Doctors treated 5 of the patients, who had ulcers and H. pylori infection, with antibiotics (clarithromycin and metronidazole). They did not give the 5 patients without ulcers antibiotics. The researchers collected fecal samples from all patients at the beginning of the study and then several days and 1 and 3 years later. They cultured enterococci from the samples and did special tests (DNA fingerprints, gene tests, and minimal inhibitory concentration) to see whether the enterococci were resistant to clarithromycin.