Vi Antigen Expression in Salmonella enterica Serovar Typhi Clinical Isolates from Pakistan

Abstract

The accurate identification of Salmonella enterica subsp. enterica serovar Typhi variants that fail to express the capsular polysaccharide, Vi, is an important and much discussed issue for medical microbiology. We have tested a multiplex PCR method which shows the presence or absence of the genetic locus required for Vi expression. Of 2,222 Salmonella serovar Typhi clinical isolates collected from patients' blood over a 4-year period in a region of Pakistan where typhoid is endemic, 12 tested negative for Vi expression by serological agglutination. However, only 1 of these 12 was Vi negative by the multiplex PCR method. This result was confirmed by immunofluorescence, the most sensitive method for Vi characterization in Salmonella serovar Typhi. The multiplex PCR described therefore represents a simple and accurate method for surveillance for Vi-negative variants of Salmonella serovar Typhi in Pakistan. Testing of clinical isolates of Salmonella serovar Typhi, before subculture, from other regions where Vi-negative Salmonella serovar Typhi has been described should be carried out so that the impact of vaccination with purified Vi antigen on the levels of Vi-negative Salmonella serovar Typhi in bacterial populations can be assessed.

Vi polysaccharide–protein conjugate vaccine for the prevention of typhoid fever in children: hope or hype?

“…However, studies that determined the incidence of Vi-deficient Salmonella typhi, using molecular genetic methods, have demonstrated an extremely low incidence of this strain among clinical isolates. Only one out of 2222 (0.045%) clinical isolates of Salmonella typhi tested negative for the ViPS by PCR and immunofluorescence [18]. This is much lower than the numbers suggested by agglutination tests with Vi antisera.…”

“…Initially dismissed as artefacts associated with poor microbiological techniques of isolation or repeated subculturing on artificial media, there is now undeniable evidence from PCR studies of clinical isolates of Salmonella typhi of the existence of Vi-negative strains [18]. The genome of these strains demonstrates an absence of one or more loci in the ten genes that constitute the viaB operon which codes for the production and transportation of the ViPS.…”

Molecular Mechanisms of Salmonella Pathogenicity: Review

“…Be lir le nen se ro tip le rin bü yük bir ço ğun lu ğun da, pa to je ni te nin mo le kü ler me ka niz ma sı üze ri ne çok az sa yı da ça lış ma bu lunmak ta dır. [1][2][3] Sal mo nel lo zun sı cak kan lı hay van lar da oluş tur du ğu sis te mik ya da sis te mik ol ma yan enfek si yon la rın mo le kü ler dü zey de araş tı rıl ma sın da S. typhi mu ri um mo del or ga niz ma ola rak kul la nıl -mak ta dır.…”

SPI-7: Salmonella’s Vi-Encoding Pathogenicity Island

“…The future of Vibased vaccines will depend on the maintenance of Vi within populations of S. Typhi; there are worrying reports that the viaB-locus can be inactivated in a number of ways. As mentioned above, spontaneous excision of SPI-7 has been observed, which would result in loss of the Vi phenotype [9,11,12]. Several other strains of S. Typhi and S. Dublin have been found to be Vinegative, yet still maintain SPI-7.…”

“…A survey of over 2,000 clinical isolates recently found only one strain which has lost SPI-7, by precise excision [11]. Loss appears to occur more frequently after storage, with S. Typhi strain Ty2 losing the majority of SPI-7 during passage, excising the island from STY4521-STY4666 [9].…”