A comparative genomics analysis revealed 702 genes present in the bacterial Gram-negative core gene set (92 species analyzed) and 959 genes in the Gram-positive core gene set (93 species analyzed). Mycoplasma genitalium, which has the smallest known genome (517 genes) of a non-symbiont, was used in a three-way reciprocal analysis with the Gram-negative core genes and the Gram-positive core genes, and 151 common bacterial core genes were found. Of these 151 core genes, 39 were putative genes encoding the 30S and 50S ribosomal subunits, whilst among recognized cell division genes, only one gene, the major ftsZ, was present. In addition, 86 reciprocal matches were identified between the 151 common bacterial genes and a previously determined 2,723 common eukaryotic core gene set. An analysis was also done to optimize the threshold bit score used to declare that genes were homologous, and a bit score cutoff of 40 was selected.