This study investigates the phylogenetic relationships of butyrate-producing bacteria in the human gut. Butyrate, a key energy source for colonic epithelial cells, is produced by anaerobic bacteria in the gut. Researchers isolated butyrate-producing bacteria from fecal samples of three individuals: an infant, an omnivore, and a vegetarian, and repeated the process one year later. Of 313 isolates, 74 produced more than 2 mM butyrate in vitro. These isolates were analyzed using 16S rDNA PCR-RFLP, revealing little overlap in ribotypes among the three individuals, with significant changes in the flora between the two isolations. Complete 16S rDNA sequences of 24 strains were determined, showing that 80% of butyrate producers belonged to the XIVa cluster of gram-positive bacteria, with the most abundant group clustering with Eubacterium rectale, Eubacterium ramulus, and Roseburia cecicola. Fifty percent of the isolates were net acetate consumers, suggesting they use the butyryl coenzyme A-acetyl coenzyme A transferase pathway for butyrate production. In contrast, only 1% of non-butyrate-producing isolates consumed acetate. The study highlights the diversity and variability of butyrate-producing bacteria in the human gut, influenced by diet and age. The results suggest that these bacteria play a crucial role in gut health, particularly in maintaining colonic epithelial cell function and preventing diseases like cancer and ulcerative colitis. The findings emphasize the need for further research to understand the roles of these bacteria in diet-related colonic fermentation and overall host health. The study also demonstrates the utility of molecular techniques in characterizing the diversity of gut microbiota.This study investigates the phylogenetic relationships of butyrate-producing bacteria in the human gut. Butyrate, a key energy source for colonic epithelial cells, is produced by anaerobic bacteria in the gut. Researchers isolated butyrate-producing bacteria from fecal samples of three individuals: an infant, an omnivore, and a vegetarian, and repeated the process one year later. Of 313 isolates, 74 produced more than 2 mM butyrate in vitro. These isolates were analyzed using 16S rDNA PCR-RFLP, revealing little overlap in ribotypes among the three individuals, with significant changes in the flora between the two isolations. Complete 16S rDNA sequences of 24 strains were determined, showing that 80% of butyrate producers belonged to the XIVa cluster of gram-positive bacteria, with the most abundant group clustering with Eubacterium rectale, Eubacterium ramulus, and Roseburia cecicola. Fifty percent of the isolates were net acetate consumers, suggesting they use the butyryl coenzyme A-acetyl coenzyme A transferase pathway for butyrate production. In contrast, only 1% of non-butyrate-producing isolates consumed acetate. The study highlights the diversity and variability of butyrate-producing bacteria in the human gut, influenced by diet and age. The results suggest that these bacteria play a crucial role in gut health, particularly in maintaining colonic epithelial cell function and preventing diseases like cancer and ulcerative colitis. The findings emphasize the need for further research to understand the roles of these bacteria in diet-related colonic fermentation and overall host health. The study also demonstrates the utility of molecular techniques in characterizing the diversity of gut microbiota.