require pragmatic and subjective approaches which do not necessitate species to be monophyletic groups’’. Recently, it has also been reported that changes in gene flow caused by ecological factors may even lead to an incipient merging of two bacterial species,
Campylobacter jejuni and C. coli, reversing the process of speciation [36]. After 10,000 years of shared adaptation to a particular habitat the genome of C. coli is becoming ever more like that of C. jejuni and the two may eventually become indistinguishable.
Despite this pessimistic view of some scientists on a reliable classification of prokaryotes, it is quite clear that gene swapping is more pronounced between closely related organisms, and mainly comprises genes that are not suitable candidates for phylogenetic analysis [28,30]. There are also other indications that support the feasibility of a stable classification of bacteria. There are many discrete well-defined genotypic clusters that are congruent with known species delineated by polyphasic approaches [19,22]. Even in the face of substantial genomic fluidity it seems that the typical geno- and phenotypic characteristics of a taxon are still maintained and, as shown in many cases, are sufficient for a reliable classification and identification of bacteria [30].
The rather conserved and ubiquitous genes of the core genome are potential phylogenetic markers for the genotypic classification of less related prokaryotes. Thus, comparative sequence analysis of certain core genes, including rRNA genes, may be useful for retrieving the phylogenetic relationships of higher taxa. For the classification of lower taxa, such as the delineation of species, various character genes may be suitable as phylogenetic markers. In particular, genes that code for key phenotypic differences may play an important role in the improved classification of bacteria. Moreover, core and/or character genes coding for functional proteins detected in the proteome could
provide some information on the characteristic pheno-typic features [8] and they can be used for defining a taxon. However, it has to be stressed that different sets of genes and phenotypic traits are required for the classification of different taxa and lineages, and there may also be some bacteria which escape a reliable classification because they do not form coherent clusters.