title: Defining natural species of bacteria: clear-cut genomic boundaries revealed by a turning point in nucleotide sequence divergence.
abstract: BACKGROUND
  Bacteria are currently classified into arbitrary species, but whether they actually exist as discrete natural species was unclear. To reveal genomic features that may unambiguously group bacteria into discrete genetic clusters, we carried out systematic genomic comparisons among representative bacteria.
  
  
  RESULTS
  We found that bacteria of Salmonella formed tight phylogenetic clusters separated by various genetic distances: whereas over 90% of the approximately four thousand shared genes had completely identical sequences among strains of the same lineage, the percentages dropped sharply to below 50% across the lineages, demonstrating the existence of clear-cut genetic boundaries by a steep turning point in nucleotide sequence divergence. Recombination assays supported the genetic boundary hypothesis, suggesting that genetic barriers had been formed between bacteria of even very closely related lineages. We found similar situations in bacteria of Yersinia and Staphylococcus.
  
  
  CONCLUSIONS
  Bacteria are genetically isolated into discrete clusters equivalent to natural species.
authors.affiliation: Genomics Research Center, The State-Province Key Laboratories of Biomedicine-Pharmaceutics of China, Harbin, China.
authors.forename: Le
authors.initials: L
authors.lastname: Tang
authors.forename: Yang
authors.initials: Y
authors.lastname: Li
authors.forename: Xia
authors.initials: X
authors.lastname: Deng
authors.forename: Randal N
authors.initials: RN
authors.lastname: Johnston
authors.forename: Gui-Rong
authors.initials: GR
authors.lastname: Liu
authors.forename: Shu-Lin
authors.initials: SL
authors.lastname: Liu
mesh_terms: D001419:Bacteria / Q000235:genetics*; D004269:DNA, Bacterial / Q000235:genetics; D019143:Evolution, Molecular; D014644:Genetic Variation / Q000235:genetics*; D023281:Genomics / Q000379:methods*; D009711:Nucleotides / Q000235:genetics*; D010802:Phylogeny; D011995:Recombination, Genetic / Q000235:genetics
chemical_list: D004269:DNA, Bacterial; D009711:Nucleotides