Luniverse est un ensemble dissyme ´trique... The universe is asymmetric and I am persuaded that life, as it is known to us, is a direct result of the asymmetry of the universe or of its indirect consequences. The universe is asymmetric... Louis Pasteur, 1874
Knowledge of the formation of d-amino acids (d-AAs) in alkali-treated proteins dates back as early as 1909. The hampered enzymatic digestion and drastically lowered nutritional utilization of the resulting products were already recognized.
Although, in the late 1930s, the presence of poly-g-d-glutamic acid in the bacterial cell envelope of the virulent Bacillus anthracis was realized, and, in the 1940s, a rapidly increasing number of microbial peptide antibiotics containing frequently d-AAs was discovered, their natural occurrence was rather considered as peculiarity of microorganisms.
Notably, g-(d,l)-polyglutamic acid (g-PGA) is the major component of the viscous sticky mucilage of Bacillus subtilis (natto) that is nowadays widely used for the mass production of traditional fermented soybean foods in Asian countries. Many species and strains of Bacillus employed in the biotechnology industry produce g-PGA with varying ratios of d-and l-enantiomers.
At the beginning of the 1970s, a highly noticed concept for the chemotaxonomy of prokaryotic organisms was developed, and it is still in use. It is based on the sequence and configuration of the AAs forming the cross-linking peptides of the so-called peptidoglycan. Remarkably, d-Ala and d-Glu, as well as several other d-AAs were common in these peptides, and essential for the structure and function of the bacterial cell envelope.
Consequently, the detection of abundant l-AA racemases and isomerases in these microorganisms was not surprising. However, in this context it is worth noting that, until the late 1980s, it had not been fully realized in the discussion on possible harmful effects related to d-AAs that the common consumption of microbially fermented foods and beverages is inevitably connected to the intake of d-AAs. This is of nutritional importance, since an abundance of foodstuffs such as fermented dairy and soybean products, sour dough bread, fermented meat, sausages, and condiments or alcoholic fermented beverages constitute a considerable part of the human diet.
To continue, probiotics, representing health food and feed, are defined as live microorganisms which when administered in adequate amounts confer a health benefit on the host. Probiotics are preparations mostly of selected lactic acid bacteria or bifidobacteria which contain d-AAs in their peptidoglycan. Thus, human beings have a steady intake of nutritional d-AAs originating from bacteria. These considerations can be extended to the indigenous bacterial flora of the gastrointestinal tract.