Acetic acid
Acetic acid is produced and excreted by acetic acid bacteria, notably the genus "Acetobacter" and "Clostridium acetobutylicum". These bacteria are found universally in foodstuffs, water, and soil, and acetic acid is produced naturally as fruits and other foods spoil. 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.