oxalate, perhaps rendering patients more susceptible to the formation of calcium oxalate kidney stones, according to researchers.

The bacterium, Oxalobacter formigenes (OF) is a commensal organism that colonizes the human colon in a large proportion of the normal adult population. OF bacteria metabolize oxalate as their sole energy source. By decreasing the availability of dietary oxalate for absorption , these bacteria could lower the risk of hyperoxaluria and thus prevent development of calcium oxalate stones.

In a prospective study of patients undergoing upper endoscopy, investigators led by David S. Goldfarb, MD, Clinical Chief, Nephrology Division, New York University (NYU) Medical Center and Professor of Medicine and Physiology at the NYU School of Medicine compared the effect of antibiotics on OF colonization in two groups: patients receiving antibiotics to treat gastric infection with Helicobacter pylori and patients without H. pylori who did not receive antibiotics (controls). The researchers identified OF colonization in stool by oxalate degradation at baseline, with confirmation by polymerase chain reaction assay. These tests were repeated after one and six months.

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The prevalence of OF intestinal colonization was 43.1% among all patients. Among the 12 patients who tested positive for OF but were not receiving antibiotics, 11 (92%) had the bacteria present on stool tests at one and six months. Of the 19 subjects who tested positive for  OF and received antibiotics, only seven (36.8%) continued to be colonized by the bacterium on follow-up stool testing at one and six months, a significant difference between the groups, according to a report in the Journal of Endourology (2011;25:1781-1785).

“Antibiotics markedly reduced the colonization rates with OF, while the control group— people who were [endo]scoped but did not have H. pylori—did not receive antibiotics and did not have elimination of colonization with OF,” said Dr. Goldfarb, Director of the Kidney Stone Prevention Program at Lenox Hill Hospital in New York. “The suggestion here is that antibiotics can leave one at increased risk of stones.”

Previous studies have shown that people who took antibiotics had less colonization by OF, but this had never been shown prospectively, he said.

Additionally, Dr. Goldfarb’s group found that a combination of amoxicillin and clarithromycin caused 62.5% of subjects to become negative for OF at one month; at six months, 56.2% remained negative.

“We did not measure urine oxalate because these patients are not stone formers and measuring oxalate would require careful attention to diets, and for this group that was not tenable,” Dr. Goldfarb noted.