Detection of Salmonella typhi by nested PCR based on the ViaB sequence.

By using nested PCR based on the nucleotide sequence encoding the Vi antigen, the ViaB region of Salmonella typhi, it has been possible to detect Salmonella typhi at the single-cell level. Amplification with two primer sets, R1, which amplified 599 bp of DNA (nucleotides 745 to 1343), and R2, which amplified 307 bp (nucleotides 877 to 1183), was associated with highly specific results for S. typhi in clinical specimens, including blood (2). Nevertheless, the sensitivity of the nested PCR based on ViaB sequences would be considered poor, as it would not detect Vi-negative strains of S. typhi. Vi-negative strains of S. typhi that are devoid of the Vi antigen and that cannot be typed with Vi phages occur globally. They were reported worldwide in the 1960s (3). In the 1970s, Vi-negative isolates were encountered in Jamaica (1), Indonesia (6), New Zealand (4), and Malaysia (5). Undoubtedly, a positive result with ViaB nested PCR (2) would have immense diagnostic value. However, an alternative approach would be required to detect Vi-negative S. typhi. The PCR based on the H1-d flagellin gene (7) may well be the solution to identify Vi-negative specimens in a group of clinical specimens, including blood.