ATTENUATED TYPHOID VACCINE SALMONELLA-TYPHI TY21A - FINGERPRINTING AND DUALITY CONTROL

Live attenuated vaccines, developed with molecular genetical technique s, require new approaches for their quality control. To develop novel quality control tests that enhanced and extended existing procedures, the attenuated vaccine strain Salmonella typhi Ty21a and its parent st rain Ty2 were characterized by pulsed-field gel electrophoresis (PFGE) and direct nucleotide sequence analysis. Mutant and parent strains we re distinguished using fingerprints generated by the resolution on PFG E of chromosomal DNA digested with each of the enzymes SfiI, SpeI or X baI. These fingerprints were stable through multiple in vitro passages of the vaccine strain and were identical from one batch of vaccine to another. It was also possible to distinguish between the mutant and p arent strains by direct nucleotide sequence analysis of the galE gene. This analysis identified two base changes in the gene from strain Ty2 1a: a single base deletion causing a frameshift that would result in a truncated gene product, accounting for the galE phenotype; and a tran sition that eliminated an AluI restriction site. The consequent change in the AluI fingerprint of the galE gene in strain Ty21a provided a r apid, PCR-based alternative to the use of differential media or bioche mical assays for the identification of the vaccine strain.