Lactic acid bacteria: living organisms that emerged 3 billion years ago.

Lactic acid bacteria (LAB) are known to be useful for humans. They contribu te to the manufacture and the preservation of many foods. Despite their imp ortance, only few phylogenetic studies adress the position and role of LAB in the evolution of life on Earth. This review examines molecular data and data of paleontology and metabolism that can be used to determine when LAB first appeared. Phylogenetic studies based on ribosomal DNA sequence compar isons and on signature-sequence analysis present in well-conserved proteins suggests an ancestral position in evolution of Gram positive bacteria with low GC%, including LAB. By these criteria, LAB may have appeared before th e photosynthetic cyanobacteria. As the latter were found in sediments dated 2.75 billion years ago, LAB may have emerged 3 billion years ago. Their po or adaptation to aerobic environments also suggests that they may have emer ged during the transition of Earth's atmosphere from anaerobic to aerobic.