Evolution of the bacterial species XXXX : a partial genomic study with reflections on prokaryotic species concept.

@entity6043 @entity16121 @entity3649 @entity15669
The species @entity6043 consists at present of three subspecies, delbrueckii, @entity16121 and bulgaricus, showing a high level of DNA-DNA hybridization similarity but presenting markedly different traits related to distinct ecological adaptation. The internal genetic heterogeneity of the bacterial species @entity6043 was analyzed. Phenotypic and several genetic traits were investigated for 61 strains belonging to this species. These included 16S rDNA sequence mutations, expression of beta-galactosidase and of the cell wall-anchored protease, the characterization of the lactose operon locus and of the sequence of lacR gene, @entity3649 metabolism, and the distribution of insertion sequences. The high genetic heterogeneity of taxa was confirmed by every trait investigated: the lac operon was completely deleted in the subsp. delbrueckii, different mutation events in the repressor gene of the operon led to a constitutive expression of lacZ in the subsp. bulgaricus. Structural differences in the same genetic locus were probably due to the presence of different IS elements in the flanking regions. The different expression of the cell wall-anchored protease, constitutive in the subsp. bulgaricus, inducible in the subsp. @entity16121 , and absent in the subsp. delbrueckii was also a consequence of mutations at the gene level. The galT gene for @entity3649 metabolism was found only in the subsp. @entity16121 , while no specific amplification product was detected in the other two subspecies. All these data, together with the absence of a specific IS element, ISL6, from the major number of strains belonging to the subsp. bulgaricus, confirmed a deep internal heterogeneity among the three subspecies. Moreover, this evidence and the directional mutations found in the 16S rDNA sequences suggested that, of the three subspecies, @entity16121 is the taxon closer to the ancestor. Limitations of the current @entity15669 were also discussed, based on presented evidences. Our results indicate the need for an accurate investigation of internal heterogeneity of bacterial species. This study has consequences on the prokaryotic species concept, since genomic flexibility of prokaryotes collides with a stable classification, necessary from a scientific and applied point of view.
