Propionic acid (PA) is an organic acid. It exists a clear liquid with a pungent and unpleasant smell somewhat resembling body odor. Propionic acid (PA) is widely used as an antifungal agent in food. It is present naturally at low levels in dairy products and occurs ubiquitously, together with other short-chain fatty acids (SCFA), in the gastro-intestinal tract of humans and other mammals as an end-product of the microbial digestion of carbohydrates. The metabolism of propionic acid begins with its conversion to propionyl coenzyme A, the usual first step in the metabolism of carboxylic acids. Since propionic acid has three carbons, propionyl-CoA cannot directly enter either beta oxidation or the citric acid cycles. In most vertebrates, propionyl-CoA is carboxylated to D-methylmalonyl-CoA, which is isomerised to L-methylmalonyl-CoA. Propionic acid has significant physiological activity in animals. Propionic acid is irritant but produces no acute systemic effects and has no demonstrable genotoxic potential (PMID 1628870). The human skin is host of several species of bacteria known as Propionibacteria, which are named after their ability to produce propionic acid. The most notable one is the Cutibacterium acnes (formerly known as Propionibacterium acnes), which lives mainly in the sebaceous glands of the skin and is one of the principal causes of acne. Propionic aciduria is one of the most frequent organic acidurias, a disease that comprise many various disorders. The outcome of patients born with Propionic aciduria is poor intellectual development patterns, with 60% having an IQ less than 75 and requiring special education. Successful liver and/or renal transplantations, in a few patients, have resulted in better quality of life but have not necessarily prevented neurological and various visceral complications. These results emphasize the need for permanent metabolic follow-up whatever the therapeutic strategy (PMID 15868474). Decreased early mortality, less severe symptoms at diagnosis, and more favorable short-term neurodevelopmental outcome were recorded in patients identified through expanded newborn screening. (PMID 16763906)‚Üµ When propionic acid is infused directly into rodents' brains, it produces hyperactivity, dystonia, social impairment, perseveration and brain changes (e.g., innate neuroinflammation, glutathione depletion) that may be used as a means to model autism in rats. Propionic acid is a metabolite of Bacteroides, Clostridium, Dialister, Megasphaera, Phascolarctobacterium, Propionibacterium, Propionigenum, Salmonella, Selenomonas and Veillonella (https://www.mdpi.com/2311-5637/3/2/21).