Significance of Vi Negative Isolates of Salmonella Enterica Serovar Typhi

Typhoid is a major global disease. The causative agent, Salmonella enterica serovar Typhi (S. Typhi) has a capsular antigen called Vi antigen which is traditionally considered to be the main cause of virulence. All the current vaccines are based on Vi antigen. However, the realization of the fact that there are S. Typhi strains which lack Vi antigen but still exist naturally and can cause disease has stirred great scientific interest. It is also interesting to note that their relative prevalence is affected by climatic conditions. Now it is established that Vi positive and Vi negative S. Typhi have different modes of pathogenesis; and as recent studies suggest, different structure of polysaccharide antigens. This means that current vaccines are not effective against a significant number of S. Typhi strains which not only affect the success of vaccination programs but also help in rapid emergence of Vi negative S. Typhi due to natural selection. The focus should be on vaccines based on antigens which are universally present in all S. Typhi. One such candidate is O-specific polysaccharides (OSPs). Successful attempts have been made to prepare conjugate vaccines based on OSPs.