Bacterial Pathogenesis

classdymg bacteria was analyzed. Among the first genotypic approaches
was the determination of the DNA base composition, which is still con-
sidered part of the standard description of bacterial taxa. Various indirect
methods to describe genotypic characteristics such as hybridization tech-
niques and restriction enzyme analysis are now partly replaced by direct
sequence analysis.
The Ad Hoc Committee on Reconciliation of Approaches to Bacterial
Systematics (Wayne et al., 1987) stated that taxonomy should be phylo-
geny determined and that the complete genome sequence should there-
fore be the standard for species delineation. In practice, whole genome
DNA-DNA hybridization studies approach the sequence standard and
represent the best applicable procedure at present. The species was there-
fore defined as a group of strains, including the type strain, sharing 70%
or greater DNA-DNA relatedness with 5°C or less AT, (T, is the melting
temperature of the hybrid as determined by stepwise denaturation; ATm is
the difference in T, in "C between the homologous and heterologous
hybrid formed in standard conditions; Wayne et al., 1987). It was also rec-
ommended that phenotypic and chemotaxonomic features should agree
with this definition and that groups of strains delineated by means of
DNA-DNA hybridization studies as distinct species, but indistinguish-
able by phenotypic characteristics, should not be named. Preferentially,
several simple and straightforward tests should endorse speciation based
on DNA hybridization values. Phenotypically similar, but genotypically
distinct groups of strains have been referred to as genomic species,
genomic groups, genospecies, genomospecies, or genomovars (Ursing et
al., 1995).
The level of DNA-DNA hybridization thus plays a key role in specia-
tion as defined by Wayne et al. (1987). Although theoretically justified, the
practice of DNA hybridization is far from simple (Vandamme et al.,
1996a). DNA-DNA hybridization studies are laborious and time-consum-
ing, and for many bacterial groups it is hardly possible to extract sufficient
DNA to perform the obligatory hybridization studies. Even if sufficient
DNA is generated, many other problems exist. Different methods are
used to determine the level of DNA-DNA hybridization, and these meth-
ods do not always give the same quantitative results. The value of 70%
DNA relatedness seems only to be indicative rather than absolute (Ursing
et al., 1995). Several alternative small-scale DNA-DNA hybridization pro-
cedures have been elaborated, but were not or were insufficiently vali-
dated by comparison with the classical DNA-DNA hybridization
techniques.
++++++ THE POLYPHASIC SPECIES CONCEPT
Over the last 50 years, an extremely wide variety of different cellular com-
ponents has been used to study relationships between bacteria and to
design classifications. The genotypic information present at the DNA
level has been analyzed by estimations of the DNA base composition and
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