I don't have it to hand at the moment, but there was a paper about six
months ago (in int. j. syst. bact. I think). O.K. I have it to hand now
....Check out IJSB Oct 1994 vol 44 no 4.
Taxonomic note: A place for DNA-DNA reassociation and 16S rRNA sequence
analysis in the present definition in bacteriology.
This follows on from the 1987 paper in IJSB (oct 1987, vol 37 no 4 p463-464)
Report of the ad hoc committee on reconsiliation of approaches to bacterial
(top ref. pages 846-849)
The story goes as follows:
First there was numerical taxonomy (of the biochemical etc. kind)
Then there was DNA-DNA hybridisation (quote from 1987 paper- The phylogenetic
definition of a species generally would include strains with approximately 70%
or greater DNA-DNA relatedness and with 5oC or less deltaTm. Both values must
Now the biggie is the 16S sequence (quote from the 1994 paper- Since today the
primary structure of the 16S rRNA is easier to determine than hybridization
between DNA strands, the strength of the sequence analysis is to recognise the
level at which DNA pairing studies needs to be performed, which certainly
applies to similarities of 97% and higher).
There is your new rule of thumb. Again don't be seduced into thinking
that this is the only criterion needed for species definition. It is
only a guideline and should be treated with EXTREME caution. Species
definition requires a multitude of different observations (biochemical and
Hope this helps
P.S. authors of first paper: Wayne, L.G. et al (12 authors)
authors of second paper: Stackebrandt, E. and Goebel, B.M.