Molecular Virulence Mechanisms of Mycobacterium bovis 107
effects of the pathogen or the efforts of the host
to control the infection. This is of importance
when considering M. bovis, a pathogen with the
ability to infect and transmit across a wide vari-
ety of hosts compared to other members of the
MTBC, and suggests an intimate ability to over-
come and exploit host defences to its own ends.
Secondly, most virulence factors are defined as
such because in their absence the pathogen is
less able to negatively affect the host. However,
these factors may not be involved in directly
harming the host, but are essential for pathogen
survival and ability to thrive, such as the ability
to acquire nutrients in vivo. Lastly the mycobac-
teria have coevolved alongside their hosts over
many millennia; one study suggested the emer-
gence of the MTBC 70,000 years ago (Comas
et al., 2013). As the mycobacteria have adapted
in order to take advantage of their animal hosts,
so too have their hosts evolved under selective
pressures to develop resistance to mycobacterial
infection. This is most evident when examining
the varying outcomes of infection with MTBC
bacilli in humans, with some infections develop-
ing into disseminated disease within a few years,
some showing latent infection that only pro-
gresses to full disease after many years, and
some infections being eradicated by the host
with no symptoms of disease evident (Lillebaek
et al., 2002). These outcomes reinforce the con-
cept that the genetic and immune status of the
host is significant in determining the outcome of
infection and hence the underlying virulence of
the mycobacteria. It is therefore important to
note that although many studies have high-
lighted the importance of particular proteins or
surface lipids in mediating disease in cultured
cells or mouse models, there is likely a great deal
that is unique to the interaction of mycobacteria
with their respective hosts.
To summarize, we define the virulence fac-
tors as the elements required by the mycobacte-
ria to survive and cause damage to the host,
directly or indirectly. In order to quantify the
virulence of an organism we can use different
measures, including host mortality, bacterial
burden and histopathology. When examining
M. bovis virulence, we are interested in both the
conserved factors that are shared by different
mycobacterial species and the unique factors
that M. bovis possesses that may allow it to affect
such a wide range of host species. These
virulence factors encompass cell wall compo-
nents involved in attachment and the interac-
tion between bacterial and host cell surfaces,
secreted proteins that may guide bacterial
localization and regulatory factors that influ-
ence the expression of a range of different fac-
tors. Insight into the identity of the genes
underlying these factors was offered through
comparative genomic analysis of M. bovis and
other mycobacteria and it is to this topic that we
shall next turn.8.2 Genomic AnalysisA key step in defining the genetic basis of viru-
lence and tropism in the MTBC was the elucida-
tion of their genome sequences. Although the
constituent MTBC species show a high degree of
genetic similarity (>99%), there are significant
differences apparent in their genomes. One of
the most striking differences first identified
between the genomes of M. bovis and M. tubercu-
losis was that M. bovis has a smaller genome,
with large regions missing from its genome com-
pared to M. tuberculosis (Garnier et al., 2003).
These deleted sections are known as regions of
difference (RD) and they can be used to trace the
evolution of the different MTBC species. It was
originally thought that M. tuberculosis had
emerged in humans following transmission of
M. bovis into the human population with the
domestication of cattle. However, the use of the
RD loci as evolutionary markers revealed instead
that M. tuberculosis more closely resembled the
common progenitor of the MTBC than M. bovis,
as the latter had suffered the greatest loss of RD
loci compared to other members of the complex
(Brosch et al., 2002; Mostowy et al., 2002). The
loss of the RD loci during the evolution of
M. bovis can be thought of as a series of deletion
events, although it is not understood what role
the loss of these loci had, if any, in granting
some selective advantage to the bacteria in colo-
nizing a wider range of hosts, or whether it was
simply the removal of deleterious or redundant
gene regions.
While multiple genome sequences of
M. bovis and M. tuberculosis strains are now
available, the original findings from the compar-
ative analysis of the first strains to be sequenced,