Human Armpit cavity provides an ideal environment for growth of bacteria. The intensity of bacteria in the armpits is mind boggling. In fact, it is reported that there are a million bacteria per square centimeter. Not only are their population so high, there are so many different types of them. They produce chemicals by breaking down the precursor chemicals present in sweat which cause the stinking smell. The odours are so embarrassing to people that millions of dollars industry of deodorants thrives due to this.
This means that in addition to ideal temperature and lodging facilities like folds of skin and hair, the armpits secretions must be supplying them with chemicals they depend upon to extract energy for survival.
Armpits contain sweat chemicals. When these chemicals are broken down into other compounds by bacteria many of these end products give of very obnoxious smells. Bacteria produce sulphur containing compounds which give off nauseating onion like smell. Then there are compounds which are degraded into organic acids. One of the acids gives the “cumin like smell” while another acid gives odour similar to the one which comes from the goats.
Then they produce pheromones which are the chemicals that attracts the opposite gender person. Two possible pheromones are Androstenol (alcohol) which smells like “musk” and other is Androstenone. (ketone) which smells differently to different persons.
Two low molecular weight acids namely iso-valeric and we’ll known acetic acid are also produced. The first one gives cheesy, sweaty foot smell and other as is commonly known as the vinegar smell.
These chemicals are produced by bacteria using different enzyme reaction routes..
The details of the odour chemicals and their precursors, enzyme and bacteria responsible for these are given in this article..