The purpose of this study is to describe the intestinal microbiomes of a cohort of healthy adult subjects over a 6-month period of time. Many questions about the human microbiota exist. Previous studies have shown that the differences among individuals are greater than the differences among different sampling sites in a single individual. Up to 60 healthy adult subjects, ages 18-45 years, from the Baltimore and University of Maryland Baltimore communities will be recruited and screened to document their health status. A brief medical history, including recent travel and antibiotic use, will be recorded. Subjects will provide a stool specimen for genomic analysis of the intestinal microbiome over a 6-month period at the following intervals: Day 0, Week 1, Week 2, Week 3, Week 4, Week 8, Week 12, Week 13, Week 14, Week 15, Week 16, Week 20, and Week 24. At Week 12, subjects will receive a 3-day course of oral Ciprofloxacin 500 mg every12 hours. Weekly stool specimens will be obtained beginning with the first day of antibiotic use and for 4 Weeks thereafter. Then monthly specimens of stool and other sites will be resumed at Week 16. The primary objective is to define the human intestinal microbiome in healthy adults in a longitudinal fashion. The secondary objective is to define the re-colonization of the intestine after treatment with a broad-spectrum antibiotic using comprehensive genomic techniques. Each subject will participate in the study for up to 7 months.