The study investigates the composition, variability, and temporal stability of the intestinal microbiota in elderly individuals. Using pyrosequencing of 16S rRNA gene V4 region amplicons, the researchers analyzed the fecal microbiota of 161 elderly subjects (65+ years) and 9 younger controls (28–46 years). The microbiota of each individual was unique and separable from others. Bacteroidetes dominated in elderly subjects (average 57%), while Firmicutes averaged 40%. The microbiota showed significant interindividual variability, with some phyla like Proteobacteria and Actinobacteria showing dramatic differences. The core microbiota of elderly subjects was distinct from that of younger adults, with higher Bacteroidetes abundance and different abundance patterns of Clostridium groups. Follow-up samples showed that 85% of subjects had microbiota compositions more similar to their time-0 samples than any other dataset. The study concludes that the fecal microbiota of the elderly shows temporal stability in most subjects but is characterized by unusual phylum proportions and extreme variability. The microbiota of elderly subjects was found to be more diverse and less dominated by Firmicutes compared to younger adults. The study highlights the importance of understanding the gut microbiota in the elderly, as it may be a determinant of health and could be modulated by dietary or other interventions. The research also shows that antibiotic use significantly affects the microbiota composition, with changes in phylum and genus abundance. The study provides insights into the variability and stability of the gut microbiota in the elderly, emphasizing the need for further research into its functional implications.The study investigates the composition, variability, and temporal stability of the intestinal microbiota in elderly individuals. Using pyrosequencing of 16S rRNA gene V4 region amplicons, the researchers analyzed the fecal microbiota of 161 elderly subjects (65+ years) and 9 younger controls (28–46 years). The microbiota of each individual was unique and separable from others. Bacteroidetes dominated in elderly subjects (average 57%), while Firmicutes averaged 40%. The microbiota showed significant interindividual variability, with some phyla like Proteobacteria and Actinobacteria showing dramatic differences. The core microbiota of elderly subjects was distinct from that of younger adults, with higher Bacteroidetes abundance and different abundance patterns of Clostridium groups. Follow-up samples showed that 85% of subjects had microbiota compositions more similar to their time-0 samples than any other dataset. The study concludes that the fecal microbiota of the elderly shows temporal stability in most subjects but is characterized by unusual phylum proportions and extreme variability. The microbiota of elderly subjects was found to be more diverse and less dominated by Firmicutes compared to younger adults. The study highlights the importance of understanding the gut microbiota in the elderly, as it may be a determinant of health and could be modulated by dietary or other interventions. The research also shows that antibiotic use significantly affects the microbiota composition, with changes in phylum and genus abundance. The study provides insights into the variability and stability of the gut microbiota in the elderly, emphasizing the need for further research into its functional implications.