I was preparing LB agar plates with my students, when one of them made an astute observation. You see, the LB medium used – a mix of salts, tryptone, and yeast extract – is designed specifically to support the growth of bacteria such as E. coli. Now, the natural habitat for E. coli is in the digestive tract of terrestrial vertebrates. The medium contains the types of nutrients that this organism would find in its natural habitat.
And that is why, as my student so aptly put it, LB medium smells like ass.