organism Mycobacterium tuberculosis, which causes the disease
tuberculosis in humans, is actually a member of a larger
Mycobacterium tuberculosis complex. This complex includes the
subspecies M. bovis, M. africanum, and M. microti,
in addition to the M. tuberculosis bacterium from which the
grouping derived its name (Figure 1).
Figure 1. Proposed phylogeny of the Mycobacterium
tuberculosis complex. The phylogeny depicts the order in which
members of the complex could have evolved from a common ancestor.
The clusters to the left of the vertical axis represent genomic
deletions that separate the subspecies.
grander scheme of organization, Mycobacterium tuberculosis
is classified under the domain Bacteria with other microorganisms that
possess cell walls and lack nuclei (are prokaryotic). The phylum
Actinobacteria, which is comprised of a single class by the same name,
consists of bacteria that vary widely in their morphological,
physiological, and genomic characteristics. Actinobacteria morphologies
range from coccoid to rod-shaped to filamentous. Furthermore, bacteria
in this grouping are Gram-positive with a high G + C (guanine and
cytosine) content in their DNA (Figure 2). Under the order Actinomycetales,
Actinobacteria take the form of rigid bacilli or filamentous cells that
have a tendency to branch; thus, M. tuberculosis belongs with the
Actinomycetales because the bacterium is rigid and rod-shaped. The
suborder Corynebacterineae denotes that, in addition to having a
Gram-positive type cell wall, M. tuberculosis bacilli are
non-motile and do not produce spores. Within the family
Mycobacteriaceae, bacterial cells typically do not branch, so any
mycelium formation tends to be undeveloped. As with M. tuberculosis,
most Mycobacteriaceae are aerobic organisms that parasitize both humans
and animals. The classification Mycobacterium, the sole genus of
the Mycobacteriaceae family, also indicates that the organism in
question is slow-growing and acid-fast. Finally, the species name
Mycobacterium tuberculosis distinguishes the bacterium from other
species within the same genus, such as Mycobacterium leprae,
which is the causative agent of the disease leprosy. The following
is the complete taxonomic classification for the species
Mycobacterium tuberculosis as stated in Bergey’s Manual of
Systematic Bacteriology 2nd edition, volume one.
Figure 2. Phylogenetic tree showing relationships among the
bacteria. The red arrow indicates the position of Mycobacterium
tuberculosis within the grouping.