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J. Bacteriol., Mar 1996, 1274-1282, Vol 178, No. 5
GG Mahairas, PJ Sabo, MJ Hickey, DC Singh and CK Stover
The live attenuated bacillus Calmette-Guerin (BCG) vaccine for the
prevention of disease associated with Mycobacterium tuberculosis was
derived from the closely related virulent tubercle bacillus, Mycobacterium
bovis. Although the BCG vaccine has been one of the most widely used
vaccines in the world for over 40 years, the genetic basis of BCG's
attenuation has never been elucidated. We employed subtractive genomic
hybridization to identify genetic differences between virulent M. bovis and
M. tuberculosis and avirulent BCG. Three distinct genomic regions of
difference (designated RD1 to RD3) were found to be deleted from BCG, and
the precise junctions and DNA sequence of each deletion were determined.
RD3, a 9.3-kb genomic segment present in virulent laboratory strains of M.
bovis and M. tuberculosis, was absent from BCG and 84% of virulent clinical
isolates. RD2, a 10.7-kb DNA segment containing a novel repetitive element
and the previously identified mpt- 64 gene, was conserved in all virulent
laboratory and clinical tubercle bacilli tested and was deleted only from
substrains derived from the original BCG Pasteur strain after 1925. Thus,
the RD2 deletion occurred after the original derivation of BCG. RD1, a
9.5-kb DNA segment found to be deleted from all BCG substrains, was
conserved in all virulent laboratory and clinical isolates of M. bovis and
M. tuberculosis tested. The reintroduction of RD1 into BCG repressed the
expression of at least 10 proteins and resulted in a protein expression
profile almost identical to that of virulent M. bovis and M. tuberculosis,
as determined by two-dimensional gel electrophoresis. These data indicate a
role for RD1 in the regulation of multiple genetic loci, suggesting that
the loss of virulence by BCG is due to a regulatory mutation. These
findings may be applicable to the rational design of a new attenuated
tuberculosis vaccine and the development of new diagnostic tests to
distinguish BCG vaccination from tuberculosis infection.
Copyright © 1996, American Society for Microbiology
Molecular analysis of genetic differences between Mycobacterium bovis BCG and virulent M. bovis
Laboratory of Tuberculosis and Molecular Microbiology, PathoGenesis Corp., Seattle, Washington 98119, USA.
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