 Although queuosine is found in the tRNA of nearly all eukaryotic organisms, it is produced exclusively by bacteria; higher organisms must obtain queuine from the diet or salvage it from symbiotic microbes—a process for which dedicated enzymatic machinery exists.As such, queuine has been described as a vitamin.Because it can be generated from guanine by some species in the human microbiome, its status as a vitamin may be analogous to that of 4-Aminobenzoic acid, a "conditional" vitamin which is only essential in the diet if the microbiome does not produce sufficient quantities.