GBTC 5212 Exam 3 (Vaccines)

What requirements should vaccine strains fulfill?

• Should be very attenuated in virulence

• Should be able to remain in the host for a sufficient time to allow the host to develop a specific immune response (producing antibodies) against future infection with a fully virulent strain

What does attenuated mean?

The pathogenic bacterium has lost most of its capacity to produce a disease

What do bacterial vaccines consist of?

• Bacterial products

• Inactivated bacteria

• Attenuated live bacteria

What are some examples of bacterial products?

• Proteins

• Peptides

• Carbohydrate capsule

What are the properties of bacterial product vaccines?

• Usually safe

• Chemically defined

• Do not spread to non-vaccinated individuals

• Usually expensive

• Produce a limited immune response

What are the properties of inactivated bacterial vaccines?

• Killed by heat or chemical treatment

• Poor immunogenicity

• Large amount is usually required

What are the properties of live bacterial vaccines?

• Specific genes are deleted or altered

• Naturally processed antigens produce a strong immune response

What are the ways live bacterial vaccines are created?

• Chemical mutagenesis

• Transposon mutagenesis

• Gene replacement

What are the types of attenuation?

• Unidentified mutation

• Defined mutation

What are the types of defined mutation?

• Disruption of metabolic genes

• Disruption of regulatory genes

• Disruption of virulence genes

What vaccines were produced by unidentified mutations?

• BCG (Bacillus Calmette-Guerin) vaccine

• Ty21a vaccine

What does the BCG vaccine protect against?

Tuberculosis

How was the BCG vaccine produced?

• Produced by in vitro passage of Mycobacterium bovis

• Resulted in multiple mutations within the chromosome of the bacteria

• The exact reason for this attenuation is not known

• The vaccine is very safe and effective

What does the Ty21a vaccine protect against?

Typhoid fever

How was the Ty21a vaccine produced?

• Salmonella typhi was subjected to chemical mutagenesis with nitrosoguandine (NTG)

• The vaccine elicits a very good response if given in 3-4 doses

What is chemical mutagenesis?

Treating the virulent strain with chemical mutagens like NTG or EMS

What are some example of DNA changes?

• Transition

• Transversion

• Deletion of a few base pairs

What are mutagenized cells screened for?

Loss of specific virulence factors

What are some example of DNA changes caused by chemical mutagenesis?

• Mutation in the promoter region of the virulence gene leading to loss of all activity

• Mutation within the structural part of the virulence gene preventing synthesis of a full biologically active protein

• Disruption of genes involved in metabolism or regulation

What is shared between most strains produced by chemical mutagenesis?

They are less likely to be successful vaccine candidates

What is the logic behind metabolic dependent mutants?

• Introduce a mutation in a gene that codes for a certain enzyme (s) in the synthetic pathway of an important metabolite

• In an environment which is restricted for the metabolite, like certain locations in the host, the mutant can not proliferate

What are the requirements for a metabolic dependent mutant?

• The metabolite must be essential for the growth of the bacterium

• The mutant can grow only when it is supplied with the metabolite (in the growth medium)

• Virulence is reduced

• Bacteria are quickly eliminated from the host

What mutations does strain 543Ty carry?

• aroA

• purA

What is aroA?

Gene that encodes enzyme involved in the aromatic amino acid synthesis pathway

What is purA?

Gene that encodes an essential enzyme for the DNA synthesis pathway

Why was 543Ty not used in vaccines?

Less immunogenic than its parent strain

What have cholera vaccines focused on?

Production of a non-toxigenic strain

What is gene replacement?

Replacement of the intact virulence gene on the chromosome with a mutated gene

How does gene replacement work?

• Gene is mutated by cloning an antibiotic resistance marker within the gene (gene interruption)

• The mutant strain cannot revert to a virulent phenotype

Where is gene replacement important?

Pathogenesis studies and vaccine development

What is the main advantage of gene replacement?

Inactivation of a specific virulence gene with no chance of reversion