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The Vi capsular polysaccharide (ViPS) protects Salmonella enterica subspecies enterica serotype Typhi (S.Typhi) in vivo by multiple mechanisms. Recent microbiological reports from typhoid endemic countries suggest that acapsulate S.Typhi may occur in nature and contribute to clinical typhoid fever that is indistinguishable from disease caused by capsulate strains. The prevalence and genetic basis of ViPS-negative S.Typhi isolates in children from Kathmandu, Nepal, were tested in 68 isolates. Although 5.9% of isolates tested negative for capsular expression by slide agglutination tests, a novel multiplex PCR assay and individual PCR analyses demonstrated the presence of all 14 genes responsible for the synthesis, transportation and regulation of the ViPS. These data suggest that phenotypically acapsulate S.Typhi may not have a genetic basis for the same. [ABSTRACT FROM AUTHOR]

The article focuses on the potential of the Vi polysaccharide (ViPS)-protein conjugate vaccine to prevent typhoid fever in children. It states that this vaccine developed in the 1980s does not produce immunological memory and is not immunogenic in children below 2 years of age. Enteric fevers caused by Salmonella paratyphi A are said to be a challenge to the use of ViPS-protein conjugate vaccine.