Photon emission by bacteria challenged with phenylacetaldehyde. A possible distinction between gram-positive and gram-negative bacteria.

With all bacteria tested, addition of phenylacetaldehyde leads to light emission. The latter is markedly stronger with gram-negative bacteria, presumably because they possess a thinner wall and an extra external lipophilic membrane. Consistent with this explanation, the bactericidal effect of phenylacetaldehyde is also stronger with gram-negative bacteria. The spectrum of the emitted light shows maximal emission in the 500 nm region and is very similar to that observed when a protein (bovine serum albumin), free amino acids or isopropylamine reacts with phenylacetaldehyde.