Acetic acid is a two-carbon, straight-chain fatty acid. It is the smallest short-chain fatty acid (SCFA) and one of the simplest carboxylic acids. is an acidic, colourless liquid and is the main component in vinegar. Acetic acid has a sour taste and pungent smell. It is an important chemical reagent and industrial chemical that is used in the production of plastic soft drink bottles, photographic film; and polyvinyl acetate for wood glue, as well as many synthetic fibres and fabrics. In households diluted acetic acid is often used as a cleaning agent. In the food industry acetic acid is used as an acidity regulator. Acetic acid is found in all organisms, from bacteria to plants to humans. The acetyl group, derived from acetic acid, is fundamental to the biochemistry of virtually all forms of life. When bound to coenzyme A (to form acetylCoA) it is central to the metabolism of carbohydrates and fats. However, the concentration of free acetic acid in cells is kept at a low level to avoid disrupting the control of the pH of the cell contents. Acetic acid is produced and excreted in large amounts by certain acetic acid bacteria, notably the Acetobacter genus and Clostridium acetobutylicum. These bacteria are found universally in foodstuffs, water, and soil. Due to their widespread presence on fruit, acetic acid is produced naturally as fruits and many other sugar-rich foods spoil. Several species of anaerobic bacteria, including members of the genus Clostridium and Acetobacterium can convert sugars to acetic acid directly. However, Clostridium bacteria are less acid-tolerant than Acetobacter. Even the most acid-tolerant Clostridium strains can produce acetic acid in concentrations of only a few per cent, compared to Acetobacter strains that can produce acetic acid in concentrations up to 20%. Acetic acid is also a component of the vaginal lubrication of humans and other primates, where it appears to serve as a mild antibacterial agent. Acetic acid can be found in other biofluids such as urine at low concentrations. Urinary acetic acid is produced by bacteria such as Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumonia, Enterobacter, Acinetobacter, Proteus mirabilis, Citrobacter frundii, Enterococcus faecalis, Streptococcus group B, Staphylococcus saprophyticus (PMID: 22292465). Acetic acid concentrations greater than 30 uM/mM creatinine in the urine can indicate a urinary tract infection, which typically suggests the presence of E. coli or Klebshiella pneumonia in the urinary tract. (PMID: 24909875) Acetic acid is also produced by other bacteria such as Akkermansia, Bacteroidetes, Bifidobacterium, Prevotella and Ruminococcus (PMID: 20444704; PMID: 22292465).