The most minimal bacteria

Today I learned about the existence of the bacteriome, a specialized organ found in some insect species which is just chock full of endosymbiotic bacteria. Most animals provide hosts for bacteria, but the critical part here is the endosymbiotic nature: these symbionts must live and reproduce within host cells. As a result, many insect endosymbionts are quite odd in genetic terms and have tiny genomes. They can only grow to a certain population size, too, as they're limited by the space available within those host cells.

One such example of the resulting genetic oddities is found in Hodgkinia symbionts from cicada bacteriomes. A report by McCutcheon et al in 2009 showed howHodgkinia cicadicolaappear to have re-coded their UGA codons to code for tryptophan rather than the usual Stop codon. This specific re-coding has been observed before, but only in very low-GC content species, of which Hodgkinia is not one (it has a GC% of more than 58 percent). This symbiont also has a crazy-small genome at 144 kb. That was the smallest bacterial genome yet sequenced butNasuia deltocephalinicola, another insect endosymbiont, has it beat by 22 kb.