culture conditions rather than its natural conditions [2]. The ability
to cause transient infection without causing disease mount effective
long-lasting immune response against the pathogen. However, this
vaccination method is not fool proof. It has its own share of
advantages and disadvantages. Because of their potential to replicate, these vaccines confer prolonged protective immunity and
produce long lasting memory cells. Consequently, a single immunization dose is sufficient to provide protection without the need of
subsequent booster doses. On the other hand, the major disadvantage of such vaccines include their ability to revert back to the
virulent form. Sequential passaging of the virulent bacterial strain
in unnatural host or in hostile conditions generate alterations in
gene sequences which lead to attenuation of the bacterial strain. For
example, in vitro passaging of Mycobacterium bovis for multiple
rounds on potato slices cooked in bile beef led to elimination of
the deletion region 1 (RD 1). Chemical mutagenesis is another
method of gene disruption in virulent bacterial strains. In pathogenic Salmonella Typhi strain Ty2, a distinctive mutation caused
inactivation of galE gene and Vi polysaccharide synthesis. This
resulted in an impressive attenuation of the bacterial strain
[3]. M01ZH09 is an S. typhi strain Ty2 derivative with deletion
mutations in aroC and ssaV [4]. Ty800, a Ty2 derivative deleted in
phoP/phoQ is another safe and immunogenic single-dose vaccine
for typhoid patients [5]. On the other hand, targeted deletion of
the ctxA gene encoding the toxic A subunit (CTA) of the cholera
toxin (CT) led to the development of Vibrio cholerae vaccine strain
CVD 103-HgR in the 1980s [6].
Viral inactivation is also performed by similar methods of passaging in animals, eggs, or cell culture or by sequential passaging in
cold-adapted conditions. Alternatively, mutagenesis is induced
either by ultraviolet radiation or by chemical means until successful
attenuation. Genetically attenuated parasitic strains developed by
targeted gene disruption is one of the major methods of development of parasite attenuated virus. A major advantage of genetically
attenuated parasites is that they exhibit a homogeneous population
with defined genetic constitution and identical attenuated phenotypes. Genetically attenuated parasite (GAP) was developed in Plasmodium falciparum by disruption of the p52 gene by single
crossover recombination through plasmid integration at targeted
sites. The most recent P. falciparum GAP constitutes deletion of
two tandem-arranged genes, p52 and p36, that results in severe
growth defects in hepatocyte infection. Irradiated sporozoites of
Plasmodium berghei confers protective immunity against
subsequent infection of viable sporozoites [7]. The subsequent
section lists the various methods of attenuation with emphasis on
specific case studies.
332
Dipasree Hajra et al.
to cause transient infection without causing disease mount effective
long-lasting immune response against the pathogen. However, this
vaccination method is not fool proof. It has its own share of
advantages and disadvantages. Because of their potential to replicate, these vaccines confer prolonged protective immunity and
produce long lasting memory cells. Consequently, a single immunization dose is sufficient to provide protection without the need of
subsequent booster doses. On the other hand, the major disadvantage of such vaccines include their ability to revert back to the
virulent form. Sequential passaging of the virulent bacterial strain
in unnatural host or in hostile conditions generate alterations in
gene sequences which lead to attenuation of the bacterial strain. For
example, in vitro passaging of Mycobacterium bovis for multiple
rounds on potato slices cooked in bile beef led to elimination of
the deletion region 1 (RD 1). Chemical mutagenesis is another
method of gene disruption in virulent bacterial strains. In pathogenic Salmonella Typhi strain Ty2, a distinctive mutation caused
inactivation of galE gene and Vi polysaccharide synthesis. This
resulted in an impressive attenuation of the bacterial strain
[3]. M01ZH09 is an S. typhi strain Ty2 derivative with deletion
mutations in aroC and ssaV [4]. Ty800, a Ty2 derivative deleted in
phoP/phoQ is another safe and immunogenic single-dose vaccine
for typhoid patients [5]. On the other hand, targeted deletion of
the ctxA gene encoding the toxic A subunit (CTA) of the cholera
toxin (CT) led to the development of Vibrio cholerae vaccine strain
CVD 103-HgR in the 1980s [6].
Viral inactivation is also performed by similar methods of passaging in animals, eggs, or cell culture or by sequential passaging in
cold-adapted conditions. Alternatively, mutagenesis is induced
either by ultraviolet radiation or by chemical means until successful
attenuation. Genetically attenuated parasitic strains developed by
targeted gene disruption is one of the major methods of development of parasite attenuated virus. A major advantage of genetically
attenuated parasites is that they exhibit a homogeneous population
with defined genetic constitution and identical attenuated phenotypes. Genetically attenuated parasite (GAP) was developed in Plasmodium falciparum by disruption of the p52 gene by single
crossover recombination through plasmid integration at targeted
sites. The most recent P. falciparum GAP constitutes deletion of
two tandem-arranged genes, p52 and p36, that results in severe
growth defects in hepatocyte infection. Irradiated sporozoites of
Plasmodium berghei confers protective immunity against
subsequent infection of viable sporozoites [7]. The subsequent
section lists the various methods of attenuation with emphasis on
specific case studies.
332
Dipasree Hajra et al.