Where Does Mycobacterium Tuberculosis Stand in Bacterial Taxonomy?
As tuberculosis continues to be a formidable infection disease
around the world, more research needs to be directed toward the
understanding of the causal agent, i.e., Mycobacterium tuberculosis.
The availability of the Mycobacterium tuberculosis genome sequence
offers an unprecedented opportunity
for advancing our knowledge about this microorganism at the molecular
level that can then be translated into new and more effective
strategies for disease prevention, diagnosis, and treatment.
Bacterial Phylogeny
M. tuberculosis belongs to Actinobacteria, which along with
the Bacillus/Clostridium group are placed under
taxon Firmicutes, according to the bacterial taxonomy
provided by NCBI (National Center for Biotechnology Information, USA).
Here, Actinobacteria correspond to the high G + C group,
the Bacillus/Clostridium group to the low G + C group,
and Firmicutes to Gram-positive bacteria
in the original reference of
the phylogenetic tree constructed based on 16S ribosomal RNA.
This arrangement suggests that
M. tuberculosis is more related to Gram-positive
than to Gram-negative bacteria, such as Escherichia and Neisseria, which
are classified as Proteobacteria.
In the latest bacterial taxonomy tree, the class Actinobacteria
is elevated to the rank of phylum. This change raises the question
about the evolutionary affinity between the low G+C and high G+C
groups.
Genome Comparison
Genome comparison is enlightening and promising in
phylogenetic analysis concerned with constructing the
evolutionary history of biological beings.
As a result, differences on the genomic scale can be identified
that may account for important properties such as pathogenicity
and drug susceptibility.
As more microbial genomes have been completely sequenced,
it can be envisioned
to construct a phylogenetic tree based on whole genomes rather
than on any selected single gene. Expectedly, a genome-based tree
is more representative of the evolutionary history of
whole organisms and less inconsistent
than single-gene trees. As such, the genome-based tree
provides a better index for interpretation and inference
about the origin and nature of species.
Our research indicates,
in contrast to the standard bacterial phylogeny based on
16S ribosomal RNA sequence comparison, genome comparison based on
shared orthologous gene content shows that M. tuberculosis is actually
more related to Gram-negative than to Gram-positive bacteria.
This finding is also supported by clinical observations that
M. tuberculosis shows sensitivity to some Gram-negative
effective antibiotics like Streptomycin, Ciprofloxacin and Amikacin but
not to Gram-positive specific antibiotics
inasmuch as pharmacological response may reflect
underlying genetic makeup.
In fact, M. tuberculosis shares some similarities with both
Gram-positive and Gram-negative bacteria and seems to have
evolved by a separate path.
It would be justified to move the M. tuberculosis associated
group (Actinomycetes) to a position independent of both
Gram-positive and negative bacteria.