Background:
It is a matter of ongoing debate whether a
universal species concept is possible for
bacteria. Indeed, it is not clear
whether closely related isolates of
bacteria typically form discrete
genotypic clusters that can be assi
gned as species. The most challenging test of whether species can
be clearly delineated is provided by analysis
of large populations of closely-related, highly
recombinogenic, bacteria that colo
nise the same body site. We ha
ve used concatenated sequences
of seven house-keeping loci from 770 strains of 11 named
Neisseria
species, and phylogenetic trees,
to investigate whether genotypic
clusters can be resolved amon
g these recombinogenic bacteria
and, if so, the extent to which they correspond to named species.
Results:
Alleles at individual loci were widely di
stributed among the named species but this
distorting effect of recombinat
ion was largely buffered by using concatenated sequences, which
resolved clusters co
rresponding to the three species most numerous in the sample,
N. meningitidis
,
N. lactamica
and
N. gonorrhoeae
. A few isolates arose from the branch that separated
N. meningitidis
from
N. lactamica
leading us to describe
these species as 'fuzzy'.
Conclusion:
A multilocus approach using large samples of closely related isolates delineates
species even in the highly recombinogenic human
Neisseria
where individual loci are inadequate for
the task. This approach should be applied by
taxonomists to large samples of other groups of
closely-related bacteria,
and especially to those where specie
s delineation has historically been
difficult, to determine whether ge
notypic clusters can be delineated
, and to guide the definition of
species.